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www..com LTC3405A-1.375
1.375V, 1.5MHz, 300mA Synchronous Step-Down Regulators in ThinSOT
FEATURES

DESCRIPTIO
High Efficiency: Up to 90% Very Low Quiescent Current: Only 20A During Operation 300mA Output Current at VIN = 3V 2.5V to 5.5V Input Voltage Range 1.5MHz Constant Frequency Operation No Schottky Diode Required Low Dropout Operation: 100% Duty Cycle Stable with Ceramic Capacitors Shutdown Mode Draws < 1A Supply Current 3% Output Voltage Accuracy Current Mode Operation for Excellent Line and Load Transient Response Overtemperature Protected Low Profile (1mm) ThinSOTTM Package
The LTC (R)3405A-1.375 is a high efficiency monolithic synchronous buck regulator using a constant frequency, current mode architecture. Supply current during operation is only 20A and drops to <1A in shutdown. The 2.5V to 5.5V input voltage range makes the LTC3405A-1.375 ideally suited for single Li-Ion battery-powered applications. 100% duty cycle provides low dropout operation, extending battery life in portable systems. Switching frequency is internally set at 1.5MHz, allowing the use of small surface mount inductors and capacitors. The LTC3405A-1.375 is specifically designed to work well with ceramic output capacitors, achieving very low output voltage ripple and a small PCB footprint. The internal synchronous switch increases efficiency and eliminates the need for an external Schottky diode. The LTC3405A-1.375 is available in a low profile (1mm) ThinSOT package. For other output voltages, refer to the LTC3405A and LTC3405A-1.5/LTC3405A-1.8 data sheets.
, LTC and LT are registered trademarks of Linear Technology Corporation. Burst Mode is a registered trademark of Linear Technology Corporation. ThinSOT is a trademark of Linear Technology Corporation. All other trademarks are the property of their respective owners. Protected by U.S. Patents, including 5481178, 6580258, 6304066, 6127815, 6498466, 6611131.
APPLICATIO S

Cellular Telephones Personal Information Appliances Wireless and DSL Modems Digital Still Cameras MP3 Players Portable Instruments
TYPICAL APPLICATIO
VIN 2.7V TO 5.5V CIN 4.7F CER VIN RUN MODE GND VOUT SW
100 90
4.7H LTC3405A-1.375
COUT 4.7F CER
EFFICIENCY (%)
VOUT 1.375V 300mA
80 70 60 50 40 30 0.1
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Figure 1a. High Efficiency Step-Down Converter
Figure 1b. Efficiency vs Load Current
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VIN = 2.7V VIN = 3.6V VIN = 4.2V VIN = 5.5V 10 100 1 OUTPUT CURRENT (mA) 1000
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ABSOLUTE
(Note 1)
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RATI GS
PACKAGE/ORDER I FOR ATIO
TOP VIEW RUN 1 GND 2 SW 3 6 MODE 5 VOUT 4 VIN
Input Supply Voltage .................................. - 0.3V to 6V MODE, RUN, VOUT Voltages....................... - 0.3V to VIN SW Voltage .................................. - 0.3V to (VIN + 0.3V) P-Channel Switch Source Current (DC) ............. 400mA N-Channel Switch Sink Current (DC) ................. 400mA Peak SW Sink and Source Current .................... 630mA Operating Temperature Range (Note 2) .. - 40C to 85C Junction Temperature (Note 3) ............................ 125C Storage Temperature Range ................ - 65C to 150C Lead Temperature (Soldering, 10 sec)................. 300C
ORDER PART NUMBER LTC3405AES6-1.375 S6 PART MARKING LTBRP
S6 PACKAGE 6-LEAD PLASTIC TSOT-23
TJMAX = 125C, JA = 250C/ W
Consult LTC Marketing for parts specified with wider operating temperature ranges.
ELECTRICAL CHARACTERISTICS
The denotes specifications which apply over the full operating temperature range, otherwise specifications are TA = 25C. VIN = 3.6V unless otherwise specified.
SYMBOL IPK VOUT VOVL VOUT VLOADREG VIN IS PARAMETER Peak Inductor Current Regulated Output Voltage Output Overvoltage Lockout Output Voltage Line Regulation Output Voltage Load Regulation Input Voltage Range Input DC Bias Current Pulse Skipping Mode Burst Mode(R) Operation Shutdown Oscillator Frequency RDS(ON) of P-Channel FET RDS(ON) of N-Channel FET SW Leakage RUN Threshold RUN Leakage Current MODE Threshold MODE Leakage Current (Note 4) VOUT = 1.238V, MODE = 3.6V, ILOAD = 0A VOUT = 1.42V, MODE = 0V, ILOAD = 0A VRUN = 0V, VIN = 4.2V VOUT = 1.375V VOUT = 0V ISW = 100mA ISW = -100mA VRUN = 0V, VSW = 0V or 5V, VIN = 5V

CONDITIONS VIN = 3V, VOUT = 1.238V, Duty Cycle < 35% MODE = 3.6V VOVL = VOVL - VOUT VIN = 2.5V to 5.5V

MIN 375 1.334 2
TYP 500 1.375 5.6 0.04 0.5
MAX 625 1.416 9.3 0.4 5.5
UNITS mA V % %/V % V A A A MHz kHz A V A V A
2.5 300 20 0.1 1.2 1.5 170 0.7 0.6 0.01 0.3 0.3 1 0.01 1.5 0.01
400 35 1 1.8 0.85 0.90 1 1.5 1 2 1
fOSC RPFET RNFET ILSW VRUN IRUN VMODE IMODE
Burst Mode is a registered trademark of Linear Technology Corporation. Note 1: Absolute Maximum Ratings are those values beyond which the life of a device may be impaired. Note 2: The LTC3405A-1.375 is guaranteed to meet performance specifications from 0C to 70C. Specifications over the -40C to 85C operating temperature 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: LTC3405A-1.375: TJ = TA + (PD)(250C/W) Note 4: Dynamic supply current is higher due to the gate charge being delivered at the switching frequency.
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TYPICAL PERFOR A CE CHARACTERISTICS
(From Figure1a) Efficiency vs Input Voltage
90 80
100 90 80 Burst Mode OPERATION
EFFICIENCY (%)
EFFICIENCY (%)
60 50 40 30
PULSE SKIPPING MODE
EFFICIENCY (%)
70 60 50 40 30 2.5 Burst Mode OPERATION
IOUT = 0.1mA IOUT = 1mA IOUT = 10mA IOUT = 100mA IOUT = 250mA
3.0
3.5 4.0 4.5 LOAD CURRENT (mA)
Oscillator Frequency vs Temperature
1.70 VIN = 3.6V 1.65
1.8
OSCILLATOR FREQUENCY (MHz)
1.55 1.50 1.45 1.40 1.35 1.30 -50 -25 50 25 75 0 TEMPERATURE (C) 100 125
1.6 1.5 1.4 1.3 1.2
OUTPUT VOLTAGE (V)
1.60
FREQUENCY (MHz)
RDS(ON) vs Input Voltage
1.2 1.1 1.0 0.9 0.8
RDS(0N) ()
0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 0 1 3 2 5 4 INPUT VOLTAGE (V) 6 7 SYNCHRONOUS SWITCH
RDS(ON) ()
UW
5.0
TA = 25C unless otherwise noted.
Efficiency vs Output Current
100 90 80 70 60 50
3.6V 4.2V 3.6V 4.2V 1 100 10 OUTPUT CURRENT (mA) 1000
Efficiency vs Output Current
70
20 10
40 30 0.1
VIN = 2.7V VIN = 3.6V VIN = 4.2V VIN = 5.5V 10 100 1 OUTPUT CURRENT (mA) 1000
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5.5
0 0.1
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Oscillator Frequency vs Supply Voltage
1.395
Output Voltage vs Load Current
1.7
1.385
Burst Mode OPERATION PULSE SKIPPING MODE
1.375
1.365
1.355
1.345 2 3 4 5 SUPPLY VOLTAGE (V) 6
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0
100
400 200 300 CURRENT LOAD (mA)
500
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RDS(ON) vs Temperature
1.2 VIN = 4.2V 1.0 V = 2.7V IN VIN = 3.6V
MAIN SWITCH
0.8 0.6 0.4 0.2 SYNCHRONOUS SWITCH MAIN SWITCH 0 -50 -25 50 25 75 0 TEMPERATURE (C) 100 125
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LTC3405A-1.375 TYPICAL PERFOR A CE CHARACTERISTICS
(From Figure 1a) Dynamic Supply Current vs Supply Voltage
1600 ILOAD = 0A 1400
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SUPPLY CURRENT (A)
SUPPLY CURRENT (A)
1000 800 600 400 200 Burst Mode OPERATION 0 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 PULSE SKIPPING MODE
220 180 140 100 60 Burst Mode OPERATION 20 0 -50 -25 50 25 75 0 TEMPERATURE (C) 100 125
SWITCH LEAKAGE (nA)
1200
SUPPLY VOLTAGE (V)
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Switch Leakage vs Input Voltage
60 RUN = 0V 50
SWITCH LEAKAGE (pA)
40 30 20
SYNCHRONOUS SWITCH
MAIN SWITCH 10 0
0
1
2 3 4 INPUT VOLTAGE (V)
Start-Up from Shutdown
VOUT 100mV/DIV AC COUPLED
RUN 2V/DIV VOUT 1V/DIV
IL 200mA/DIV VIN = 3.6V ILOAD = 200mA 40s/DIV
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4
UW
5 6
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TA = 25C unless otherwise noted.
Dynamic Supply Current vs Temperature
340
160
Switch Leakage vs Temperature
VIN = 5.5V 140 RUN = 0V
VIN = 3.6V 300 ILOAD = 0A PULSE SKIPPING MODE
260
120 100 80 60 40 20 0 -50 -25 SYNCHRONOUS SWITCH MAIN SWITCH
50 25 75 0 TEMPERATURE (C)
100
125
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Burst Mode Operation
Pulse Skipping Mode Operation
SW 5V/DIV VOUT 100mV/DIV AC COUPLED IL 100mA/DIV
SW 5V/DIV
VOUT 10mV/DIV
IL 100mA/DIV
VIN = 3.6V ILOAD = 20mA
5s/DIV
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VIN = 3.6V ILOAD = 20mA
500ns/DIV
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Load Step
IL 200mA/DIV
ILOAD 200mA/DIV VIN = 3.6V 20s/DIV ILOAD = 0mA TO 250mA PULSE SKIPPING MODE
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TYPICAL PERFOR A CE CHARACTERISTICS
(From Figure 1a) Load Step
VOUT 100mV/DIV AC COUPLED
VOUT 100mV/DIV AC COUPLED IL 200mA/DIV
ILOAD 200mA/DIV VIN = 3.6V 20s/DIV ILOAD = 20mA TO 250mA PULSE SKIPPING MODE
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PI FU CTIO S
RUN (Pin 1): Run Control Input. Forcing this pin above 1.5V enables the part. Forcing this pin below 0.3V shuts down the device. In shutdown, all functions are disabled drawing <1A supply current. Do not leave RUN floating. GND (Pin 2): Ground Pin. SW (Pin 3): Switch Node Connection to Inductor. This pin connects to the drains of the internal main and synchronous power MOSFET switches. VIN (Pin 4): Main Supply Pin. Must be closely decoupled to GND, Pin 2, with a 2.2F or greater ceramic capacitor. VOUT (Pin 5): Output Voltage Feedback Pin. An internal resistive divider divides the output voltage down for comparison to the internal 0.9V reference voltage. MODE (Pin 6): Mode Select Input. To select pulse skipping mode, tie to VIN. Grounding this pin selects Burst Mode operation. Do not leave this pin floating.
UW
TA = 25C unless otherwise noted.
Load Step
VOUT 100mV/DIV AC COUPLED
Load Step
IL 200mA/DIV
IL 200mA/DIV
ILOAD 200mA/DIV VIN = 3.6V 20s/DIV ILOAD = 20mA TO 250mA Burst Mode OPERATION
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ILOAD 200mA/DIV VIN = 3.6V 20s/DIV ILOAD = 0mA TO 250mA Burst Mode OPERATION
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FU CTIO AL DIAGRA
MODE 6 SLOPE COMP OSC OSC
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VOUT 5 R1 95k R2 180k
FREQ SHIFT
0.9V VFB
EA
VIN RUN 1 0.9V REF 0.95V SHUTDOWN
-
OVDET
+
IRCMP
OPERATIO
(Refer to Functional Diagram)
Main Control Loop The LTC3405A-1.375 uses a constant frequency, current mode step-down architecture. The main (P-channel MOSFET) and synchronous (N-channel MOSFET) switches are internal. During normal operation, the internal top power MOSFET is turned on each cycle when the oscillator sets the RS latch, and turned off when the current comparator, ICOMP, resets the RS latch. The peak inductor current at which ICOMP resets the RS latch, is controlled by the output of error amplifier EA. When the load current increases, the output voltage decreases which causes a slight decrease in VFB relative to the 0.9V reference, which in turn, causes the EA amplifier's output voltage to increase until the average inductor current matches the new load current. While the top MOSFET is off, the bottom MOSFET is turned on until either the inductor current starts to reverse, as indicated by the current reversal comparator IRCMP, or the beginning of the next clock cycle.
Comparator OVDET guards against transient overshoots > 5.6% by turning the main switch off and keeping it off until the fault is removed. Burst Mode Operation The LTC3405A-1.375 is capable of Burst Mode operation in which the internal power MOSFETs operate intermittently based on load demand. To enable Burst Mode operation, simply connect the MODE pin to GND. To disable Burst Mode operation and enable PWM pulse skipping mode, connect the MODE pin to VIN or drive it with a logic high (VMODE > 1.5V). In this mode, the efficiency is lower at light loads, but becomes comparable to Burst Mode operation when the output load exceeds 25mA. The advantage of pulse skipping mode is lower output ripple and less interference to audio circuitry.
6
+
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0.65V 4 VIN
-
+
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0.4V
- +
EN SLEEP
-
BURST Q Q SWITCHING LOGIC AND BLANKING CIRCUIT
ICOMP
+
5
S R
RS LATCH
ANTISHOOTTHRU
3 SW
OV
2 GND
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OPERATIO
When the converter is in Burst Mode operation, the peak current of the inductor is set to approximately 100mA regardless of the output load. Each burst event can last from a few cycles at light loads to almost continuously cycling with short sleep intervals at moderate loads. In between these burst events, the power MOSFETs and any unneeded circuitry are turned off, reducing the quiescent current to 20A. In this sleep state, the load current is being supplied solely from the output capacitor. As the output voltage droops, the EA amplifier's output rises above the sleep threshold signaling the BURST comparator to trip and turn the top MOSFET on. This process repeats at a rate that is dependent on the load demand. Short-Circuit Protection When the output is shorted to ground, the frequency of the oscillator is reduced to about 210kHz, 1/7 the nominal
APPLICATIO S I FOR ATIO
The basic LTC3405A-1.375 application circuit is shown in Figure 1. External component selection is driven by the load requirement and begins with the selection of L followed by CIN and COUT. Inductor Selection For most applications, the inductor value will fall in the range of 2.2H to 10H. Its value is determined by the desired ripple current. Large value inductors lower ripple current and small value inductors result in higher ripple currents. Higher VIN or VOUT also increases the ripple current as shown in equation 1. A reasonable starting point for setting ripple current is IL = 120mA (40% of 300mA).
IL =
( )( )
V VOUT 1 - OUT VIN fL 1
The DC current rating of the inductor should be at least equal to the maximum load current plus half the ripple current to prevent core saturation. Thus, a 360mA rated inductor should be enough for most applications (300mA + 60mA). For better efficiency, choose a low DC-resistance inductor.
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(Refer to Functional Diagram)
frequency. This frequency foldback ensures that the inductor current has more time to decay, thereby preventing runaway. The oscillator's frequency will progressively increase to 1.5MHz when VOUT rises above 0V. Slope Compensation and Inductor Peak Current Slope compensation provides stability in constant frequency architectures by preventing subharmonic oscillations at high duty cycles. It is accomplished internally by adding a compensating ramp to the inductor current signal at duty cycles in excess of 40%. Normally, this results in a reduction of maximum inductor peak current for duty cycles > 40%. However, the LTC3405A-1.375 uses a patented scheme that counteracts this compensating ramp, which allows the maximum inductor peak current to remain unaffected throughout all duty cycles.
The inductor value also has an effect on Burst Mode operation. The transition to low current operation begins when the inductor current peaks fall to approximately 100mA. Lower inductor values (higher IL) will cause this to occur at lower load currents, which can cause a dip in efficiency in the upper range of low current operation. In Burst Mode operation, lower inductance values will cause the burst frequency to increase. Inductor Core Selection Different core materials and shapes will change the size/ current and price/current relationship of an inductor. Toroid or shielded pot cores in ferrite or permalloy materials are small and don't radiate much energy, but generally cost more than powdered iron core inductors with similar electrical characteristics. The choice of which style inductor to use often depends more on the price vs size requirements and any radiated field/EMI requirements than on what the LTC3405A-1.375 requires to operate.
(1)
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APPLICATIO S I FOR ATIO
Table 1 shows some typical surface mount inductors that work well in LTC3405A-1.375 applications.
Table 1. Representative Surface Mount Inductors
MANUFACTURER PART NUMBER Taiyo Yuden LB2016T2R2M LB2012T2R2M LB2016T3R3M ELT5KT4R7M LB2016T4R7M ELT5KT6R8M ELT5KT100M MAX DC VALUE CURRENT DCR HEIGHT 2.2H 2.2H 3.3H 4.7H 4.7H 6.8H 10H 315mA 240mA 280mA 950mA 450mA 210mA 760mA 680mA 620mA 0.13 1.6mm 0.23 1.25mm 0.2 1.6mm 0.2 1.2mm 0.2 2mm 0.25 1.6mm 0.3 1.2mm 0.36 1.2mm 0.23 1.2mm
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Panasonic Murata Taiyo Yuden Panasonic Panasonic Sumida
LQH32CN2R2M33 4.7H
CMD4D116R8MC 6.8H
CIN and COUT Selection In continuous mode, the source current of the top MOSFET is a square wave of duty cycle VOUT/VIN. To prevent large voltage transients, a low ESR input capacitor sized for the maximum RMS current must be used. The maximum RMS capacitor current is given by:
CIN required IRMS IOMAX
[V (V
OUT
IN - VOUT
VIN
This formula has a maximum at VIN = 2VOUT, 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 the capacitor manufacturer's ripple current ratings are often based on 2000 hours of life. This makes it advisable to further derate the capacitor, or choose a capacitor rated at a higher temperature than required. Always consult the manufacturer if there is any question. The selection of COUT is driven by the required effective series resistance (ESR). Typically, once the ESR requirement for COUT has been met, the RMS current rating generally far exceeds the IRIPPLE(P-P) requirement. The output ripple VOUT is determined by:
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1 VOUT IL ESR + 8 fCOUT
where f = operating frequency, COUT = output capacitance and IL = ripple current in the inductor. For a fixed output voltage, the output ripple is highest at maximum input voltage since IL increases with input voltage. Aluminum electrolytic and dry tantalum capacitors are both available in surface mount configurations. In the case of tantalum, it is critical that the capacitors are surge tested for use in switching power supplies. An excellent choice is the AVX TPS series of surface mount tantalum. These are specially constructed and tested for low ESR so they give the lowest ESR for a given volume. Other capacitor types include Sanyo POSCAP, Kemet T510 and T495 series, and Sprague 593D and 595D series. Consult the manufacturer for other specific recommendations. Using Ceramic Input and Output Capacitors Higher values, lower cost ceramic capacitors are now becoming available in smaller case sizes. Their high ripple current, high voltage rating and low ESR make them ideal for switching regulator applications. Because the LTC3405A-1.375's control loop does not depend on the output capacitor's ESR for stable operation, ceramic capacitors can be used freely to achieve very low output ripple and small circuit size. Care must be taken when ceramic capacitors are used at the input and the output. When a ceramic capacitor is used at the input and the power is supplied by a wall adapter through long wires, a load step at the output can induce ringing at the input, VIN. At best, this ringing can couple to the output and be mistaken as loop instability. At worst, a sudden inrush of current through the long wires can potentially cause a voltage spike at VIN, large enough to damage the part. When choosing the input and output ceramic capacitors, choose the X5R or X7R dielectric formulations. These dielectrics have the best temperature and voltage characteristics of all the ceramics for a given value and size.
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APPLICATIO S I FOR ATIO
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 the 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, two main sources usually account for most of the losses in LTC3405A-1.375 circuits: VIN quiescent current and I2R losses. The VIN quiescent current loss dominates the efficiency loss at very low load currents whereas the I2R loss dominates the efficiency loss at medium to high load currents. In a typical efficiency plot, the efficiency curve at very low load currents can be misleading since the actual power lost is of no consequence as illustrated in Figure 2.
1 VIN = 3.6V
0.1
POWER LOST (W)
0.01
0.001
0.0001 0.1
1 100 10 LOAD CURRENT (mA)
1000
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Figure 2. Power Lost vs Load Current
1. The VIN quiescent current is due to two components: the DC bias current as given in the electrical characteristics and the internal main switch and synchronous switch gate charge currents. The gate charge current results from switching the gate capacitance of the internal power MOSFET switches. Each time the gate is switched from high to low to high again, a packet of charge, dQ, moves from VIN to ground. The resulting dQ/dt is the current out of VIN that is typically larger than
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the DC bias current. In continuous mode, IGATECHG = f(QT + QB) where QT and QB are the gate charges of the internal top and bottom switches. Both the DC bias and gate charge losses are proportional to VIN and thus their effects will be more pronounced at higher supply voltages. 2. I2R losses are calculated from the resistances of the internal switches, RSW, and external inductor RL. In continuous mode, the average output current flowing through inductor L is "chopped" between the main switch and the synchronous switch. Thus, the series resistance looking into the SW pin is a function of both top and bottom MOSFET RDS(ON) and the duty cycle (DC) as follows: RSW = (RDS(ON)TOP)(DC) + (RDS(ON)BOT)(1 - DC) The RDS(ON) for both the top and bottom MOSFETs can be obtained from the Typical Performance Charateristics curves. Thus, to obtain I2R losses, simply add RSW to RL and multiply the result by the square of the average output current. Other losses including CIN and COUT ESR dissipative losses and inductor core losses generally account for less than 2% total additional loss. Thermal Considerations In most applications, the LTC3405A-1.375 does not dissipate much heat due to its high efficiency. But, in applications where they run at high ambient temperature with low supply voltage, the heat dissipated may exceed the maximum junction temperature of the part. If the junction temperature reaches approximately 150C, both power switches will be turned off and the SW node will become high impedance. To keep the LTC3405A-1.375 from exceeding the maximum junction temperature, the user will need to do some thermal analysis. The goal of the thermal analysis is to determine whether the power dissipated exceeds the maximum junction temperature of the part. The temperature rise is given by: TR = (PD)(JA)
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APPLICATIO S I FOR ATIO
where PD is the power dissipated by the regulator and JA is the thermal resistance from the junction of the die to the ambient temperature. The junction temperature, TJ, is given by: TJ = TA + TR where TA is the ambient temperature. As an example, consider the LTC3405A-1.375 with an input voltage of 2.7V, a load current of 300mA and an ambient temperature of 70C. From the typical performance graph of switch resistance, the RDS(ON) of the Pchannel switch at 70C is approximately 0.94 and the RDS(ON) of the N-channel synchronous switch is approximately 0.75. The series resistance looking into the SW pin is: RSW = 0.95 (0.51) + 0.75 (0.49) = 0.85 Therefore, power dissipated by the part is: PD = ILOAD2 * RSW = 76.5mW For the SOT-23 package, the JA is 250C/ W. Thus, the junction temperature of the regulator is: TJ = 70C + (0.0765)(250) = 89.1C which is well below the maximum junction temperature of 125C. Note that at higher supply voltages, the junction temperature is lower due to reduced switch resistance (RDS(ON)). 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 acts to return VOUT to its steadystate value. During this recovery time VOUT can be monitored for overshoot or ringing that would indicate a stability problem. For a detailed explanation of switching control loop theory, see Application Note 76.
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PC Board Layout Checklist When laying out the printed circuit board, the following checklist should be used to ensure proper operation of the LTC3405A-1.375. These items are also illustrated graphically in Figures 3 and 4. Check the following in your layout: 1. The power traces, consisting of the GND trace, the SW trace and the VIN trace should be kept short, direct and wide. 2. Does the (+) plate of CIN connect to VIN as closely as possible? This capacitor provides the AC current to the internal power MOSFETs. 3. Keep the (-) plates of CIN and COUT as close as possible.
1 RUN MODE 6 LTC3405A-1.375 2
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VOUT COUT
GND
VOUT VIN CIN
5
+
3 L1
SW
4
VIN
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BOLD LINES INDICATE HIGH CURRENT PATHS
Figure 3. LTC3405A-1.375 Layout Diagram
Design Example As a design example, assume the LTC3405A-1.375 is used in a single lithium-ion battery-powered cellular phone application. The VIN will be operating from a maximum of 4.2V down to about 2.7V. The load current requirement is a maximum of 0.15A but most of the time it will be in standby mode, requiring only 2mA. Efficiency at both low and high load currents is important. Output voltage is 1.375V. With this information we can calculate L using equation (1),
L=
( )( )
V VOUT 1 - OUT VIN f IL 1
(3)
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APPLICATIO S I FOR ATIO
Substituting VOUT = 1.375V, VIN = 4.2V, IL = 60mA and f = 1.5MHz in equation (3) gives:
L=
EFFICIENCY (%)
1.375V 1.375V 1 - 10H 1.5MHz(60mA) 4.2V
VIA TO VIN VOUT PIN 1 LTC3405A-1.375 L1 SW VIN
COUT GND
CIN
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Figure 4. LTC3405A-1.375 Suggested Layout
For best efficiency choose a 200mA or greater inductor with less than 0.3 series resistance. CIN will require an RMS current rating of at least 0.125A ILOAD(MAX)/2 at temperature and COUT will require an ESR of less than 0.5. In most cases, a ceramic capacitor will satisfy this requirement. Figure 5 shows the complete circuit along with its efficiency curve.
VIN 2.7V TO 4.2V 4 CIN*** 2.2F CER VIN RUN MODE GND 2 VOUT 5 SW 3 10H* VOUT 1.375V COUT** 10F CER
LTC3405A-1.375 1 6
* MURATA LQHMCN10002 ** MURATA 0603 GRM188R60G106ME47B *** MURATA 0603 GRM188R61A225KE34B
3405A1375 F05a
Figure 5a. Small Footprint Application
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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100 90 80 70 60 50 40 30 20 10 0 0.1 1 VIN = 2.7V VIN = 3.6V VIN = 4.2V 10 100 LOAD CURRENT (mA) 1000
3405A1375 F05b
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Figure 5b. LTC3405A-1.375 Small Footprint Efficiency
VOUT 100mV/DIV AC COUPLED IL 200mA/DIV
ILOAD 200mA/DIV VIN = 3.6V 20s/DIV ILOAD = 100mA TO 300mA Burst Mode OPERATION
3405A1375 F05c
Figure 5c.
3405a1375f
11
LTC3405A-1.375
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PACKAGE DESCRIPTIO
0.754 0.854 0.127 0.20 BSC DATUM `A' 3.254 2.80 BSC 1.50 - 1.75 (NOTE 4) PIN ONE ID 0.95 BSC 0.80 - 0.90 0.30 - 0.50 REF 0.09 - 0.20 (NOTE 3)
1.9 BSC RECOMMENDED SOLDER PAD LAYOUT
NOTE: 1. DIMENSIONS ARE IN MILLIMETERS 2. DRAWING NOT TO SCALE 3. DIMENSIONS ARE INCLUSIVE OF PLATING
4. DIMENSIONS ARE EXCLUSIVE OF MOLD FLASH AND METAL BURR 5. MOLD FLASH SHALL NOT EXCEED 0.254mm 6. JEDEC PACKAGE REFERENCE IS MO-193
RELATED PARTS
PART NUMBER LT1616 LT1676 LT1765 LT1776 LTC1878 LTC1879 LTC3404 LTC3405/LTC3405A LTC3405A-1.5 LTC3405A-1.8 LTC3406/LTC3406B LTC3411 LTC3412 LTC3413 LT3430 LTC3440 DESCRIPTION 500mA (IOUT), 1.4MHz, High Efficiency Step-Down DC/DC Converter 450mA (IOUT), 100kHz, High Efficiency Step-Down DC/DC Converter 25V, 2.75A (IOUT), 1.25MHz, High Efficiency Step-Down DC/DC Converter 500mA (IOUT), 200kHz, High Efficiency Step-Down DC/DC Converter 600mA (IOUT), 550kHz, Synchronous Step-Down DC/DC Converter 1.20A (IOUT), 550kHz, Synchronous Step-Down DC/DC Converter 600mA (IOUT), 1.4MHz, Synchronous Step-Down DC/DC Converter 300mA (IOUT), 1.5MHz, Synchronous Step-Down DC/DC Converters 600mA (IOUT) 1.5MHz, Synchronous Step-Down DC/DC Converter 1.25A (IOUT), 4MHz, Synchronous Step-Down DC/DC Converter 2.5A (IOUT), 4MHz, Synchronous Step-Down DC/DC Converter 3A (IOUT), Sink/Source, 2MHz, Monolithic Synchronous Regulator for DDR/QDR Memory Termination 60V, 2.75A (IOUT), 200kHz, High Efficiency Step-Down DC/DC Converter 600mA (IOUT), 2MHz, Synchronous Buck-Boost DC/DC Converter COMMENTS 90% Efficiency, VIN = 3.6V to 25V, VOUT = 1.25V, IQ = 1.9mA ISD = <1A, ThinSOT Package 90% Efficiency, VIN = 7.4V to 60V, VOUT = 1.24V, IQ = 3.2mA ISD = 2.5A, S8 Package 90% Efficiency, VIN = 3.0V to 25V, VOUT = 1.20V, IQ = 1mA ISD = 15A, S8, TSSOP16E Packages 90% Efficiency, VIN = 7.4V to 40V, VOUT = 1.24V, IQ = 3.2mA ISD = 30A, N8,S8 Packages 95% Efficiency, VIN = 2.7V to 6V, VOUT = 0.8V, IQ = 10A ISD = <1A, MS8 Package 95% Efficiency, VIN = 2.7V to 10V, VOUT = 0.8V, IQ = 15A ISD = <1A, TSSOP16 Package 95% Efficiency, VIN = 2.7V to 6V, VOUT = 0.8V, IQ = 10A ISD = <1A, MS8 Package 95% Efficiency, VIN = 2.7V to 6V, VOUT = 0.8V, IQ = 20A ISD = <1A, ThinSOT Package 95% Efficiency, VIN = 2.5V to 5.5V, VOUT = 0.6V, IQ = 20A ISD = <1A, ThinSOT Package 95% Efficiency, VIN = 2.5V to 5.5V, VOUT = 0.8V, IQ = 60A ISD = <1A, MS10 Package 95% Efficiency, VIN = 2.5V to 5.5V, VOUT = 0.8V, IQ = 60A ISD = <1A, TSSOP16E Package 90% Efficiency, VIN = 2.25V to 5.5V, VOUT = VREF/2, IQ = 280A ISD = <1A, TSSOP16E Package 90% Efficiency, VIN = 5.5V to 60V, VOUT = 1.20V, IQ = 2.5mA ISD = 25A, TSSOP16E Package 95% Efficiency, VIN = 2.5V to 5.5V, VOUT = 2.5V, IQ = 25A ISD = <1A, MS Package
3405a1375f LT/LT 0305 * PRINTED IN USA
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Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 FAX: (408) 434-0507
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S6 Package 6-Lead Plastic TSOT-23
(Reference LTC DWG # 05-08-1636)
2.90 BSC (NOTE 4) 1.00 MAX 0.95 BSC 0.30 - 0.45 TYP 6 PLCS (NOTE 3) 0.30 - 0.45 TYP 6 PLCS (NOTE 3) 1.90 BSC
S6 TSOT-23 0801
0.01 - 0.10
www.linear.com
(c) LINEAR TECHNOLOGY CORPORATION 2002

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