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LTC1265/LTC1265-3.3/LTC1265-5 1.2A, High Efficiency Step-Down DC/DC Converter
FEATURES
s s
DESCRIPTIO
s s s s s s s s
High Efficiency: Up to 95% Current Mode Operation for Excellent Line and Load Transient Response Internal 0.3 Power Switch (VIN = 10V) Short-Circuit Protection Low Dropout Operation: 100% Duty Cycle Low-Battery Detector Low 160A Standby Current at Light Loads Active-High Micropower Shutdown: IQ < 15A Peak Inductor Current Independent of Inductor Value Available in 14-pin SO Package
The LTC(R)1265 is a monolithic step-down current mode DC/DC converter featuring Burst Mode TM operation at low output current. The LTC1265 incorporates a 0.3 switch (VIN =10V) allowing up to 1.2A of output current. Under no load condition, the converter draws only 160A. In shutdown it typically draws a mere 5A making this converter ideal for current sensitive applications. In dropout the internal P-channel MOSFET switch is turned on continuously maximizing the life of the battery source. The LTC1265 incorporates automatic power saving Burst Mode operation to reduce gate charge losses when the load currents drop below the level required for continuous operation. The inductor current is user-programmable via an external current sense resistor. Operation up to 700kHz permits the use of small surface mount inductors and capacitors.
, LTC and LT are registered trademarks of Linear Technology Corporation. Burst Mode is a trademark of Linear Technology Corporation.
APPLICATIO S
s s s s s s s s
5V to 3.3V Conversion Distributed Power Systems Step-Down Converters Inverting Converters Memory Backup Supply Portable Instruments Battery-Powered Equipment Cellular Telephones
TYPICAL APPLICATIO
VIN 5.4V TO 12V
+
CIN 68F 20V
0.1F
PWR VIN SHDN
VIN SW
L1* 33H D1
RSENSE** 0.1
1k
LTC1265-5 ITH CT PGND SENSE+
VOUT 5V 1A
EFFICIENCY (%)
+
COUT 220F 10V
3900pF 130pF
1000pF SENSE SGND
-
* COILTRONICS CTX33-4 ** IRC LRC2010-01-R100-J MBRS130LT3 AVX TPSE227K010 AVX TPSE686K020
LTC1265-FO1
Figure 1. High Efficiency Step-Down Converter
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LTC1265-5 Efficiency
100 95 90 VIN = 12V 85 80 75 70 0.01 L = 33H VOUT = 5V RSENSE = 0.1 CT = 130pF 0.10 LOAD CURRENT (A) 1.00
LTC1265 TA01
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VIN = 6V VIN = 9V
1
LTC1265/LTC1265-3.3/LTC1265-5
ABSOLUTE MAXIMUM RATINGS
(Voltages Refer to GND Pin) (Note 1)
PACKAGE/ORDER INFORMATION
TOP VIEW PWR VIN 1 VIN 2 LBOUT 3 LBIN 4 CT 5 ITH 6 SENSE- 7 14 SW 13 PWR VIN 12 PGND 11 SGND 10 SHDN 9 8 N/C (VFB*) SENSE+
Input Supply Voltage (Pins 1, 2, 13) ..........- 0.3V to 13V DC Switch Current (Pin 14) .................................... 1.2A Peak Switch Current (Pin 14) ................................. 1.6A Switch Voltage (Pin 14) .................................. VIN - 13.0 Operating Temperature Range LTC1265C ............................................... 0 to 70C LTC1265I ........................................ - 40C to 85C Junction Temperature (Note 2) ............................. 125C Storage Temperature Range ....................- 65 to 150C Lead Temperature (Soldering, 10 sec).................. 300C
ORDER PART NUMBER
LTC1265CS LTC1265CS-5 LTC1265CS-3.3 LTC1265IS
S PACKAGE 14-LEAD PLASTIC SO *ADJUSTABLE OUTPUT VERSION TJMAX = 125C, JA = 110C/W
Consult factory for Military grade parts.
The q denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. VIN = 10V, VSHDN = 0V, unless otherwise specified.
SYMBOL IFB VFB VOUT VOUT PARAMETER Feedback Current into Pin 9 Feedback Voltage Regulator Output Voltage Output Voltage Line Regulation Output Voltage Load Regulation Burst Mode Operation Output Ripple IQ Input DC Supply Current (Note 3) CONDITIONS LTC1265 LTC1265C VIN = 9V, LTC1265I LTC1265-3.3: ILOAD = 800mA LTC1265-5: ILOAD = 800mA VIN = 6.5V to 10V, ILOAD = 800mA LTC1265-3.3: 10mA < ILOAD < 800mA LTC1265-5: 10mA < ILOAD < 800mA ILOAD = 0mA Active Mode: 3.5V < VIN < 10V Sleep Mode: 3.5V < VIN < 10V Sleep Mode: 5V < VIN < 10V (LTC1265-5) Shutdown: VSHDN = VIN, 3.5V < VIN < 10V 1.15 VLBOUT = 0.4V, VLBIN = 0V VLBOUT = 5V, VLBIN = 10V LTC1265: VSENSE- = 5V, V9 = VOUT/4 + 25mV (Forced) VSENSE- = 5V, V9 = VOUT/4 - 25mV (Forced) LTC1265-3.3: VSENSE- = VOUT + 100mV (Forced) VSENSE- = VOUT - 100mV (Forced) LTC1265-5: VSENSE- = VOUT + 100mV (Forced) VSENSE- = VOUT - 100mV (Forced) LTC1265C LTC1265I VOUT in Regulation, VSENSE- = VOUT VOUT = 0V CT = 390pF, ILOAD = 800mA (LTC1265C) CT = 390pF, ILOAD = 800mA (LTC1265I)
q q q q q q q
ELECTRICAL CHARACTERISTICS
MIN 1.22 1.20 3.22 4.9 -40
TYP 0.2 1.25 1.25 3.3 5 0 40 60 50 1.8 160 160 5 1.25 1.0 25 150 25 150 25 150 0.3 0.3
MAX 1 1.28 1.30 3.40 5.2 40 65 100 2.4 230 230 15 1.35 0.5 1.5 1.0 180 180 180 0.60 0.70 100 10 6 7
UNITS A V V V V mV mV mV mVP-P mA A A A V A mA A mV mV mV mV mV mV A A s s
VLBTRIP ILBIN ILBOUT V8 - V 7
Low-Battery Trip Point Current into Pin 4 Current Sunk by Pin 3 Current Sense Threshold Voltage
0.5
130 130 130
RON I5 tOFF
ON Resistance of Switch CT Pin Discharge Current Switch Off Time (Note 4)
40 4 3.5
60 2 5 5
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LTC1265/LTC1265-3.3/LTC1265-5
The q denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. VIN = 10V, VSHDN = 0V, unless otherwise specified.
SYMBOL VIH VIL I10 PARAMETER Shutdown Pin High Shutdown Pin Low Shutdown Pin Input Current CONDITIONS Min Voltage at Pin 10 for Device to be in Shutdown Max Voltage at Pin 10 for Device to be Active VSHDN = 8V MIN 1.2 0.6 0.5 Note 3: Dynamic supply current is higher due to the gate charge being delivered at the switching frequency. Note 4: In applications where RSENSE is placed at ground potential, the off time increases by approximately 40%. TYP MAX UNITS V V A
ELECTRICAL CHARACTERISTICS
Note 1: Absolute Maximum Ratings are those values beyond which the life of the device may be impaired. Note 2: TJ is calculated from the ambient temperature TA and power dissipation PD according to the following formulas: LTC1265CS, LTC1265CS-3.3, LTC1265CS-5: TJ = TA + (PD * 110C/W)
TYPICAL PERFORMANCE CHARACTERISTICS
Efficiency vs Load Current
100 95
VIN = 5V
EFFICIENCY (%)
EFFICIENCY (%)
VIN = 9V 85 80 75 70 0.01 VIN = 12V LTC1265-3.3 VOUT = 3.3V RSENSE = 0.1 CT = 130pF COIL = CTX33-4 0.10 LOAD CURRENT (A) 1.00
1265 G01
92 90 88 86 84 82 80 4
EFFICIENCY (%)
90
Operating Frequency vs (VIN - VOUT)
1.2 1.0 0C 70C
RDS(ON) ()
NORMALIZED FREQUENCY
LEAKAGE CURRENT (nA)
0.8 0.6 0.4 0.2
0 0 1 2 5678 (VIN - VOUT) VOLTAGE (V) 4 3 9 10
UW
25C
1265 G04
Efficiency vs Input Voltage (VOUT = 5V)
100 98 96 94 ILOAD = 250mA ILOAD = 800mA 100 98 96 94 92 90 88 86 LTC1265-5 RSENSE = 0.1 CT = 130pF COIL = CTX33-4 5 6 7 9 10 11 8 INPUT VOLTAGE (V) 12 13 84 82 80
Efficiency vs Input Voltage (VOUT = 3.3V)
LTC1265-3.3 RSENSE = 0.1 CT = 130pF COIL = CTX33-4 ILOAD = 250mA
ILOAD = 800mA
4
5
6
7 9 10 11 8 INPUT VOLTAGE (V)
12
13
1265 G02
LTC1265 G03
Switch Resistance
0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 3 4 5 6 7 8 9 10 11 12 13 INPUT VOLTAGE (V)
1265 G05
Switch Leakage Current
300 270 240 VIN = 12V
TJ = 125C TJ = 70C TJ = 25C TJ = 0C
210 180 150 120 90 60 30 0 0 20 60 40 TEMPERATURE (C) 80 100
1265 G06
3
LTC1265/LTC1265-3.3/LTC1265-5 TYPICAL PERFORMANCE CHARACTERISTICS
DC Supply Current
2.1 1.8 SUPPLY CURRENT (mA) 1.5 1.2 0.9 0.6 0.3 0 0 2 10 4 8 6 INPUT VOLTAGE (V) 12 14
1265 G07
DOES NOT INCLUDE GATE CHARGE SUPPLY CURRENT (A) ACTIVE MODE
6 5 4 3 2 1 0 3 4 5 6 7 8 9 10 11 12 13 INPUT VOLTAGE (V)
1265 G08
SWITCHING CURRENT (mA)
SLEEP MODE
PIN FUNCTIONS
PWR VIN (Pins 1, 13): Supply for the Power MOSFET and its Driver. Must decouple this pin properly to ground. Must always tie Pins 1 and 13 together. VIN (Pin 2): Main Supply for All the Control Circuitry in the LTC1265. LBOUT (Pin 3): Open-Drain Output of the Low-Battery Comparator. This pin will sink current when Pin 4 (LBIN) goes below 1.25V. During shutdown, this pin is high impedance. LBIN (Pin 4): The (-) Input of the Low-Battery Comparator. The (+) input is connected to a reference voltage of 1.25V. CT (Pin 5): External capacitor CT from Pin 5 to ground sets the switch off time. The operating frequency is dependent on the input voltage and CT. ITH (Pin 6): Feedback Amplifier Decoupling Point. The current comparator threshold is proportional to Pin 6 voltage. SENSE - (Pin 7): Connect to the (-) input of the current comparator. For LTC1265-3.3 and LTC1265-5, it also connects to an internal resistive divider which sets the output voltage. SENSE+ (Pin 8): The (+) Pin to the Current Comparator. A built-in offset between Pins 7 and 8 in conjunction with RSENSE sets the current trip threshold. N/C,VFB (Pin 9): For the LTC1265 adjustable version, this pin serves as the feedback pin from an external resistive divider used to set the output voltage. On the LTC1265-3.3 and LTC1265-5 versions, this pin is not used. SHDN (Pin 10): Pulling this pin HIGH keeps the internal switch off and puts the LTC1265 in micropower shutdown. Do not float this pin. SGND (Pin 11): Small-Signal Ground. Must be routed separately from other grounds to the (-) terminal of COUT. PGND (Pin 12): Switch Driver Ground. Connects to the (-) terminal of CIN. Anode of the Schottky diode must be connected close to this pin. SW (Pin 14): Drain of the P-Channel MOSFET Switch. Cathode of the Schottky diode must be connected close to this pin.
4
UW
Supply Current in Shutdown
8 7
5.5
Gate Charge Losses
5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0 0 200 400 600 FREQUENCY (kHz) 800 1000
1265 G09
SHUTDOWN = 3V TA = 25C
VIN = 12V
VIN = 9V
VIN = 6V
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LTC1265/LTC1265-3.3/LTC1265-5
FUNCTIONAL DIAGRA
V
SLEEP R Q S VTH2 VTH1 C
25mV TO 150mV 5pF VOS 13k
T
+
5 CT OFF-TIME CONTROL
2 VIN
3 LB0UT
SENSE- VFB 11 SGND
A3
OPERATION
(Refer to Functional Diagram)
The LTC1265 uses a constant off-time architecture to switch its internal P-channel power MOSFET. The off time is set by an external timing capacitor at CT (Pin 5). The operating frequency is then determined by the off time and the difference between VIN and VOUT. The output voltage is set by an internal resistive divider (LTC1265-3.3 and LTC1265-5) connected to SENSE - (Pin 7) or an external divider returned to V FB (Pin 9 for LTC1265). A voltage comparator V, and a gain block G, compare the divided output voltage with a reference voltage of 1.25V. To optimize efficiency, the LTC1265 automatically switches between continuous and Burst Mode operation. The voltage comparator is the primary control element when the device is in Burst Mode operation, while the gain block controls the output voltage in continuous mode. When the load is heavy, the LTC1265 is in continuous operation. During the switch ON time, current comparator C monitors the voltage between Pins 7 and 8 connected across an external shunt in series with the inductor. When
the voltage across the shunt reaches the comparator's threshold value, its output signal will change state, setting the flip flop and turning the internal P-channel MOSFET off. The timing capacitor connected to Pin 5 is now allowed to discharge at a rate determined by the off-time controller. When the voltage on the timing capacitor has discharged past VTH1, comparator T trips, sets the flip flop and causes the switch to turn on. Also, the timing capacitor is recharged. The inductor current will again ramp up until the current comparator C trips. The cycle then repeats. When the load current increases, the output voltage decreases slightly. This causes the output of the gain stage (Pin 6) to increase the current comparator threshold, thus tracking the load current. When the load is relatively light, the LTC1265 automatically goes into Burst Mode operation. The current loop is interrupted when the output voltage exceeds the desired regulated value. The hysteretic voltage comparator V trips when VOUT is above the desired output voltage, shutting off the switch and causing the capacitor to discharge. This
+
-
ITH 6
G 100k
-
S
-
+
-
+
-
+
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(Pin 9 connection shown for LTC1265-3.3 and LTC1265-5; change create LTC1265)
1, 13 PWR VIN SENSE + 8 14 SW 12 PGND 9 VFB ADJUSTABLE VERSION SENSE - 7
REFERENCE
10 SHDN
4 LBIN
1265 FD
5
LTC1265/LTC1265-3.3/LTC1265-5
OPERATION
capacitor discharges past VTH1 until its voltage drops below VTH2. Comparator S then trips and a sleep signal is generated. The circuit now enters into sleep mode with the power MOSFET turned off. In sleep mode, the LTC1265 is in standby and the load current is supplied by the output capacitor. All unused circuitry is shut off, reducing quiescent current from 2mA to 160A. When the output capacitor discharges by the amount of the hysteresis of the comparator V, the P-channel switch turns on again and the process repeats itself. During Burst Mode operation the peak inductor current is set at 25mV/RSENSE.
APPLICATIONS INFORMATION
The basic LTC1265 application circuit is shown in Figure 1. External component selection is driven by the load requirement, and begins with the selection of RSENSE. Once RSENSE is known, CT and L can be chosen. Next, the Schottky diode D1 is selected followed by CIN and COUT. RSENSE Selection for Output Current RSENSE is chosen based on the required output current. With the current comparator monitoring the voltage developed across RSENSE, the threshold of the comparator determines the peak inductor current. Depending on the load current condition, the threshold of the comparator lies between 25mV/RSENSE and 150mV/RSENSE. The maximum output current of the LTC1265 is:
IOUT(MAX) = 150mV - IRIPPLE (Amps) RSENSE 2
where IRIPPLE is the peak-to-peak inductor ripple current. At a relatively light load, the LTC1265 is in Burst Mode operation. In this mode the peak inductor current is set at 25mV/RSENSE. To fully benefit from Burst Mode operation, the inductor current should be continuous during burst periods. Hence, the peak-to-peak inductor ripple current must not exceed 25mV/RSENSE. To account for light and heavy load conditions, the IOUT(MAX) is then given by:
RSENSE ()
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(Refer to Functional Diagram)
To avoid the operation of the current loop interfering with Burst Mode operation, a built-in offset VOS is incorporated in the gain stage. This prevents the current from increasing until the output voltage has dropped below a minimum threshold. Using constant off-time architecture, the operating frequency is a function of the voltage. To minimize the frequency variation as dropout is approached, the off-time controller increases the discharge current as VIN drops below VOUT + 2V. In dropout the P-channel MOSFET is turned on continuously (100% duty cycle) providing low dropout operation with VOUT VIN.
25mV (Amps) IOUT(MAX) = 150mV - RSENSE 2 * RSENSE = 137.5mV (Amps) RSENSE
Solving for RSENSE and allowing a margin of variations in the LTC1265 and extended component values yields:
RSENSE = 100mV () IOUT(MAX)
The LTC1265 is rated with a capability to supply a maximum of 1.2A of output current. Therefore, the minimum value of RSENSE that can be used is 0.083. A graph for selecting RSENSE versus maximum output is given in Figure 2.
0.5
0.4
0.3
0.2
0.1
0
0
0.2 0.6 0.8 0.4 MAXIMUM OUTPUT CURRENT (A)
1
1265 G10
Figure 2. Selecting RSENSE
LTC1265/LTC1265-3.3/LTC1265-5
APPLICATIONS INFORMATION
Under short-circuit condition, the peak inductor current is determined by: 2V, the LTC1265 reduces tOFF by increasing the discharge current in CT. This prevents audible operation prior to dropout. (See shelving effect shown in the Operating Frequency curve under Typical Performance Characteristics.) To maintain continuous inductor current at light load, the inductor must be chosen to provide no more than 25mV/ RSENSE of peak-to-peak ripple current. This results in the following expression for L: L 5.2(105)RSENSE (CT)VREG Using an inductance smaller than the above value will result in the inductor current being discontinuous. A consequence of this is that the LTC1265 will delay entering Burst Mode operation and efficiency will be degraded at low currents. Inductor Core Selection With the value of L selected, the type of inductor must be chosen. Basically, there are two kinds of losses in an inductor; core and copper losses. Core losses are dependent on the peak-to-peak ripple current and core material. However it is independent of the physical size of the core. By increasing the inductance, the peak-to-peak inductor ripple current will decrease, therefore reducing core loss. Utilizing low core loss material, such as molypermalloy or Kool M(R) will allow user to concentrate on reducing copper loss and preventing saturation. Although higher inductance reduces core loss, it increases copper loss as it requires more windings. When space is not at a premium, larger wire can be used to reduce the wire resistance. This also prevents excessive heat dissipation. CATCH DIODE SELECTION Losses in the catch diode depend on forward drop and switching times. Therefore Schottky diodes are a good choice for low drop and fast switching times. The catch diode carries load current during the off time. The average diode current is therefore dependent on the
Kool M is a registered trademark of Magnetics, Inc.
ISC(PK) = 150mV (Amps) RSENSE
In this condition, the LTC1265 automatically extends the off time of the P-channel MOSFET to allow the inductor current to decay far enough to prevent any current buildup. The resulting ripple current causes the average shortcircuit current to be approximately IOUT(MAX). CT and L Selection for Operating Frequency The LTC1265 uses a constant off-time architecture with tOFF determined by an external capacitor CT. Each time the P-channel MOSFET turns on, the voltage on CT is reset to approximately 3.3V. During the off time, CT is discharged by a current that is proportional to VOUT. The voltage on CT is analogous to the current in inductor L, which likewise, decays at a rate proportional to VOUT. Thus the inductor value must track the timing capacitor value. The value of CT is calculated from the desired continuous mode operating frequency: CT = 1 1.3(104)f
)
VIN - VOUT VIN + VD
)
(Farads)
where VD is the drop across the Schottky diode. As the operating frequency is increased, the gate charge losses will reduce efficiency. The complete expression for operating frequency is given by:
f VIN - VOUT (Hz) tOFF VIN + VD 1
)
)
where: tOFF = 1.3(104)CT
))
VREG (sec) VOUT
VREG is the desired output voltage (i.e. 5V, 3.3V). VOUT is the measured output voltage. Thus VREG/VOUT = 1 in regulation. Note that as VIN decreases, the frequency decreases. When the input-to-output voltage differential drops below
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LTC1265/LTC1265-3.3/LTC1265-5
APPLICATIONS INFORMATION
P-channel switch duty cycle. At high input voltages, the diode conducts most of the time. As VIN approaches VOUT, the diode conducts 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 ISC(PK) at close to 100% duty cycle. Most LTC1265 circuits will be well served by either a 1N5818 or a MBRS130LT3 Schottky diode. An MBRS0520 is a good choice for IOUT(MAX) 500mA. CIN In continuous mode, the input current of the converter is a square wave of duty cycle VOUT/ VIN. To prevent large voltage transients, a low ESR input capacitor must be used. In addition, the capacitor must handle a high RMS current. The CIN RMS current is given by:
IOUT [VOUT (VIN - VOUT)] /2 (ARMS) IRMS VIN
1
This formula has a maximum at VIN = 2VOUT, where IRMS = IOUT/2. This simple worst case is commonly used for design because even significant deviations do not offer much relief. Note that capacitor manufacturer's ripple current ratings are often based on only 2000 hours lifetime. This makes it advisable to further derate the capacitor, or to choose a capacitor rated at a higher temperature than required. Do not underspecify this component. An additional 0.1F ceramic capacitor is also required on PWR VIN for high frequency decoupling. COUT The selection of COUT is based upon the effective series resistance (ESR) for proper operation of the LTC1265. The required ESR of COUT is: ESRCOUT < 50mV/IRIPPLE where IRIPPLE is the ripple current of the inductor. For the case where the IRIPPLE is 25mV/RSENSE, the required ESR of COUT is: ESRCOUT < 2(RSENSE) To avoid overheating, the output capacitor must be sized to handle the ripple current generated by the inductor. The
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worst-case RMS ripple current in the output capacitor is given by:
IRMS
150mV (ARMS) 2(RSENSE)
Generally, once the ESR requirement for COUT has been met, the RMS current rating far exceeds the IRIPPLE(P-P) requirement. ESR is a direct function of the volume of the capacitor. Manufacturers such as Nichicon, AVX and Sprague should be considered for high performance capacitors. The OS-CON semiconductor dielectric capacitor available from Sanyo has the lowest ESR for its size at a somewhat higher price. In surface mount applications, multiple capacitors may have to be paralleled to meet the capacitance, ESR or RMS current handling requirement of the application. Aluminum electrolyte and dry tantalum capacitors are both available in surface mount configurations. In the case of tantalum, it is critical that the capacitors are both available in surface mount configuration and are surge tested for use in switching power supplies. An excellent choice is the AVX TPS series of surface mount tantalums, available in case heights ranging from 2mm to 4mm. Consult the manufacturer for other specific recommendations. When the capacitance of COUT is made too small, the output ripple at low frequencies will be large enough to trip the voltage comparator. This causes Burst Mode operation to be activated when the LTC1265 would normally be in continuous operation. The effect will be most pronounced with low value of RSENSE and can be improved at higher frequencies with lower values of L. Low-Battery Detection The low-battery comparator senses the input voltage through an external resistive divider. This divided voltage connects to the (-) input of a voltage comparator (Pin 4) which is compared with a 1.25V reference voltage. Neglecting Pin 4 bias current, the following expression is used for setting the trip limit:
R4 VLB_TRIP = 1.25 1 + R3
)
)
LTC1265/LTC1265-3.3/LTC1265-5
APPLICATIONS INFORMATION
The output, Pin 3, is an N-channel open drain that goes low when the battery voltage is below the threshold set by R3 and R4. In shutdown, the comparator is disabled and Pin 3 is in a high impedance state.
VIN R4 LTC1265 4
- +
3
R3 1.25V REFERENCE
LTC1265 F03
Figure 3. Low-Battery Comparator
LTC1265 ADJUSTABLE APPLICATIONS The LTC1265 develops a 1.25V reference voltage between the feedback (Pin 9) terminal and signal ground (see Figure 4). By selecting resistor R1, a constant current is caused to flow through R1 and R2 to set overall output voltage. The regulated output voltage is determined by:
R2 VOUT = 1.25 1 + R1
)
)
VOUT R2
For most applications a 30k resistor is suggested for R1. To prevent stray pickup, a 100pF capacitor is suggested across R1 located close to the LTC1265.
LTC1265 VFB SGND 11
9 100pF
R1
LTC1265 F04
Figure 4. LTC1265 Adjustable Configuration
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THERMAL CONSIDERATIONS In a majority of applications, the LTC1265 does not dissipate much heat due to its high efficiency. However, in applications where the switching regulator is running at high duty cycles or the part is in dropout with the switch turned on continuously (DC), the user will need to do some thermal analysis. The goal of the thermal analysis is to determine whether the power dissipated by the regulator exceeds the maximum junction temperature of the part. The temperature rise is given by: TR = P(JA) where P 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 is simply given by: TJ = TR + TA As an example, consider the LTC1265 is in dropout at an input voltage of 4V with a load current of 0.5A. From the Typical Performance Characteristics graph of Switch Resistance, the ON resistance of the P-channel is 0.55. Therefore power dissipated by the part is: P = I2(RDSON) = 0.1375W For the SO package, the JA is 110C/W. Therefore the junction temperature of the regulator when it is operating in ambient temperature of 25C is: TJ = 0.1375(110) + 25 = 40.1C Remembering that the above junction temperature is obtained from a RDSON at 25C, we need to recalculate the junction temperature based on a higher RDSON since it increases with temperature. However, we can safely assume that the actual junction temperature will not exceed the absolute maximum junction temperature of 125C. Now consider the case of a 1A regulator with VIN = 4V and TA = 65C. Starting with the same 0.55 assumption for RDSON, the TJ calculation will yield 125C. But from the graph, this will increase the RDSON to 0.76, which when used in the above calculation yields an actual TJ > 148C. Therefore the LTC1265 would be unsuitable for a 4V input, 1A output regulator operating at TA = 65C.
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LTC1265/LTC1265-3.3/LTC1265-5
APPLICATIONS INFORMATION
Board Layout Checklist When laying out the printed circuit board, the following checklist should be used to ensure proper operation of the LTC1265. These items are also illustrated graphically in the layout diagram of Figure 5. Check the following in your layout: 1. Are the signal and power grounds segregated? The LTC1265 signal ground (Pin 11) must return to the (-) plate of COUT. The power ground (Pin 12) returns to the anode of the Schottky diode, and the (-) plate of CIN, whose leads should be as short as possible. 2. Does the (+) plate of the CIN connect to the power VIN (Pins 1,13) as close as possible? This capacitor provides the AC current to the internal P-channel MOSFET and its driver. 3. Is the input decoupling capacitor (0.1F) connected closely between power VIN (Pins 1,13) and power ground (Pin 12)? This capacitor carries the high frequency peak currents. 4. Is the Schottky diode closely connected between the power ground (Pin 12) and switch (Pin 14)? 5. Does the LTC1265 SENSE - (Pin 7) connect to a point close to RSENSE and the (+) plate of COUT? In adjustable applications, the resistive divider, R1 and R2, must be connected between the (+) plate of COUT and signal ground. 6. Are the SENSE - and SENSE + leads routed together with minimum PC trace spacing? The 1000pF capacitor between Pins 7 and 8 should be as close as possible to the LTC1265. 7. Is SHDN (Pin 10) actively pulled to ground during normal operation? The SHDN pin is high impedance and must not be allowed to float.
1
PWR VIN VIN
2
VIN
LTC1265 3 1k 1000pF 4 LBOUT LBIN PGND 12
3900pF
5
CT ITH SENSE-
6
7
SENSE+
8
R2 VOUT
1000pF
OUTPUT DIVIDER REQUIRED WITH ADJUSTABLE VERSION ONLY BOLD LINES INDICATE HIGH PATH CURRENTS
Figure 5. LTC1265 Layout Diagram (See Board Layout Checklist)
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+
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SW PWR VIN
14 13 D1
+
CIN 0.1F L
SGND
11 10
SHDN 9 N/C (VFB)
SHDN
R1 COUT RSENSE
LTC1265 F05
LTC1265/LTC1265-3.3/LTC1265-5
APPLICATIONS INFORMATION
Troubleshooting Hints Since efficiency is critical to LTC1265 applications, it is very important to verify that the circuit is functioning correctly in both continuous and Burst Mode operation. As the LTC1265 is highly tolerant of poor layout, the output voltage will still be regulated. Therefore, monitoring the output voltage will not tell you whether you have a good or bad layout. The waveform to monitor is the voltage on the timing capacitor Pin 5. In continuous mode the voltage on the CT pin is a sawtooth with approximately 0.9VP-P swing. This voltage should never dip below 2V as shown in Figure 6a. When the load currents are low (ILOAD < IBURST) Burst Mode operation occurs. The voltage on CT pin now falls to ground for periods of time as shown in Figure 6b. During this time the LTC1265 is in sleep mode with quiescent current reduced to 160A. The inductor current should also be monitored. If the circuit is poorly decoupled, the peak inductor current will be haphazard as in Figure 7a. A well decoupled LTC1265 has a clean inductor current as in Figure 7b.
VOLTAGE AT CT (PIN 5)
VOLTAGE AT CT (PIN 5)
3.3V 2.4V
0V TIME (a) CONTINUOUS MODE OPERATION
LTC1265 F06a
(a) POORLY DECOUPLED LTC1265
Figure 7. Inductor Waveforms
U
W
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SLEEP MODE 3.3V 2.4V
0V TIME (b) Burst Mode OPERATION
LTC1265 F06b
Figure 6. CT Waveforms
(b) WELL DECOUPLED LTC1265
11
LTC1265/LTC1265-3.3/LTC1265-5
APPLICATIONS INFORMATION
Design Example As a design example, assume VIN = 5V, VOUT = 3.3V, IMAX = 0.8A and f = 250kHz. With this information we can easily calculate all the important components. From (1), RSENSE = 100mV/0.8 = 0.125 From (2) and assuming VD = 0.4V, CT 100pF Using (3), the value of the inductor is: L 5.2(105)(0.125)(100pF)3.3V = 22H For the catch diode, a MBRS130LT3 or 1N5818 will be sufficient in this application. CIN will require an RMS current rating of at least 0.4A at temperature, and COUT will require an ESR of (from 5): ESRCOUT < 0.25 The inductor ripple current is given by:
V + VD tOFF = 0.22A IRIPPLE = OUT L
EFFICIENCY (% )
100 95 90 85 80 75 70 0.01 L = DALE LPT4545-220 (22H) VOUT = 3.3V CT = 100pF
)
)
At light loads the peak inductor current is at: IPEAK = 25mV/0.125 = 0.2A Therefore, at load current less than 0.1A the LTC1265 will be in Burst Mode operation. Figure 8 shows the complete circuit and Figure 9 shows the efficiency curve with the above calculated component values.
12
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VIN 5V
+
CIN 0.1F PWR VIN SHDN 1k ITH 3900pF 100pF CT SENSE - SGND VIN SW D1 22H 0.125 VOUT 3.3V 0.8A
LTC1265-3.3 PGND SENSE +
+
COUT
1000pF
LTC1265 F08
Figure 8. Design Example Circuit
0.1 LOAD CURRENT (mA)
1.0
1265 G11
Figure 9. Design Example Efficiency Curve
LTC1265/LTC1265-3.3/LTC1265-5
TYPICAL APPLICATIONS
High Efficiency 5V to 3.3V Converter
VIN 5V 4 3 270pF 5
3900pF
1k
*AVX TPSD107K010 **AVX TPSE227K010 COILCRAFT D03316-473 DALE WSL2010-0.1-1%
*AVX TPSD226K025 **AVX TPSD107K010 L1 SELECTION MANUFACTURER COILCRAFT COILTRONICS DALE SUMIDA PART NO. DO3316-473 CTX50-4 LPT4545-500LA CD74-470
IRC LRC2010-01-R100-J D1= MBRS130LT3
VIN (V) 3.5 4.0 5.0 6.0 7.0 7.5
IOUT(MAX) (mA) 360 430 540 630 720 740
7
SENSE -
SENSE +
8 RSENSE 0.1
1000pF
+
U
6 7
2 VIN LBIN LBOUT CT
1, 13 PWR VIN SW PGND SGND SHDN 0.1F 14 12 11 10 9 8 SHDN
+
CIN* 100F 10V
L1 47H
RSENSE 0.1
LTC1265-3.3
MBRS130LT1
+
VOUT 3.3V COUT** 1A 220F 10V
ITHR SENSE -
NC SENSE +
1000pF
LTC1265 TA02
Positive-to-Negative (-5V) Converter
VIN 3.5V TO 7.5V 4 3 220pF 5
2 VIN LBIN LBOUT CT
1, 13 TP0610L PWR VIN 14 SW PGND SGND 12 11
SHDN L1 50H D1 0.1F
+
CIN* 22F 25V x2 VOUT -5V
LTC1265-5
2200pF
1k
6
ITHR
SHDN
10
100k
COUT** 100F 10V
LTC1265 TA03
13
LTC1265/LTC1265-3.3/LTC1265-5
TYPICAL APPLICATIONS
VIN (V) 3.5 4.0 5.0 6.0 7.0 7.5 L1B
IOUT(MAX) (mA) 240 275 365 490 610 665
VIN 3.5V TO 7.5V 4 3 75pF
*
L1A 3 2 3300pF L1A TOP VIEW 4 1*
L1B
*SANYO OS-CON CAPACITOR **IRC LRC2010-01-R162-J L1A, L2A SELECTION MANUFACTURER COILTRONICS DALE PART NO. CTX33-4 LPT4545-330LA
VIN (V) 4.0 5.0 6.0 7.0 8.0 9.0 10.0 11.0 12.0 L1B
IOUT(MAX) (mA) 40 60 80 100 115 130 150 165 180 L1A 3 2 TOP VIEW 4 1*
VIN 4V TO 12V 2 4 3 75pF 5 VIN LBIN LTC1265 LBOUT CT PGND SGND SHDN 1k 6 7 ITHR SENSE - VFB SENSE + 1, 13 PWR VIN SW 14 12 11 10 9 8 SHDN 0.1F 1N914
L1A 50H 2 1 MBRS130LT3
SI19430DY 4
*
*
L1B 50H
301k
3300pF L1A
3
L1B
*AVX TPSE686K020 **AVX TPSE336K025 IRC LRC2010-01-R162-J L1A,L2A SELECTION MANUFACTURER COILTRONICS DALE PART NO. CTX50-4 LPT4545-500LA
0.01F
RSENSE* 0.162
100pF
34k
LTC1265 TA05
14
+
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5V Buck-Boost Converter
2 VIN LBIN LTC1265 LBOUT CT
1, 13 PWR VIN SW PGND SGND SHDN 0.1F 14 12 11 10 9 8 SHDN
+
CIN* 100F 16V
33F 10V*
L1A 33H 1 1N5818 2
+
VOUT 5V
5
4
*
L1B 33H
75k
3
+
1k
6 7
ITHR SENSE -
VFB SENSE +
COUT* 100F 10V
0.01F
RSENSE** 0.162
100pF
25k
LTC1265 F09
9V to 12V and - 12V Outputs
MBRS130LT3 VOUT -12V COUT* 68F 20V VOUT 12V
+
CIN* 68F 20V
33F** 25V
+
+
COUT* 68F 20V
LTC1265/LTC1265-3.3/LTC1265-5
TYPICAL APPLICATIONS
VIN 3.5V TO 12.5V 4 3 51pF 5
3300pF
1k
7
SENSE -
SENSE +
8 RSENSE 0.20
1000pF
LTC1265 TA06
Logic Selectable 0V/3.3V/5V 700mA Regulator
*DALE 593D68X0020E2W **DALE 593D107X0010D2W IRC LRC2010-01-R15-J L1 SELECTION MANUFACTURER COILCRAFT COILTRONICS DALE SUMIDA
VIN 3.5V TO 12.5V 0V: VOUT = 5V 5V: VOUT = 3.3V 75pF 5 4 3
2 VIN LBIN LTC1265 LBOUT CT
1, 13 PWR VIN SW PGND SGND SHDN 0.1F 14 12 11 10 9 8 VSHDN
+
CIN* 68F 20V MBRS130LT3
VSHDN = 0V: VOUT = 3.3V/5V = 5V: VOUT = 0V
3300pF
100pF
L1 33H
1k
6 7
ITHR SENSE -
VFB SENSE +
1000pF
RSENSE 0.15
56.2k
75k VOUT 0V/3.3V/5V 700mA
LTC1265 TA07
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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6
2.5mm Max Height 5V-to-3.3V (500mA)
2 VIN LBIN LBOUT CT
1, 13 PWR VIN SW PGND SGND SHDN 0.1F 14 12 11 10 9 SHDN
+
CIN* 15F 10V x 2
*AVX TAJB156K010 **AVX TAJB226K06 IRC LRC2010-01-R200-J SUMIDA CLS62-180
LTC1265-3.3
MBRS0520LT1
L1 18H
ITHR
N/C
COUT** 22F 6.3V x 2
VOUT 3.3V 500mA
PART NO. DO3316-333 CTX33-4 LPT4545-330LA CD74-330
45.3k COUT** 100F 10V
15
LTC1265/LTC1265-3.3/LTC1265-5
TYPICAL APPLICATIONS
4-NiCad Battery Charger
*DALE 593D226X0025D2W **DALE 593D107X0016E2W DALE WSL2010-0.10-1% L1 SELECTION VIN 8V TO 12.5V 4 51 3 5 FAST CHARGE: = 0V TRICKLE CHARGE: > 2V 2 VIN LBIN LTC1265 LBOUT CT PGND SGND SHDN 6 VN2222L 3300pF 1k 7 ITHR SENSE - VFB SENSE + 1, 13 PWR VIN SW 0.1F 14 12 11 10 9 8 RSENSE 0.10 138k MBRS130LT3 VOUT 4 NICAD 1A FAST CHARGE 0.1A TRICKLE CHARGE CHARGER ON/OFF 100pF L1 100H 30k COUT** 100F 10V MBRS130LT3
270pF
1000pF
PACKAGE DESCRIPTION Dimension in inches (millimeters) unless otherwise noted.
0.337 - 0.344* (8.560 - 8.738)
0.010 - 0.020 x 45 (0.254 - 0.508) 0.008 - 0.010 (0.203 - 0.254)
0.053 - 0.069 (1.346 - 1.752)
0 - 8 TYP
0.004 - 0.010 (0.101 - 0.254)
14
13
12
0.016 - 0.050 (0.406 - 1.270)
0.014 - 0.019 (0.355 - 0.483) TYP
0.050 (1.270) BSC
0.228 - 0.244 (5.791 - 6.197)
*DIMENSION DOES NOT INCLUDE MOLD FLASH. MOLD FLASH SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE **DIMENSION DOES NOT INCLUDE INTERLEAD FLASH. INTERLEAD FLASH SHALL NOT EXCEED 0.010" (0.254mm) PER SIDE
1
2
3
RELATED PARTS
PART NUMBER LTC1143 LTC1147 LTC1148HV LTC1174 LTC1574 LTC1622 LTC1627 LTC1772 DESCRIPTION Dual Step-Down Switching Regulator Controller Step-Down Switching Regulator Controller Step-Down Switching Regulator Controller Step-Down Switching Regulator with Internal 0.5A Switch Step-Down Switching Regulator with Internal 0.5A Switch and Schottky Diode Low Input Voltage Step-Down DC/DC Controller Monolithic Synchronous Step-Down Switching Regulator Constant Frequency Step-Down DC/DC Controller COMMENTS Dual Version of LTC1147 Nonsynchronous, 8-Pin, VIN 16V Synchronous, VIN 20V VIN 18.5V, Comparator/Low Battery Detector Monolithic, IQ = 40A, 400mA, MS8 VIN 18.5V, Comparator Constant Frequency, 2V to 10V VIN, MS8 Constant Frequency, IOUT to 500mA, 2.65V to 8.5V VIN SOT-23, 2.2V to 9.8V VIN
126535fa LT/TP 1299 2K REV A * PRINTED IN USA
LTC1474/LTC1475 Low Quiescent Current Step-Down Regulators
16
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408)432-1900 q FAX: (408) 434-0507 q www.linear-tech.com
+
U
U
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CIN* 22F, 25V
MANUFACTURER COILCRAFT COILTRONICS SUMIDA
PART NO. DO3316-104 CTX100-4P CD105-101
LTC1265 TA08
11
10
9
8
0.150 - 0.157** (3.810 - 3.988)
S14 1298
4
5
6
7
(c) LINEAR TECHNOLOGY CORPORATION 1995

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