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LTC3737 Dual 2-Phase, No RSENSETM, DC/DC Controller with Output Tracking
FEATURES
s s
DESCRIPTIO
s s s s s s s
s
s s s s s s
Sense Resistor Optional Out-of-Phase Controllers Reduce Required Input Capacitance Programmable Output Voltage Tracking Constant Frequency Current Mode Architecture Wide VIN Range: 2.75V to 9.8V Wide VOUT Range: 0.6V to VIN 0.6V 1.5% Reference Low Dropout Operation: 100% Duty Cycle True PLL for Frequency Locking or Adjustment (Frequency Range 250kHz to 850kHz) Selectable Burst Mode(R) or Pulse Skipping Operation at Light Loads Internal Soft-Start Circuitry Selectable Maximum Peak Current Sense Threshold Power Good Output Voltage Monitor Output Overvoltage Protection Micropower Shutdown: IQ = 9A Tiny 4mm x 4mm QFN and 24-Lead SSOP Packages
The LTC(R)3737 is a 2-phase dual step-down switching regulator controller that requires few external components. The constant frequency current mode architecture provides excellent AC and DC load and line regulation. MOSFET VDS sensing eliminates the need for current sense resistors and improves efficiency. Power loss and noise due to the ESR of the input capacitance are minimized by operating the two controllers out of phase. Burst Mode operation provides high efficiency operation at light loads. 100% duty cycle provides low dropout operation and extends battery operating time. Switching frequency can be programmed up to 750kHz, allowing the use of small surface mount inductors and capacitors. For noise sensitive applications, the LTC3737 can be externally synchronized from 250kHz to 850kHz. Other features include a power good output voltage monitor, a tracking input and internal soft-start. The LTC3737 is available in the low profile thermally enhanced (4mm x 4mm) QFN package or a 24-lead SSOP narrow package.
, LTC and LT are registered trademarks of Linear Technology Corporation. Burst Mode is a registered trademark of Linear Technology Corporation. No RSENSE is a trademark of Linear Technology Corporation. U.S. patent numbers 5481178, 5731694, 5929620, 6144194,6580258, 5994885
APPLICATIO S
s s s s
One or Two Lithium-Ion Powered Devices Notebook and Palmtop Computers, PDAs Portable Instruments Distributed DC Power Systems
TYPICAL APPLICATIO
187k SW1 59k 15k VFB1 ITH1 220pF 220pF 59k 118k
SENSE1+ PVIN1 PGATE1
M1
2.2H
VIN 2.75V TO 9.8V
+
D1 47F
VOUT1 2.5V
EFFICIENCY (%)
PGOOD VIN LTC3737 PGND SGND TRACK ITH2 VFB2 SW2 RUN/SS PVIN2 SENSE2+ M2
3737 F01
10F x2
15k
2.2H
Figure 1. High Efficiency, 2-Phase, 550kHz Dual Step-Down Converter
+
PGATE2
D2
47F VOUT2 1.8V
U
Efficiency vs Load Current
100 95 90 85 80 75 70 65 60 55 50 1 10 100 1000 LOAD CURRENT (mA) 10000
3737 F01b
U
U
VIN = 3.3V
VOUT = 2.5V
VOUT = 1.8V
3737f
1
LTC3737
ABSOLUTE AXI U RATI GS
Input Supply Voltage (VIN), PVIN1, PVIN2, SENSE1+, SENSE2+ .................................. - 0.3V to 10V PGATE1, PGATE2, PLLLPF, RUN/SS, SYNC/MODE, TRACK, IPRG1, IPRG2 Voltages .... - 0.3V to (VIN + 0.3V) VFB1, VFB2, ITH1, ITH2 Voltages .................. - 0.3V to 2.4V SW1, SW2 Voltages ............ -2V to VIN + 1V or 10V Max PGOOD ..................................................... - 0.3V to 10V PGATE1, PGATE2 Peak Output Current (<10s) ......... 1A
PACKAGE/ORDER I FOR ATIO
TOP VIEW SW1 (SENSE1-) IPRG1 VFB1 ITH1 IPRG2 PLLLPF SGND VIN TRACK 1 2 3 4 5 6 7 8 9 24 SENSE1+ 23 PVIN1 22 NC 21 SYNC/MODE 20 PGATE1 19 PGND 18 PGATE2 17 RUN/SS 16 NC
IPRG1 SW1 (SENSE1-) SENSE1+
PVIN1
ORDER PART NUMBER
VFB1
LTC3737EGN
ITH1 1 IPRG2 2 PLLLPF 3 SGND 4 VIN 5 TRACK 6
NC
PGOOD SW2 (SENSE2-) SENSE2+
ITH2
ITH2 11 PGOOD 12
14 SENSE2 SW2 13 (SENSE2-)
+
PVIN2
VFB2
VFB2 10
15 PVIN2
GN PACKAGE 24-LEAD (NARROW) PLASTIC SSOP
TJMAX = 125C, JA = 130C/W
Consult LTC Marketing for parts specified with wider operating temperature ranges.
ELECTRICAL CHARACTERISTICS
PARAMETER Main Control Loops Input DC Supply Current Sleep Mode Shutdown UVLO Undervoltage Lockout Threshold Shutdown Threshold at RUN/SS Start-Up Current Source Regulated Feedback Voltage Output Voltage Line Regulation (Note 4)
The q denotes specifications that apply over the full operating temperature range, otherwise specifications are at TA = 25C. VIN = 4.2V unless otherwise specified.
CONDITIONS MIN TYP MAX UNITS
RUN/SS = 0V VIN < UVLO Threshold VIN Falling VIN Rising RUN/SS = 0V 0C to 85C (Note 5) -40C to 85C 2.75V < VIN < 9.8V (Note 5)
q q q
2
U
U
W
WW U
W
(Note 1)
Operating Temperature Range (Note 2) ... -40C to 85C Storage Ambient Temperature Range QFN Package .................................... -65C to 125C SSOP Package .................................. -65C to 150C Junction Temperature (Note 3) ............................ 125C Lead Temperature (Soldering, 10sec) LTC3737EGN ................................................... 300C
TOP VIEW
ORDER PART NUMBER LTC3737EUF
SYNC/ 18 MODE 17 PGATE1
24 23 22 21 20 19
25
16 PGND 15 PGATE2 14 RUN/SS 13 NC
7
8
9 10 11 12
UF PART MARKING 3737
UF PACKAGE 24-LEAD (4mm x 4mm) PLASTIC QFN TJMAX = 125C, JA = 37C/W EXPOSED PAD IS PGND (PIN 25) MUST BE SOLDERED TO PCB
220 9 3 1.95 2.15 0.45 0.5 0.591 0.588 2.25 2.45 0.65 0.7 0.6 0.6 0.05
325 20 10 2.55 2.75 0.85 1 0.609 0.612 0.2
A A A V V V A V V mV/V
3737f
LTC3737
ELECTRICAL CHARACTERISTICS
PARAMETER Output Voltage Load Regulation VFB1,2 Input Current TRACK Input Current Overvoltage Protect Threshold Overvoltage Protect Hysteresis Auxiliary Feedback Threshold Gate Drive 1, 2 Rise Time Gate Drive 1, 2 Fall Time Maximum Current Sense Voltage (SENSE+ - SW)(VSENSE(MAX)) Soft-Start Time Oscillator and Phase-Locked Loop Oscilator Frequency
The q denotes specifications that apply over the full operating temperature range, otherwise specifications are at TA = 25C. VIN = 4.2V unless otherwise specified.
CONDITIONS ITH = 0.9V (Note 5) ITH = 1.7V (Note 5) TRACK = 0.6V Measured at VFB SYNC/MODE Ramping Negative CL = 3000pF CL = 3000pF IPRG = Floating (Note 6) IPRG = 0V (Note 6) IPRG = VIN (Note 6) Time for VFB1 to Ramp from 0.05V to 0.55V Unsynchronized (SYNC/MODE Not Clocked) VPLLLPF = Floating VPLLLPF = 0V VPLLLPF = VIN SYNC/MODE Clocked Minimum Synchronizable Frequency Maximum Synchronizable Frequency fOSC > fSYNC/MODE fOSC < fSYNC/MODE IPGOOD Sinking 1mA VFB with Respect to Set Output Voltage VFB < 0.6V, Ramping Positive VFB < 0.6V, Ramping Negative VFB > 0.6V, Ramping Negative VFB > 0.6V, Ramping Positive -13 -16 13 16
q q q
MIN
TYP 0.12 -0.12 10 10
MAX 0.5 -0.5 50 50 0.7 0.675
UNITS % % nA nA V mV V ns ns
0.66 0.525
0.68 20 0.6 40 40
110 70 185 0.667
125 85 204 0.833
140 100 223 1
mV mV mV ms
q q q q q
480 260 650
550 300 750 200 1150 -4 4 125 -10.0 -13.3 10.0 13.3
600 340 825 250
kHz kHz kHz kHz kHz A A mV
Phase-Locked Loop Lock Range
850
Phase Detector Output Current Sinking Sourcing PGOOD Output PGOOD Voltage Low PGOOD Trip Level
-7 -10 7 10
% % % %
Note 1: Absolute Maximum Ratings are those values beyond which the life of a device may be impaired. Note 2: The LTC3737E is guaranteed to meet specified performance from 0C to 70C. Specifications over the -40C to 85C operating range are assured by design, characterization and correlation with statistical process controls. Note 3: TJ is calculated from the ambient temperature TA and power dissipation PD according to the following formula: TJ = TA + (PD * JAC/W)
Note 4: Dynamic supply current is higher due to gate charge being delivered at the switching frequency. Note 5: The LTC3737 is tested in a feedback loop that servos ITH to a specified voltage and measures the resultant VFB voltage. Note 6: Peak current sense voltage is reduced dependent on duty cycle to a percentage of value as shown in Figure 2.
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3
LTC3737 TYPICAL PERFOR A CE CHARACTERISTICS
Efficiency vs Load Current
100 95 90
Burst Mode OPERATION (SYNC/MODE = VIN)
EFFICIENCY (%)
EFFICIENCY (%)
85 80 75 70 65 60 55 50 1 TA = 25C VIN = 3.3V VOUT = 2.5V FIGURE 13 CIRCUIT 100 1000 10 LOAD CURRENT (mA) 10000
3737 G01
PULSE SKIPPING (SYNC/MODE = 0V)
Load Step (Pulse Skipping Mode)
VOUT AC-COUPLED 100mV/DIV
IL 2A/DIV
VIN = 3.3V 200s/DIV VOUT = 1.8V ILOAD = 300mA TO 3A SYNC/MODE = 0V FIGURE 13 CIRCUIT
Regulated Feedback Voltage vs Temperature
0.609 0.607 MAXIMUM CURRENT SENSE THRESHOLD (mV)
FEEDBACK VOLTAGE (V)
0.605 0.603 0.601 0.599 0.597 0.595 0.593 0.591 20 40 60 -60 -40 -20 0 TEMPERATURE (C) 80 100
RUN/SS VOLTAGE (V)
4
UW
3737 G07
Efficiency vs Load Current
100 TA = 25C 95 VOUT = 2.5V 90 85 80 75 70 65 60 55 50 1 100 1000 10 LOAD CURRENT (mA) 10000
3737 G02
Load Step (Burst Mode Operation)
VOUT AC-COUPLED 100mV/DIV
VIN = 3.3V
VIN = 5V
VIN = 4.2V
IL 2A/DIV
200s/DIV VIN = 3.3V VOUT = 1.8V ILOAD = 300mA TO 3A SYNC/MODE = VIN FIGURE 13 CIRCUIT
3737 G03
Tracking Start-Up with Internal Soft-Start (CSS = 0nF)
VOUT1 2.5V VOUT2 1.8V 500mV/ DIV
Tracking Start-Up with External Soft-Start (CSS = 10nF)
VOUT1 2.5V VOUT2 1.8V 500mV/ DIV
3737 G04
VIN = 4.2V 250s/DIV RLOAD1 = RLOAD2 = 1 FIGURE 13 CIRCUIT
3737 G05
VIN = 4.2V 2.5ms/DIV RLOAD1 = RLOAD2 = 1 FIGURE 13 CIRCUIT
3737 G06
Maximum Current Sense Threshold vs Temperature
135 IPRG = FLOAT
1.0 0.9
Shutdown (RUN) Threshold vs Temperature
130
0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1
125
120
115 -60 -40 -20 0 20 40 60 TEMPERATURE (C)
80
100
0 -60 -40 -20 0 20 40 60 TEMPERATURE (C)
80
100
3737 G08
3737 G09
3737f
LTC3737 TYPICAL PERFOR A CE CHARACTERISTICS
Undervoltage Lockout Threshold vs Temperature
2.50
RUN/SS PULL-UP CURRENT (A)
1.0
2.45
INPUT (VIN) VOLTAGE (V)
NROMALIZED FREQUENCY (%)
VIN RISING
2.40 2.35 2.30 VIN FALLING 2.25 2.20 2.15 2.10 20 40 60 -60 -40 -20 0 TEMPERATURE (C) 80 100
Oscillator Frequency vs Input Voltage
5
SHUTDOWN CURRENT (A)
3 2 1 0 -1 -2 -3 -4 -5 2 3 4 8 6 5 7 INPUT VOLTAGE (V) 9 10
16 14 12 10 8 6 4 2 0 2 3 4 8 6 5 7 INPUT VOLTAGE (V) 9 10
RUN/SS PIN START-UP CURRENT (A)
NORMALIZED FREQUENCY SHIFT (%)
4
TA = 25C
UW
3737 G10
3737 G13
RUN/SS Pull-Up Current vs Temperature
10 8
0.9 0.8 0.7 0.6 0.5 0.4 -60 -40 -20 0 20 40 60 TEMPERATURE (C)
Oscillator Frequency vs Temperature
6 4 2 0 -2 -4 -6 -8
80
100
-10 -60 -40 -20 0 20 40 60 TEMPERATURE (C)
80
100
3737 G11
3737 G12
Shutdown Quiescent Current vs Input Voltage
20 TA = 25C 18 RUN/SS = 0V 0.9
RUN/SS Start-Up Current vs Input Voltage
TA = 25C 0.8 RUN/SS = 0V 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 2 3 4 6 7 5 8 INPUT VOLTAGE (V) 9 10
3737 G14
3737 G15
3737f
5
LTC3737
PI FU CTIO S
ITH1, ITH2 (Pins 1, 8/Pins 4, 11): Current Threshold and Error Amplifier Compensation Point. Nominal operating range on these pins is from 0.7V to 2V. The voltage on this pin determines the threshold of the main current comparator. PLLLPF (Pin 3/Pin 6): Frequency Set/PLL Lowpass Filter. When synchronizing to an external clock, this pin serves as the lowpass filter point for the phase-locked loop. Normally, a series RC is connected between this pin and ground. When not synchronizing to an external clock, this pin serves as the frequency select input. Tying this pin to GND selects 300kHz operation; tying this pin to VIN selects 750kHz operation. Floating this pin selects 550kHz operation. SGND (Pin 4/Pin 7): Signal Ground. This pin serves as the ground connection for most internal circuits. VIN (Pin 5/Pin 8): Chip Signal Power Supply. This pin powers the entire chip except for the gate drivers. Externally filtering this pin with a lowpass RC network (e.g., R = 10, C = 1F) is suggested to minimize noise pickup, especially in high load current applications. TRACK (Pin 6/Pin 9): Tracking Input for Second Controller. This pin allows the start-up of VOUT2 to "track" that of VOUT1 according to a ratio established by a resistor divider on VOUT1 connected to the TRACK pin. For one-to-one tracking of VOUT1 and VOUT2 during start-up, a resistor divider with values equal to those connected to VFB2 from VOUT2 should be used to connect to TRACK from VOUT1. PGOOD (Pin 9/Pin 12): Power Good Output Voltage Monitor Open-Drain Logic Output. This pin is pulled to ground when the voltage on either feedback pin (VFB1, VFB2) is not within 13.3% of its nominal set point. NC (Pins 13, 19/Pins 16, 22): No Connect. RUN/SS (Pin 14/Pin 17): Run Control Input and Optional External Soft-Start Input. Forcing this pin below 0.65V shuts down the chip (both channels). Driving this pin to VIN or releasing this pin enables the chip to start-up with the internal soft-start. An external soft-start can be programmed by connecting a capacitor between this pin and ground.
6
U
U
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(QFN/SSOP)
PGND (Pin 16/Pin 19): Power Ground. This pin serves as the ground connection for the gate drivers. PGATE1, PGATE2 (Pins 17, 15/Pins 20, 18): Gate Drives for External P-Channel MOSFETs. These pins have an output swing from PGND to SENSE+. SYNC/MODE (Pin 18/Pin 21): External Clock Synchronization and Burst Mode/Pulse Skipping Select. Applying a clock with frequency between 250kHz to 850kHz causes the internal oscillator to phase lock to the external clock, and disables Burst Mode operation but allows pulse skipping at low load currents. Forcing this pin high enables Burst Mode operation. Forcing this pin low enables pulseskipping mode. In these cases, the frequency of the internal oscillator is set by the voltage on the PLLLPF pin. Do not let this pin float. PVIN1, PVIN2 (Pins 20, 12/Pins 23, 15): Powers of the Gate Drivers. SENSE1+, SENSE2+ (Pins 21, 11/Pins 24, 14): Positive Inputs to Differential Current Comparators. Normally connected to the sources of the external P-channel MOSFETs. SW1 (SENSE1-), SW2 (SENSE2-) (Pins 22, 10/Pins 1, 13): Switch Node Connections to Inductors. Also the negative inputs to differential peak current comparators. Normally connected to the drains of the external P-Channel MOSFETs and the inductor when not using a sense resistor. When a sense resistor is used, it will be connected between SW and SENSE+. IPRG1, IPRG2 (Pins 23, 2/Pins 2, 5): Three-State Pins to Select Maximum Peak Sense Voltage Threshold. These pins select the maximum allowed voltage drop between the SENSE+ and SW pins (i.e., the maximum allowed drop across the external P-channel MOSFET) for each channel. Tie high, low or float to select 204mV, 85mV or 125mV, respectively. VFB1, VFB2 (Pins 24, 7/Pins 3, 10): Each receives the remotely sensed feedback voltage for its controller from an external resistive divider across the output. Exposed Pad (Pin 25/NA): Exposed Pad is PGND and must be soldered to PCB.
3737f
LTC3737
FU CTIO AL DIAGRA
VIN CIN VIN
VOLTAGE REFERENCE
VREF 0.6V
UNDERVOLTAGE LOCKOUT
UV UVSD CMSD VIN OV1 OVP
VIN 0.15V
0.5A RUN/SS EXTSS CSS t = 1ms INTERNAL SOFT-START INTSS
- + +
MUX
SYNC/MODE
BURST DEFEAT CLOCK DETECT
BURSTDIS PHASE DETECTOR
PLLLPF VOLTAGE CONTROLLED CLK1 OSCILLATOR CLK2 VIN PGOOD UVSD SGND PGOOD1 PGOOD2 VOUT2 SLOPE1 SLOPE2 VFB2 R2B
SLOPE COMP
W
M1 L1 VOUT1 D1 COUT SENSE1 + SLOPE1 IPROG1 PVIN1 SWITCHING LOGIC AND BLANKING CIRCUIT SW1
U
U
-
ICMP
CLK1
S Q R
PGATE1 PGND
+ + +
SLEEP1
- +
0.68V
-
0.3V BURSTDIS SC1 SCP
R1B 0.12V ITH1 RC CC
RTRACKB
+
SOFTSTART
-
-
PGOOD1 OV1
+ -
0.54V
DUPLICATE FOR SECOND CHANNEL VFB1 VREF = 0.6V SOFT-START R1A
VREF = 0.6V TRACK R2A
- + +
ITH1 EAMP2 ITH2
EAMP1
- + +
RTRACKA
3737 BD
3737f
7
LTC3737
OPERATIO
Main Control Loop The LTC3737 uses a constant frequency, current mode architecture with the two controller channels operating 180 degrees out of phase. During normal operation, each external P-channel power MOSFET is turned on when the clock for that channel sets the RS latch, and turned off when the current comparator (ICMP) resets the latch. The peak inductor current at which ICMP resets the RS latch is determined by the voltage on the ITH pin, which is the output of each error amplifier (EAMP). The VFB pin receives the output voltage feedback signal from an external resistor divider. This feedback signal is compared to the internal 0.6V reference voltage by the EAMP. When the load current increases, it causes a slight decrease in VFB relative to the 0.6V reference, which in turn, causes the ITH voltage to increase until the average inductor current matches the new load current. Shutdown, Soft-Start and Tracking Start-Up (RUN/SS and TRACK Pins) The LTC3737 is shut down by pulling the RUN/SS pin low. In shutdown, all controller functions are disabled and the chip draws only 9A. The PGATE outputs are held high (off) in shutdown. Releasing RUN/SS allows an internal 0.7A current source to charge up the RUN/SS pin. When the RUN/SS pin reaches 0.65V, the LTC3737's two controllers are enabled. The start-up of VOUT1 is controlled by the LTC3737's internal soft-start. During soft-start, the error amplifier EAMP compares the feedback signal VFB1 to the internal soft-start ramp (instead of the 0.6V reference), which rises linearly from 0V to 0.6V in about 1ms. This allows the output voltage to rise smoothly from 0V to its final value, while maintaining control of the inductor current. The 1ms soft-start time can be increased by connecting the optional external soft-start capacitor, CSS, between the RUN/SS and SGND pins. As the RUN/SS pin continues to rise linearly from approximately 0.65V to 1.3V (being charged by the internal 0.7A current source), the EAMP regulates VFB1 linearly from 0V to 0.6V. The start-up of VOUT2 is controlled by the voltage on the TRACK pin. When the voltage on the TRACK pin is less
8
U
(Refer to Functional Diagram)
than the 0.6V internal reference, the LTC3737 regulates the VFB2 voltage to the TRACK pin instead of the 0.6V reference. Typically, a resistor divider on VOUT1 is connected to the TRACK pin to allow the start-up of VOUT2 to "track" that of VOUT1. For one-to-one tracking during startup, the resistor divider would have the same values as the divider on VOUT2 that is connected to VFB2. If no tracking function is desired, then the TRACK pin can be tied to VIN. Note, however, that in this situation, there would be no (internal or external) soft-start on VOUT2. Light Load Operation (Burst Mode Operation or Pulse Skipping Mode) (SYNC/MODE Pin) The LTC3737 can be enabled to enter high efficiency Burst Mode operation at low load currents. To select Burst Mode operation, tie the SYNC/MODE pin to a DC voltage above 0.6V (e.g., VIN). To disable Burst Mode operation and enable PWM pulse skipping mode, connect SYNC/MODE to a DC voltage below 0.6V (e.g., SGND). In this mode, the efficiency is lower at light loads. However, pulse skipping mode has the advantages of lower output ripple and less interference to audio circuitry. When a controller is in Burst Mode operation, the peak current in the inductor is set to approximate one-fourth of the maximum sense voltage even when the voltage on the ITH pin indicates a lower value. If the average inductor current is greater than the load current, the EAMP will decrease the voltage on the ITH pin. When the ITH voltage drops below 0.85V, the internal SLEEP signal goes high and the external MOSFET is turned off. In sleep mode, much of the internal circuitry is turned off, reducing the quiescent current that the LTC3737 draws. The load current is supplied by the output capacitor. As the output voltage decreases, the EAMP increases the ITH voltage. When the ITH voltage reaches 0.925V, the SLEEP signal goes low and the controller resumes normal operation by turning on the external P-channel MOSFET on the next cycle of the internal oscillator. When the SYNC/MODE pin is clocked by an external clock source to use the phase-locked loop (see Frequency Selection and Phase-Locked Loop), the LTC3737 operates in PWM pulse skipping mode at light loads.
3737f
LTC3737
OPERATIO
When a controller is in pulse skipping operation, an internal offset at the current comparator input will assure that the current comparator remains tripped even at zero load current and the regulator will start to skip cycles, as it must, in order to maintain regulation. Short-Circuit Protection When one of the outputs is shorted to ground (VFB < 0.12V), the switching frequency of that controller is reduced to 1/3 of the normal operating frequency. The other controller is unaffected and maintains normal operation. The short-circuit threshold on VFB2 is based on the smaller of 0.12V and a fraction of the voltage on the TRACK pin. This also allows VOUT2 to start up and track VOUT1 more easily. Note that if VOUT1 is truly short circuited (VOUT1 = VFB1 = 0V), then the LTC3737 will try to regulate VOUT2 to 0V if a resistor divider on VOUT1 is connected to the TRACK pin. Output Overvoltage Protection As a further protection, the overvoltage comparator (OVP) guards against transient overshoots, as well as other more serious conditions, that may overvoltage the output. When the feedback voltage on the VFB pin has risen 13.33% above the reference voltage of 0.6V, the external P-channel MOSFET is turned off until the overvoltage is cleared. Frequency Selection and Phase-Locked Loop (PLLLPF and SYNC/MODE Pins) The selection of switching frequency is a tradeoff between efficiency and component size. Low frequency operation increases efficiency by reducing MOSFET switching losses, but requires larger inductance and/or capacitance to maintain low output ripple voltage. The switching frequency of the LTC3737's controllers can be selected using the PLLLPF pin. If the SYNC/MODE pin is not being driven by an external clock source, the PLLLPF pin can be floated, tied to VIN or tied to SGND to select 550kHz, 750kHz or 300kHz, respectively.
U
(Refer to Functional Diagram)
A phase-locked loop (PLL) is available on the LTC3737 to synchronize the internal oscillator to an external clock source that is connected to the SYNC/MODE pin. In this case, a series RC should be connected between the PLLLPF pin and SGND to serve as the PLL's loop filter. The LTC3737 phase detector adjusts the voltage on the PLLLPF pin to align the turn-on of controller 1's external P-channel MOSFET to the rising edge of the synchronizing signal. Thus, the turn-on of controller 2's external P-channel MOSFET is 180 degrees out of phase to the rising edge of the external clock source. The typical capture range of the LTC3737's phase-locked loop is from approximately 200kHz to 1MHz, with a guarantee over all variations and temperature to be between 250kHz and 850kHz. In other words, the LTC3737's PLL is guaranteed to lock to an external clock source whose frequency is between 250kHz and 850kHz. Dropout Operation When the input supply voltage (VIN) decreases towards the output voltage, the rate of change of the inductor current while the external P-channel MOSFET is on (ON cycle) decreases. This reduction means that the P-channel MOSFET will remain on for more than one oscillator cycle if the inductor current has not ramped up to the threshold set by the EAMP on the ITH pin. Further reduction in the input supply voltage will eventually cause the P-channel MOSFET to be turned on 100%; i.e., DC. The output voltage will then be determined by the input voltage minus the voltage drop across the P-channel MOSFET and the inductor. Undervoltage Lockout To prevent operation of the P-channel MOSFET below safe input voltage levels, an undervoltage lockout is incorporated in the LTC3737. When the input supply voltage (VIN) drops below 2.25V, the external P-channel MOSFET and all internal circuitry are turned off except for the undervoltage block, which draws only a few microamperes.
3737f
9
LTC3737
OPERATIO
Peak Current Sense Voltage Selection and Slope Compensation (IPRG1 and IPRG2 Pins) When a controller is operating below 20% duty cycle, the peak current sense voltage (between the SENSE+ and SW pins) allowed across the external P-channel MOSFET is determined by:
VSENSE(MAX) = A( VITH - 0.7 V ) 10
where A is a constant determined by the state of the IPRG pins. Floating the IPRG pin selects A = 1; tying IPRG to VIN selects A = 5/3; tying IPRG to SGND selects A = 2/3. The maximum value of VITH is typically about 1.98V, so the maximum sense voltage allowed across the external P-channel MOSFET is 125mV, 85mV or 204mV for the three respective states of the IPRG pin. The peak sense voltages for the two controllers can be independently selected by the IPRG1 and IPRG2 pins. However, once the controller's duty cycle exceeds 20%, slope compensation begins and effectively reduces the peak sense voltage by a scale factor given by the curve in Figure 2.
110 100 90 80
SF = I/IMAX (%)
70 60 50 40 30 20 10 0 0 10 20 30 40 50 60 70 80 90 100 DUTY CYCLE (%)
3737 F02
Figure 2. Maximum Peak Current vs Duty Cycle
10
U
(Refer to Functional Diagram)
The peak inductor current is determined by the peak sense voltage and the on-resistance of the external P-channel MOSFET:
IPK =
VSENSE(MAX) RDS(ON)
Power Good (PGOOD) Pin A window comparator monitors both feedback voltages and the open-drain PGOOD output pin is pulled low when either or both feedback voltages are not within 10% of the 0.6V reference voltage. PGOOD is low when the LTC3737 is shutdown or in undervoltage lockout. 2-Phase Operation Why the need for 2-phase operation? Until recently, constant frequency dual switching regulators operated both controllers in phase (i.e., single phase operation). This means that both topside MOSFETs (P-channel) are turned on at the same time, causing current pulses of up to twice the amplitude of those from a single regulator to be drawn from the input capacitor. These large amplitude pulses increase the total RMS current flowing in the input capacitor, requiring the use of larger and more expensive input capacitors, and increase both EMI and power losses in the input capacitor and input power supply. With 2-phase operation, the two controllers of the LTC3737 are operated 180 degrees out of phase. This effectively interleaves the current pulses coming from the topside MOSFET switches, greatly reducing the time where they overlap and add together. The result is a significant reduction in the total RMS current, which in turn allows the use of smaller, less expensive input capacitors, reduces shielding requirements for EMI and improves real world operating efficiency. Figure 3 shows qualitatively example waveforms for a single phase dual controller versus a 2-phase LTC3737 system. In this case, 2.5V and 1.8V outputs, each drawing
3737f
LTC3737
OPERATIO
Single Phase Dual Controller
SW1 (V)
SW2 (V)
IL1
IL2
INPUT CAPACITOR RMS CURRENT
IIN
Figure 3. Example Waveforms for a Single Phase Dual Controller vs the 2-Phase LTC3737
a load current of 2A, are derived from a 7V (e.g., a 2-cell Li-Ion battery) input supply. In this example, 2-phase operation would reduce the RMS input capacitor current from 1.79ARMS to 0.91ARMS. While this is an impressive reduction by itself, remember that power losses are proportional to IRMS2, meaning that actual power wasted is reduced by a factor of 3.86. The reduced input ripple current also means that less power is lost in the input power path, which could include batteries, switches, trace/connector resistances, and protection circuitry. Improvements in both conducted and radiated EMI also directly accrue as a result of the reduced
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(Refer to Functional Diagram)
2-Phase Dual Controller
RMS input current and voltage. Significant cost and board footprint savings are also realized by being able to use smaller, less expensive, lower RMS current-rated, input capacitors. Of course the improvement afforded by 2-phase operation is a function of the relative duty cycles of the two controllers, which in turn are dependent upon the input supply voltage. Figure 4 depicts how the RMS input current varies for single phase and 2-phase dual controllers with 2.5V and 1.8V outputs over a wide input voltage range.
2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0 2 VOUT1 = 2.5V/2A VOUT2 = 1.8V/2A 3 4 8 6 5 7 INPUT VOLTAGE (V) 9 10 2-PHASE DUAL CONTROLER SINGLE PHASE DUAL CONTROLER
3737 F03
3737 F04
Figure 4. RMS Input Current Comparison
It can be readily seen that the advantages of 2-phase operation are not limited to a narrow operating range, but in fact extend over a wide region. A good rule of thumb for most applications is that 2-phase operation will reduce the input capacitor requirement to that for just one channel operating at maximum current and 50% duty cycle.
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The typical LTC3737 application circuit is shown in Figure 1. External component selection for each of the LTC3737's controllers is driven by the load requirement and begins with the selection of the inductor (L) and the power MOSFET M1. Next, the output diode D1 is selected. Finally CIN and COUT are chosen. Power MOSFET Selection An external P-channel MOSFET must be selected for use with each channel of the LTC3737. The main selection criteria for the power MOSFET are the breakdown voltage VBR(DSS), threshold voltage V GS(TH), on-resistance RDS(ON), reverse transfer capacitance CRSS and the total gate charge QG. The gate drive voltage is the input supply voltage. Since the LTC3737 is designed for operation down to low input voltages, a sublogic level MOSFET (RDS(ON) guaranteed at VGS = 2.5V) is required for applications that work close to this voltage. When these MOSFETs are used, make sure that the input supply to the LTC3737 is less than the absolute maximum MOSFET VGS rating, which is typically 8V. The P-channel MOSFET's on-resistance is chosen based on the required load current. The maximum average output load current, IOUT(MAX), is equal to the peak inductor current minus half the peak-to-peak ripple current, IRIPPLE. The LTC3737's current comparator monitors the drain-to-source voltage, VDS, of the P-channel MOSFET, which is sensed between the SENSE+ and SW pins. The peak inductor current is limited by the current threshold, set by the voltage on the ITH pin, of the current comparator. The voltage on the ITH pin is internally clamped, which limits the maximum current sense threshold VSENSE(MAX) to approximately 125mV when IPRG is floating (85mV when IPRG is tied low; 204mV when IPRG is tied high). The output current that the LTC3737 can provide is given by:
T NORMALIZED ON RESISTANCE
IOUT(MAX) =
VSENSE(MAX) IRIPPLE - RDS(ON) 2
where IRIPPLE is the inductor peak-to-peak ripple current (see Inductor Value Calculation).
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A reasonable starting point is setting ripple current IRIPPLE to be 40% of IOUT(MAX). Rearranging the above equation yields:
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5 VSENSE(MAX) RDS(ON)(MAX) = * for Duty Cycle < 20% 6 IOUT(MAX)
However, for operation above 20% duty cycle, slope compensation has to be taken into consideration to select the appropriate value of RDS(ON) to provide the required amount of load current:
RDS(ON)(MAX) = VSENSE(MAX) 5 * SF * 6 IOUT(MAX)
where SF is a scale factor whose value is obtained from the curve in Figure 2. These must be further derated to take into account the significant variation in on-resistance with temperature. The following equation is a good guide for determining the required RDS(ON)MAX at 25C (manufacturer's specification), allowing some margin for variations in the LTC3737 and external component values:
RDS(ON)(MAX) =
VSENSE(MAX) 5 * 0.9 * SF * 6 IOUT(MAX) * T
The T is a normalizing term accounting for the temperature variation in on-resistance, which is typically about 0.4%/C, as shown in Figure 5. Junction to case temperature TJC is
2.0
1.5
1.0
0.5
0 - 50
50 100 0 JUNCTION TEMPERATURE (C)
150
3737 F05
Figure 5. RDS(ON) vs Temperature
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about 10C in most applications. For a maximum ambient temperature of 70C, using 80C ~ 1.3 in the above equation is a reasonable choice. The power dissipated in the MOSFET strongly depends on its respective duty cycles and load current. When the LTC3737 is operating in continuous mode, the duty cycles for the MOSFET are:
Duty Cycle =
VOUT + VD VIN + VD
The MOSFET power dissipations at maximum output current are: PP = VOUT + VD * IOUT(MAX) VIN + VD
(
)2 * T * RDS(ON) + k *
VIN2 * IOUT(MAX) * T * RDS(ON) The MOSFET has I2R losses and the PP equation includes an additional term for transition losses, which are largest at high input voltages. The constant k = 2A-1 can be used to estimate the amount of transition loss. Using a Sense Resistor A sense resistor RSENSE can be connected between SENSE+ and SW to sense the output load current. In this case, the source of the P-channel MOSFET is connected to the SW pin and the drain is not connected to any pin of the LTC3737. Therefore, the current comparator monitors the voltage developed across RSENSE instead of VDS of the P-channel MOSFET. The output current that the LTC3737 can provide in this case is given by: IOUT(MAX) = VSENSE(MAX) IRIPPLE - RSENSE 2
Setting ripple current as 40% of IOUT(MAX) and using Figure 2 to choose SF, the value of RSENSE is:
RSENSE =
VSENSE(MAX) 5 * SF * 6 IOUT(MAX)
(See the RDS(ON) selection in Power MOSFET Selection).
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Variation in the resistance of a sense resistor is much smaller than the variation in on-resistance of the external MOSFET. Therefore the load current is well controlled, and the system is more stable with a sense resistor. However the sense resistor causes extra I2R losses in addition to the I2R losses of the MOSFET. Therefore, using a sense resistor lowers the efficiency of LTC3737, especially for large load current. Operating Frequency and Synchronization The choice of operating frequency, fOSC, is a tradeoff between efficiency and component size. Low frequency operation improves efficiency by reducing MOSFET switching losses, both gate charge loss and transition loss. However, lower frequency operation requires more inductance for a given amount of ripple current. The internal oscillator for each of the LTC3737's controllers runs at a nominal 550kHz frequency when the PLLLPF pin is left floating and the SYNC/MODE pin is a DC low or high. Pulling the PLLLPF to VIN selects 750kHz operation; pulling the PLLLPF to GND selects 300kHz operation. Alternatively, the LTC3737 will phase lock to a clock signal applied to the SYNC/MODE pin with a frequency between 250kHz and 850kHz (see Phase-Locked Loop and Frequency Synchronization). Inductor Value Calculation Given the desired input and output voltages, the inductor value and operating frequency, fOSC, directly determine the inductor's peak-to-peak ripple current:
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( VOUT + VD ) / ( VIN + VD ) IRIPPLE = ( VIN - VOUT ) fOSC * L
Lower ripple current reduces core losses in the inductor, ESR losses in the output capacitors, and output voltage ripple. Thus, highest efficiency operation is obtained at low frequency with a small ripple current. Achieving this, however, requires a large inductor. A reasonable starting point is to choose a ripple current that is about 40% of IOUT(MAX). Note that the largest ripple current occurs at the highest input voltage. To guarantee
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VIN - VOUT VOUT + VD * fOSC * IRIPPLE VIN + VD
that ripple current does not exceed a specified maximum, the inductor should be chosen according to:
L
Burst Mode Operation Considerations The choice of RDS(ON) and inductor value also determines the load current at which the LTC3737 enters Burst Mode operation. When bursting, the controller clamps the peak inductor current to approximately:
IBURST(PEAK) = 1 VSENSE(MAX) * 4 RDS(ON)
The corresponding average current depends on the amount of ripple current. Lower inductor values (higher IRIPPLE) will reduce the load current at which Burst Mode operation begins. The ripple current is normally set so that the inductor current is continuous during the burst periods. Therefore, IRIPPLE IBURST(PEAK) This implies a minimum inductance of:
LMIN VIN - VOUT V +V * OUT D fOSC * IBURST(PEAK) VIN + VD
A smaller value than LMIN could be used in the circuit, although the inductor current will not be continuous during burst periods, which will result in slightly lower efficiency. In general, though, it is a good idea to keep IRIPPLE comparable to IBURST(PEAK). Inductor Core Selection Once the value of L is known, the type of inductor must be selected. High efficiency converters generally cannot afford the core loss found in low cost powdered iron cores, forcing the use of more expensive ferrite, molypermalloy or Kool M(R) cores. Actual core loss is independent of core size for a fixed inductor value, but is very dependent on the inductance selected. As inductance increases, core losses go down. Unfortunately, increased inductance requires
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more turns of wire and therefore copper losses will increase. Ferrite designs have very low core losses and are preferred at high switching frequencies, so design goals can concentrate on copper loss and preventing saturation. Ferrite core material saturates "hard", which means that inductance collapses abruptly when the peak design current is exceeded. Core saturation results in an abrupt increase in inductor ripple current and consequent output voltage ripple. Do not allow the core to saturate! Molypermalloy (from Magnetics, Inc.) is a very good, low loss core material for toroids, but is more expensive than ferrite. A reasonable compromise from the same manufacturer is Kool M. Toroids are very space efficient, especially when several layers of wire can be used, while inductors wound on bobbins are generally easier to surface mount. However, designs for surface mount that do not increase the height significantly are available from Coiltronics, Coilcraft, Dale and Sumida. Output Diode Selection The catch diode carries load current during the switch off time of the power MOSFETs . The average diode current is therefore dependent on the P-channel MOSFET duty cycle. At high input voltages, the diode conducts most of the time. As VIN approaches VOUT, the diode conducts for only a small fraction of the time. The most stressful condition for the diode is when the output is short circuited. Under this condition, the diode must safely handle IPEAK at close to 100% duty cycle. Therefore, it is important to adequately specify the diode peak current and average power dissipation so as not to exceed the diode's ratings. Under normal conditions, the average current conducted by the diode is:
ID = VIN - VOUT * IOUT VIN + VD
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The allowable forward voltage drop in the diode is calculated from the maximum short-circuit current as:
VF
PD IPEAK
Kool M is a registered trademark of Magnetics, Inc.
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where PD is the allowable power dissipation and will be determined by efficiency and/or thermal requirements. A Schottky diode is a good choice for low forward drop and fast switching time. Remember to keep lead length short and observe proper grounding to avoid ringing and increased dissipation. CIN and COUT Selection The selection of CIN is simplified by the 2-phase architecture and its impact on the worst-case RMS current drawn through the input network (battery/fuse/capacitor). It can be shown that the worst-case capacitor RMS current occurs when only one controller is operating. The controller with the highest VOUT * IOUT product needs to be used in the formula below to determine the maximum RMS capacitor current requirement. Increasing the output current drawn from the other controller will actually decrease the input RMS ripple current from its maximum value. The out-of-phase technique typically reduces the input capacitor's RMS ripple current by a factor of 30% to 70% when compared to a single phase power supply solution. In continuous mode, the source current of the P-channel MOSFET is a square wave of duty cycle (VOUT + VD)/ (VIN + VD). To prevent large voltage transients, a low ESR capacitor sized for the maximum RMS current of one channel must be used. The maximum RMS capacitor current is given by:
CIN Required IRMS
1/ 2 IMAX ( VOUT + VD )( VIN - VOUT ) VIN + VD
[
]
This formula has a maximum at VIN = 2VOUT + VD, where IRMS = IOUT/2. This simple worst-case condition is commonly used for design because even significant deviations do not offer much relief. Note that capacitor manufacturers' ripple current ratings are often based on only 2000 hours of life. This makes it advisable to further derate the capacitor or to choose a capacitor rated at a higher temperature than required. Several capacitors may be
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paralleled to meet size or height requirements in the design. Due to the high operating frequency of the LTC3737, ceramic capacitors can also be used for CIN. Always consult the manufacturer if there is any question. The benefit of the LTC3737 2-phase operation can be calculated by using the equation above for the higher power controller and then calculating the loss that would have resulted if both controller channels switched on at the same time. The total RMS power lost is lower when both controllers are operating due to the reduced overlap of current pulses required through the input capacitor's ESR. This is why the input capacitor's requirement calculated above for the worst-case controller is adequate for the dual controller design. Also, the input protection fuse resistance, battery resistance, and PC board trace resistance losses are also reduced due to the reduced peak currents in a 2-phase system. The overall benefit of a multiphase design will only be fully realized when the source impedance of the power supply/battery is included in the efficiency testing. The sources of the P-channel MOSFETs should be placed within 1cm of each other and share a common CIN(s). Separating the sourced and CIN may produce undesirable voltage and current resonances at VIN. A small (0.1F to 1F) bypass capacitor between the chip VIN pin and ground, placed close to the LTC3737, is also suggested. A 10 resistor placed between CIN and the VIN pin provides further isolation between the two channels. The selection of COUT is driven by the effective series resistance (ESR). Typically, once the ESR requirement is satisfied, the capacitance is adequate for filtering. The output ripple (VOUT) is approximated by:
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1 VOUT IRIPPLE ESR + 8 fCOUT
where f is the operating frequency, COUT is the output capacitance and IRIPPLE is the ripple current in the inductor. The output ripple is highest at maximum input voltage since IRIPPLE increases with input voltage.
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Setting Output Voltage
The LTC3737 output voltages are each set by an external feedback resistor divider carefully placed across the output, as shown in Figure 6. The regulated output voltage is determined by:
R VOUT = 0.6 V * 1 + B RA
VOUT 1/2 LTC3737 VFB RA
3737 F06
RB
Figure 6. Setting Output Voltage
Run/Soft Start Function The RUN/SS pin is a dual purpose pin that provides the optional external soft-start function and a means to shut down the LTC3737. Pulling the RUN/SS pin below 0.65V puts the LTC3737 into a low quiescent current shutdown mode (IQ = 9A). If RUN/SS has been pulled all the way to ground, there will be a delay before the LTC3737 comes out of shutdown and is given by:
tDELAY = 0.65V *
CSS = 0.93 s / F * CSS 0.7A
This pin can be driven directly from logic as shown in Figure 7. Diode D1 in Figure 7 reduces the start delay but allows CSS to ramp up slowly providing the soft-start function. This diode (and capacitor) can be deleted if the external soft-start is not needed.
3.3V OR 5V D1 CSS CSS
3737 F07
RUN/SS
RUN/SS
RTRACKB
Figure 7. RUN/SS Pin Interfacing
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During soft-start, the start-up of VOUT1 is controlled by slowly ramping the positive reference to the error amplifier from 0V to 0.6V, allowing VOUT1 to rise smoothly from 0V to its final value. The default internal soft-start time is 1ms. This can be increased by placing a capacitor between the RUN/SS pin and SGND. In this case, the soft start time will be approximately:
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tSS1 = CSS *
Tracking
600mV 0.7A
The start-up of VOUT2 is controlled by the voltage on the TRACK pin. Normally this pin is used to allow the start-up of VOUT2 to track that of VOUT1 as shown qualitatively in Figures 8a and 8b. When the voltage on the TRACK pin is less than the internal 0.6V reference, the LTC3737 regulates the VFB2 voltage to the TRACK pin voltage instead of 0.6V. The start-up of VOUT2 may ratiometrically track that of VOUT1, according to a ratio set by a resistor divider (Figure 8c):
VOUT1 R2A R + RTRACKB * TRACKA = VOUT2 RTRACKA R2B + R2A
For coincident tracking (VOUT1 = VOUT2 during start-up), R2A = RTRACKA R2B = RTRACKB The ramp time for VOUT2 to rise from 0V to its final value is:
tSS2 = tSS1 *
RTRACKA R1A + R1B * R1A RTRACKA + RTRACKB
VOUT1 VOUT2 LTC3737 VFB1 R1A VFB2 R2A R2B
R1B
TRACK
3737 F08a
RTRACKA
Figure 8a. Using the TRACK Pin
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OUTPUT VOLTAGE
VOUT2
OUTPUT VOLTAGE
TIME
(8b) Coincident Tracking
Figures 8b and 8c. Two Different Modes of Output Voltage Tracking
For coincident tracking,
tSS2 = tSS1 *
VOUT2F VOUT1F
FREQUENCY (kHz)
where VOUT1F and VOUT2F are the final, regulated values of VOUT1 and VOUT2. VOUT1 should always be greater than VOUT2 when using the TRACK pin. If no tracking function is desired, then the TRACK pin may be tied to VIN. However, in this situation there would be no (internal nor external) soft-start on VOUT2. Phase-Locked Loop and Frequency Synchronization The LTC3737 has a phase-locked loop (PLL) comprised of an internal voltage controlled oscillator (VCO) and a phase detector. This allows the turn-on of the external P-channel MOSFET of controller 1 to be locked to the rising edge of an external clock signal applied to the SYNC/MODE pin. The turn-on of controller 2's external P-channel MOSFET is thus 180 degrees out of phase with the external clock. The phase detector is an edge sensitive digital type that provides zero degrees phase shift between the external and internal oscillators. This type of phase detector does not exhibit false lock to harmonics of the external clock. The output of the phase detector is a pair of complementary current sources that charge or discharge the external filter network connected to the PLLLPF pin. The relationship between the voltage on the PLLLPF pin and operating frequency, when there is a clock signal applied to SYNC/ MODE, is shown in Figure 9 and specified in the electrical characteristics table. Note that the LTC3737 can only be synchronized to an external clock whose frequency is
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VOUT1 VOUT1 VOUT2 TIME
3737 F08b,c
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(8c) Ratiometric Tracking
within range of the LTC3737's internal VCO, which is nominally 200kHz to 1MHz. This is guaranteed over temperature and variations to be between 250kHz and 850kHz. A simplified block diagram is shown in Figure 10.
1400 1200 1000 800 600 400 200 0 0 0.5 1 1.5 2 PLLLPF PIN VOLTAGE (V) 2.4
3737 F09
Figure 9. Relationship Between Oscillator Frequency and Voltage at the PLLLPF Pin
2.4V RLP CLP SYNC/ MODE EXTERNAL OSCILLATOR PLLLPF DIGITAL PHASE/ FREQUENCY DETECTOR
OSCILLATOR
3737 F08
Figure 10. Phase-Locked Loop Block Diagram
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If the external clock frequency is greater than the internal oscillator's frequency, fOSC, then current is sourced continuously from the phase detector output, pulling up the PLLLPF pin. When the external clock frequency is less than fOSC, current is sunk continuously, pulling down the PLLLPF pin. If the external and internal frequencies are the same but exhibit a phase difference, the current sources turn on for an amount of time corresponding to the phase difference. The voltage on the PLLLPF pin is adjusted until the phase and frequency of the external oscillators are identical. At the stable operating point, the phase detector output is high impedance and the filter capacitor CLP holds the voltage. The loop filter components, CLP and RLP, smooth out the current pulses from the phase detector and provide a stable input to the voltage controlled oscillator. The filter components CLP and RLP determine how fast the loop acquires lock. Typically RLP = 10k and CLP is 2200pF to 0.01F. Typically, the external clock (on the SYNC/MODE pin) input high level is 1.6V, while the input low level is 1.2V. These levels are guaranteed to be TTL/CMOS compatible: 0.8V is guaranteed low, while 2.0V is guaranteed high. Table 1 summarizes the different states in which the PLLLPF pin can be used.
Table 1
PLLLPF PIN 0V Floating VIN RC Loop Filter SYNC/MODE PIN DC Voltage DC Voltage DC Voltage Clock Signal FREQUENCY 300kHz 550kHz 750kHz Phase-Locked to External Clock
NORMALIZED VOLTAGE OR CURRENT (%)
Fault Condition: Short Circuit and Current Limit To prevent excessive heating of the catch diode, foldback current limiting can be added to reduce the current in proportion to the severity of the fault. Foldback current limiting is implemented by adding diodes DFB1 and DFB2 between the output and the ITH pin as shown in Figure 11. In a hard short (VOUT = 0V), the current will be reduced to approximately 50% of the maximum output current.
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VOUT 1/2 LTC3737 ITH VFB R1 DFB2 R2
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+
DFB1
3737 F11
Figure 11. Foldback Current Limiting
Low Supply Operation Although the LTC3737 can function down to below 2.4V, the maximum allowable output current is reduced as VIN decreases below 3V. Figure 12 shows the amount of change as the supply is reduced down to 2.4V. Also shown is the effect on VREF.
105 100 95 90 85 80 75 2.0 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 3.0 INPUT VOLTAGE (V)
3737 F12
VREF
MAXIMUM SENSE VOLTAGE
Figure 12. Line Regulation of VREF and Maximum Sense Voltage
Minimum On-Time Considerations Minimum on-time, tON(MIN), is the smallest amount of time that the LTC3737 is capable of turning the top P-channel MOSFET on and then off. It is determined by internal timing delays and the gate charge required to turn on the top MOSFET. The minimum on-time for the LTC3737 is typically about 250ns. Low duty cycle and high frequency applications may approach the minimum on-time limit and care should be taken to ensure that:
tON(MIN) < VOUT + VD fOSC * ( VIN + VD )
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If the duty cycle falls below what can be accommodated by the minimum on-time, the LTC3737 will begin to skip cycles. The output voltage will continue to be regulated, but the ripple current and ripple voltage will increase. Efficiency Considerations The efficiency of a switching regulator is equal to the output power divided by the input power times 100%. It is often useful to analyze individual losses to determine what is limiting efficiency and which change would produce the most improvement. Efficiency can be expressed as: Efficiency = 100% - (L1 + L2 + L3 + ...) where L1, L2, etc. are the individual losses as a percentage of input power. Although all dissipative elements in the circuit produce losses, five main sources usually account for most of the losses in LTC3737 circuits: 1) LTC3737 DC bias current, 2) MOSFET gate charge current, 3) I2R losses, 4) voltage drop of the output diode and 5) transition losses. 1) The VIN (pin) current is the DC supply current, given in the electrical characteristics, that excludes MOSFET driver currents. VIN current results in a small loss that increases with VIN. 2) MOSFET gate charge current results from switching the gate capacitance of the power MOSFET. Each time a MOSFET gate is switched from low to high to low again, a packet of charge dQ moves from PVIN to ground. The resulting dQ/dt is a current out of PVIN, which is typically much larger than the DC supply current. In continuous mode, IGATECHG = f * QP. 3) I2R losses are calculated from the DC resistances of the MOSFET, inductor and sense resistor. In continuous mode, the average output current flows through L but is "chopped" between the P-channel MOSFET and the output diode. The MOSFET RDS(ON) multiplied by duty cycle can be summed with the resistance of L to obtain I2R losses. 4) The output diode is a major source of power loss at high currents and is worse at high input voltages. The diode loss is calculated by multiplying the forward voltage times the load current times the diode duty cycle.
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5) Transition losses apply to the external MOSFET and increase with higher operating frequencies and input voltages. Transition losses can be estimated from: Transition Loss = 2(VIN)2 * IO(MAX) * CRSS(f) Other losses, including CIN and COUT ESR dissipative losses and inductor core losses, generally account for less than 2% total additional loss. Checking Transient Response The regulator loop response can be checked by looking at the load transient response. Switching regulators take several cycles to respond to a step in load current. When a load step occurs, VOUT immediately shifts by an amount equal to (ILOAD)(ESR), where ESR is the effective series resistance of COUT. ILOAD also begins to charge or discharge COUT, which generates a feedback error signal. The regulator loop then returns VOUT to its steady-state value. During this recovery time, VOUT can be monitored for overshoot or ringing. OPTI-LOOP(R) compensation allows the transient response to be optimized over a wide range of output capacitance and ESR values. The ITH series RC-CC filter (see Functional Diagram) sets the dominant pole-zero loop compensation. The ITH external components shown in the Figure 1 circuit will provide an adequate starting point for most applications. The values can be modified slightly (from 0.2 to 5 times their suggested values) to optimize transient response once the final PC layout is done and the particular output capacitor type and value have been determined. The output capacitors need to be decided upon because the various types and values determine the loop feedback factor gain and phase. An output current pulse of 20% to 100% of full load current having a rise time of 1s to 10s will produce output voltage and ITH pin waveforms that will give a sense of the overall loop stability. The gain of the loop will be increased by increasing RC, and the bandwidth of the loop will be increased by decreasing CC. The output voltage settling behavior is related to the stability of the closedloop system and will demonstrate the actual overall supply performance. For a detailed explanation of optimizing the compensation components, including a review of control loop theory, refer to Application Note 76.
OPTI-LOOP is a registered trademark of Linear Technology Corporation.
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A second, more severe transient is caused by switching in loads with large (>1F) supply bypass capacitors. The discharged bypass capacitors are effectively put in parallel with COUT, causing a rapid drop in VOUT. No regulator can deliver enough current to prevent this problem if the load switch resistance is low and it is driven quickly. The only solution is to limit the rise time of the switch drive so that the load rise time is limited to approximately (25)(CLOAD). Thus a 10F capacitor would require a 250s rise time, limiting the charging current to about 200mA. Design Example As a design example, assume VIN will be operating from a maximum of 4.2V down to a minimum of 2.7V (powered by a single lithium-ion battery). Load current requirement is a maximum of 2.5A, but most of the time it will be in a standby mode requiring only 2mA. Efficiency at both low and high load currents is important. Burst Mode operation at light loads is desired. Output voltage is 2.5V. The IPRG pin will be tied to VIN, so the maximum current sense threshold VSENSE(MAX) is approximately 204mV.
Maximum Duty Cycle = VOUT + VD = 93% VIN(MIN) + VD
From Figure 2, SF = 57%.
VSENSE(MAX) 5 RDS(ON)(MAX) = * 0.9 * SF * 6 IOUT(MAX) * T = 0.027
A 0.025 Si3473DV P-channel MOSFET is close to this value. The PLLLPF pin will be left floating, so the LTC3737 will operate at its default frequency of 550kHz. For continuous Burst Mode operation, the required minimum inductor value is:
LMIN = 4.2V - 2.5V 2.5V + 0.3V = 1.40H 0.051V 4.2V + 0.3V 550kHz 0.025
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PC Board Layout Checklist When laying out the printed circuit board, use the following checklist to ensure proper operation of the LTC3737. * The power loop (input capacitor, MOSFET, inductor, output diode, output capacitor) of each channel should be as small as possible and isolated as much as possible from the other channel's power loop. It is better to have two separate, smaller valued input capacitors (e.g., two 10F--one for each channel) than it is to have a single larger valued capacitor (e.g., one 22F) that the channels share with a common connection. * The signal and power grounds should be kept separate. The signal ground consists of the feedback resistor dividers, ITH compensation networks and the SGND pin. The power grounds consist of the (-) terminal of the input and output capacitors, the anode of the Schottky diodes and the PGND pins. Each channel should have its own power ground for its power loop as described above. The power grounds for the two channels should connect together at a common point. It is most important to keep the ground paths with high switching currents away from each other. * Put the feedback resistors close to the VFB pins. The ITH compensation components should also be very close to the LTC3737. * The current sense traces (SENSE+ and SENSE-/SW) should be Kelvin connections right at the P-channel MOSFET source and drain. * Keep the switch nodes (SW1, SW2) and the gate driver nodes (PGATE1, PGATE2) away from the small-signal components, especially the opposite channel's feedback resistors, ITH compensation components and the current sense pins (SENSE+ and SENSE-/SW).
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100pF 220pF 187k 118k 59k 23 2 59k 1F 4 10 1M 59k 5 9 3 15k
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VFB1 ITH1 SW1 SENSE1+ PVIN1 NC IPRG1 IPRG2 PLLLPF LTC3737EUF SGND VIN PGOOD PGATE2 RUN/SS NC SYNC/MODE PGATE1 PGND 18 17 16 15 14 13 D2 D1 C1 150F
VFB2 ITH2 TRACK SW2 SENSE2+ PVIN2 7 220pF 118k 15k 8 6 10 11 12 M2 CSS 10nF
L2 1.5H VOUT2 1.8V 5A
100pF C1, C2: SANYO 4TPB150MC C3: TAIYO YUDEN LMK325BJ106K-T D1, D2: SBM540 L1, L2: VISHAY IHLP-2525CZ-01-1.5 M1, M2: FDC602P
Figure 13. 2-Phase, 550kHz, Dual Output Step-Down DC/DC Converter
TYPICAL APPLICATIO S
2-Phase, 750kHz, Burst Mode Dual Output Step-Down DC/DC Converter
100pF 220pF 187k 118k 59k 23 2 59k 1F 4 10 1M 59k 5 9 3 L1 2.2H
15k M1 24 1 22 21 20 19 18 17 16 15 14 13 D2 VFB1 ITH1 SW1 SENSE1+ PVIN1 NC IPRG1 IPRG2 PLLLPF SGND VIN PGOOD LTC3737EUF SYNC/MODE PGATE1 PGND PGATE2 RUN/SS NC D1
C1 47F
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VFB2 ITH2 TRACK SW2 SENSE2+ PVIN2 7 220pF 118k 15k 8 6 10 11 12 M2
L2 2.2H VOUT2 1.8V 3A
100pF C1, C2: SANYO 6TPA47M C3: TAIYO YUDEN LMK325BJ106K-T D1, D2: IR 10BQ015 L1, L2: COILCRAFT D03316P-22 M1, M2: Si9803DY
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C2 47F
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VIN 5V L1 1.5H VOUT1 2.5V 5A M1 1 22 21 20 19
W
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C3 22F x2
C2 150F
3737 F13
VIN 5V VOUT1 2.5V 3A C3 10F x2
3737 TA01
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LTC3737
TYPICAL APPLICATIO S
2-Phase, Synchronizable Dual Output Step-Down DC/DC Converter
187k 118k 59k 23 2 59k 10nF 10k 3 4 10 1F 1M 59k 5 9
PGOOD
+
NC
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VFB2 ITH2 TRACK SW2 SENSE2 PVIN2 7 220pF 118k 15k 8 6 10 11 12 M2
L2 2.2H VOUT2 1.8V 2A
3737 TA02
100pF
C1, C2: SANYO 6TPA47M C3: TAIYO YUDEN LMK325BJ106K-T D1, D2: CENTRAL CMSH1-20ML
L1, L2: COILCRAFT D03316P-22 M1, M2: Si9803DY
2-Phase, 550kHz, Single Output Step-Down DC/DC Converter (3.3VIN to 1.8VOUT at 8A)
VIN 3.3V 220pF 15k M1 24 23 2 3 4 10 1F 1M 59k 5 9 1 22 21 20 19 18 17 16 15 14 13 D2 C2 150F C1 150F VFB1 ITH1 SW1 SENSE1+ PVIN1 NC IPRG1 IPRG2 PLLLPF LTC3737EUF SGND VIN PGOOD PGATE2 RUN/SS NC SYNC/MODE PGATE1 PGND D1 L1 1.5H VOUT 1.8V 8A
+
VFB2 ITH2 TRACK SW2 SENSE2+ PVIN2 7 220pF 118k 59k 118k 8 6 10 11 12 M2
L2 1.5H
3737 TA04
C1, C2: SANYO 4TPR150MC C3: TAIYO YUDEN LMK325BJ106K-T D1, D2: SBM540 L1, L2: VISHAY IHLP-2525CZ-01-1.5 M1, M2: FDC602P
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100pF 220pF L1 2.2H
VIN 3.3V 15k M1 24 1 22 21 20 19 18 17 16 15 14 D2 C2 47F C1 47F VFB1 ITH1 SW1 SENSE1+ PVIN1 NC IPRG1 IPRG2 PLLLPF LTC3737EUF SGND VIN PGATE2 RUN/SS SYNC/MODE PGATE1 PGND D1 VOUT1 2.5V 2A
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C3 10F x2
C3 10F x2
LTC3737
PACKAGE DESCRIPTIO U
GN Package 24-Lead Plastic SSOP (Narrow .150 Inch)
(Reference LTC DWG # 05-08-1641)
.337 - .344* (8.560 - 8.738) 24 23 22 21 20 19 18 17 16 15 1413 .045 .005 .033 (0.838) REF .150 - .165 .229 - .244 (5.817 - 6.198) .150 - .157** (3.810 - 3.988) .0250 BSC 1 2 3 4 5 6 7 8 9 10 11 12 .0532 - .0688 (1.35 - 1.75) .004 - .0098 (0.102 - 0.249) .015 .004 x 45 (0.38 0.10) .0075 - .0098 (0.19 - 0.25) .016 - .050 (0.406 - 1.270) 0 - 8 TYP .008 - .012 (0.203 - 0.305) TYP .0250 (0.635) BSC
GN24 (SSOP) 0204
.254 MIN
.0165 .0015 RECOMMENDED SOLDER PAD LAYOUT
NOTE: *DIMENSION DOES NOT INCLUDE MOLD FLASH. MOLD FLASH 1. CONTROLLING DIMENSION: INCHES SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE INCHES 2. DIMENSIONS ARE IN (MILLIMETERS) **DIMENSION DOES NOT INCLUDE INTERLEAD FLASH. INTERLEAD FLASH SHALL NOT EXCEED 0.010" (0.254mm) PER SIDE 3. DRAWING NOT TO SCALE
UF Package 24-Lead Plastic QFN (4mm x 4mm)
(Reference LTC DWG # 05-08-1697)
0.75 0.05 BOTTOM VIEW--EXPOSED PAD 0.23 TYP (4 SIDES) 23 24 0.38 0.10 1 2 4.50 0.05 2.45 0.05 3.10 0.05 (4 SIDES) 2.45 0.10 (4-SIDES)
4.00 0.10 (4 SIDES) 0.70 0.05 PIN 1 TOP MARK (NOTE 6)
R = 0.115 TYP
PACKAGE OUTLINE 0.25 0.05 0.50 BSC RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS NOTE: 1. DRAWING PROPOSED TO BE MADE A JEDEC PACKAGE OUTLINE MO-220 VARIATION (WGGD-X)--TO BE APPROVED 2. DRAWING NOT TO SCALE 3. ALL DIMENSIONS ARE IN MILLIMETERS 4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE, IF PRESENT 0.200 REF 0.00 - 0.05
(UF24) QFN 1103
0.25 0.05 0.50 BSC
5. EXPOSED PAD SHALL BE SOLDER PLATED 6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION ON THE TOP AND BOTTOM OF PACKAGE
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Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
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LTC3737
TYPICAL APPLICATIO
2-Phase, 300kHz, Resistor Sensing, Dual Output Step-Down DC/DC Converter
100pF 220pF 187k L1 2.2H VIN 2.75V TO 9.8V 15k M1 24 59k 23 2 3 1F 10 1M 59k 4 5 9 1 22 21 20 19 VFB1 ITH1 SW1 SENSE1+ PVIN1 NC IPRG1 IPRG2 PLLLPF LTC3737EUF SGND VIN PGOOD PGATE2 RUN/SS NC SYNC/MODE PGATE1 PGND R1 0.03 18 17 16 15 14 13 C2 D2 47F D1 C1 47F VOUT1 2.5V 2A
VFB2 ITH2 TRACK SW2 SENSE2+ PVIN2 7 220pF 118k 15k 8 6 10 11 12
R2 0.03 M2
L2 2.2H VOUT2 1.8V 2A
3737 TA03
100pF
C1, C2: SANYO 6TPA47M C3: TAIYO YUDEN LMK325BJ106K-T D1, D2: CENTRAL CMSH1-20ML
L1, L2: COILCRAFT D03316P-22 M1, M2: Si3867DV
RELATED PARTS
PART NUMBER LTC1628/LTC3728 LTC1629/LTC3729 LTC1702A LTC1735 LTC1772 LTC1773 LTC1778 LTC1872 LTC1929 LTC3411 LTC3412 LTC3700 LTC3701 LTC3708 LTC3736 DESCRIPTION Dual High Efficiency, 2-Phase Synchronous Step Down Controllers 20A TO 200A PolyPhaseTM High Efficiency Controllers No RSENSETM 2-Phase Dual Synchronous Controller High Efficiency Synchronous Step-Down Controller Constant Frequency Current Mode Step-Down DC/DC Controller Synchronous Step-Down Controller No RSENSE Synchronous Step-Down Controller Constant Frequency Current Mode Step-Up Controller Constant Frequency Current Mode 2-Phase Synchronous Controller 1.25A (IOUT), 4MHz, Synchronous Step-Down DC/DC Converter 2.5A (IOUT), 4MHz, Synchronous Step-Down DC/DC Converter Constant Frequency Step-Down Controller with LDO Regulator 2-Phase, Low Input Voltage Dual Step-Down DC/DC Controller 2-Phase, Dual Synchronous Controller with Output Tracking 2-Phase, Dual Synchronous Controller with Output Tracking COMMENTS Constant Frequency, Standby, 5V and 3.3V LDOs, VIN to 36V, 28-Lead SSOP Expandable Up to 12 Phases, No Heat Sinks, VIN to 36V, 28-Lead SSOP 550kHz, No Sense Resistor, GN24, VIN to 7V Burst Mode Operation, 16-Pin Narrow SSOP, Fault Protection, 3.5V VIN 36V 2.5V VIN 9.8V, IOUT Up to 4A, SOT-23 Package, 550kHz 2.65V VIN 8.5V, IOUT Up to 4A, 10-Lead MSOP Current Mode Operation Without Sense Resistor, Fast Transient Response, 4V VIN 36V 2.5V VIN 9.8V, SOT-23 Package, 550kHz Up to 42A, No Heat Sink, 3.5V VIN 36V 95% Efficiency, VIN: 2.5V to 5.5V, VOUT = 0.8V, IQ = 60A, ISD = <1A, MS Package 95% Efficiency, VIN: 2.5V to 5.5V, VOUT = 0.8V, IQ = 60A, ISD = <1A, TSSOP-16E Package 2.65 VIN 9.8V, 550kHz, 10-Lead SSOP 2.5V VIN 9.8V, 550kHz, PGOOD, PLL, 16-Lead SSOP Constant On-Time Dual Controller, VIN Up to 36V, Very Low Duty Cycle Operation, 5mm x 5mm QFN Package 2.75V VIN 9.8V, 0.6V VOUT VIN, 4mm x 4mm QFN
PolyPhase and No RSENSE are trademarks of Linear Technology Corporation.
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Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 q FAX: (408) 434-0507
q
www.linear.com
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C3 10F x2
LT/TP 0404 1K * PRINTED IN USA
(c) LINEAR TECHNOLOGY CORPORATION 2004

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