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FEATURES
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LTC3548A Dual Synchronous 400mA/800mA, 2.25MHz Step-Down DC/DC Regulator DESCRIPTION
The LTC(R)3548A is a dual, constant frequency, synchronous step-down DC/DC converter. Intended for low power applications, it operates from a 2.5V to 5.5V input voltage range and has a constant 2.25MHz switching frequency, enabling the use of tiny, low cost capacitors and inductors 1mm or less in height. Each output voltage is adjustable from 0.6V to 5V. Internal synchronous 0.35, 0.7A/1.2A power switches provide high efficiency without the need for external Schottky diodes. A user-selectable mode input is provided to allow the user to trade-off ripple noise for low power efficiency. Burst Mode(R) operation provides the highest efficiency at light loads, while pulse-skipping mode provides the lowest ripple noise at light loads. To further maximize battery life, the P-channel MOSFETs are turned on continuously in dropout (100% duty cycle), and both channels draw a total quiescent current of only 40A. In shutdown, the device draws <1A.
L, LT, LTC, LTM, Linear Technology, the Linear logo and Burst Mode are registered trademarks 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.
High Efficiency: Up to 95% Low Ripple (<35mVP-P) Burst Mode Operation; IQ = 40A 2.25MHz Constant Frequency Operation High Switch Current: 0.7A and 1.2A No Schottky Diodes Required Low RDS(ON) Internal Switches: 0.35 Current Mode Operation for Excellent Line and Load Transient Response Short-Circuit Protected Low Dropout Operation: 100% Duty Cycle Ultralow Shutdown Current: IQ <1A Output Voltages from 5V down to 0.6V Power-On Reset Output Externally Synchronizable Oscillator Optional External Soft-Start Small Thermally Enhanced MSOP and 3mm x 3mm DFN Packages
APPLICATIONS
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PDAs/Palmtop PCs Digital Cameras Cellular Phones Wireless and DSL Modems
TYPICAL APPLICATION
2.5V/1.8V at 400mA/800mA Step-Down Regulators
VIN 2.5V TO 5.5V 10F EFFICIENCY (%) RUN/SS2 VIN RUN/SS1 MODE/SYNC VOUT2 2.5V 400mA 4.7H SW2 22pF LTC3548A SW1 POR 2.2H 22pF 100k RESET VOUT1 1.8V 800mA 100 90 80 70 60 50 40 30 20 4.7F 887k VFB2 280k GND VFB1 442k 887k 10F
3548A TA01
Efficiency/Power Loss Curves
2.5V 1.8V 0.1 POWER LOSS (W) 1
0.01
0.001 VIN = 3.3V Burst Mode OPERATION NO LOAD ON OTHER CHANNEL 1 10 100 LOAD CURRENT (mA)
10 0
0.0001 1000
3548A TA02
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LTC3548A ABSOLUTE MAXIMUM RATINGS (Note 1)
VIN Voltage ...................................................- 0.3V to 6V VFB1, VFB2 Voltages ................................... -0.3V to 1.5V RUN/SS1, RUN/SS2 Voltages ...........-0.3V to VIN + 0.3V MODE/SYNC Voltage.........................-0.3V to VIN + 0.3V SW1, SW2 Voltages ......................... -0.3V to VIN + 0.3V POR Voltage ................................................. -0.3V to 6V Operating Junction Temperature Range (Note 2) LTC3548AE .......................................... -40C to 85C LTC3548AI ......................................... -40C to 125C Storage Temperature Range................... -65C to 150C Lead Temperature (Soldering, 10 sec) MSOP ............................................................... 300C
PIN CONFIGURATION
TOP VIEW VFB1 RUN/SS1 VIN SW1 NC 1 2 3 4 5 11 10 VFB2 9 RUN/SS2 8 POR 7 SW2 6 MODE/SYNC TOP VIEW VFB1 RUN/SS1 VIN SW1 NC 1 2 3 4 5 10 9 8 7 6 VFB2 RUN/SS2 POR SW2 MODE/SYNC
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DD PACKAGE 10-LEAD (3mm 3mm) PLASTIC DFN TJMAX = 125C, JA = 45C/W, JC = 10C/W EXPOSED PAD (PIN 11) IS PGND, MUST BE SOLDERED TO GND
MSE PACKAGE 10-LEAD PLASTIC MSOP TJMAX = 125C, JA = 45C/W, JC = 10C/W EXPOSED PAD (PIN 11) IS PGND, MUST BE SOLDERED TO GND
ORDER INFORMATION
LEAD FREE FINISH LTC3548AEDD#PBF LTC3548AEMSE#PBF LTC3548AIDD#PBF LTC3548AIMSE#PBF TAPE AND REEL LTC3548AEDD#TRPBF LTC3548AEMSE#TRPBF LTC3548AIDD#TRPBF LTC3548AIMSE#TRPBF PART MARKING* LFKV LTFKW LFKV LTFKW PACKAGE DESCRIPTION 10-Lead (3mm x 3mm) Plastic DFN 10-Lead Plastic MSOP 10-Lead (3mm x 3mm) Plastic DFN 10-Lead Plastic MSOP TEMPERATURE RANGE -40C to 85C -40C to 85C -40C to 125C -40C to 125C
Consult LTC Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container. For more information on lead free part marking, go to: http://www.linear.com/leadfree/ For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/
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LTC3548A ELECTRICAL CHARACTERISTICS
SYMBOL VIN IFB VFB VLINE REG VLOAD REG IS PARAMETER Operating Voltage Range Feedback Pin Input Current Feedback Voltage (Note 3) Reference Voltage Line Regulation Output Voltage Load Regulation Input DC Supply Current Active Mode Sleep Mode Shutdown Oscillator Frequency Synchronization Frequency Peak Switch Current Limit Channel 1 Peak Switch Current Limit Channel 2 Top Switch On-Resistance Bottom Switch On-Resistance Switch Leakage Current Power-On Reset Threshold Power-On Reset On-Resistance Power-On Reset Delay VRUN IRUN VMODE RUN/SS Threshold Low RUN/SS Threshold High RUN/SS Leakage Current MODE Threshold Low MODE Threshold High

The l denotes the specifications which apply over the full operating junction temperature range, otherwise specifications are at TA = 25C. VIN = 3.6V, unless otherwise specified. (Note 2)
CONDITIONS

MIN 2.5 0.588 0.585
TYP
MAX 5.5 30
UNITS V nA V V %/V %
0C TA 85C -40C TA 125C (Note 2) VIN = 2.5V to 5.5V (Note 3) MODE/SYNC = 0V (Note 3) (Note 4) VFB1 = VFB2 = 0.5V VFB1 = VFB2 = 0.63V, MODE/SYNC = 3.6V RUN = 0V, VIN = 5.5V, MODE/SYNC = 0V VFBX = 0.6V VIN = 3V, VFBX = 0.5V, Duty Cycle < 35% VIN = 3V, VFBX = 0.5V, Duty Cycle < 35% (Note 6) (Note 6) VIN = 5V, VRUN = 0V, VFBX = 0V VFBX Ramping Up, MODE/SYNC = 0V VFBX Ramping Down, MODE/SYNC = 0V
0.6 0.6 0.3 0.5 700 40 0.1
0.612 0.612 0.5
950 60 1 2.7 1.6 0.9 0.45 0.45 1
A A A MHz MHz A A A % %
fOSC fSYNC ILIM RDS(ON) ISW(LKG) POR
1.8 1 0.6
2.25 2.25 1.2 0.7 0.35 0.30 0.01 8.5 -8.5 100 65,536
200 1.5 2 1 0.5 VIN
Cycles V V A V V
0.3
1 0.01
0 VIN - 0.5
Note 1: Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. Exposure to any Absolute Maximum Rating condition for extended periods may affect device reliability and lifetime. Note 2: The LTC3548AE is guaranteed to meet specified performance from 0C to 85C. Specifications over the -40C and 85C operating junction temperature range are assured by design, characterization and correlation with statistical process controls. The LTC3548AI is guaranteed over the full -40C to 125C operating junction temperature range.
Note 3: The LTC3548A is tested in a proprietary test mode that connects VFB to the output of the error amplifier. Note 4: Dynamic supply current is higher due to the internal gate charge being delivered at the switching frequency. Note 5: TJ is calculated from the ambient, TA, and power dissipation, PD, according to the following formula: TJ = TA + (PD * JA). Note 6: The DFN switch on-resistance is guaranteed by correlation to wafer level measurements.
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LTC3548A TYPICAL PERFORMANCE CHARACTERISTICS
Burst Mode Operation
SW 5V/DIV VOUT 50mV/DIV
TA = 25C unless otherwise specified.
Pulse-Skipping Mode
SW 5V/DIV VOUT 10mV/DIV
IL 200mA/DIV
IL 100mA/DIV
2s/DIV VIN = 3.6V VOUT = 1.8V ILOAD = 50mA FIGURE 3 CIRCUIT
3548A G01
1s/DIV VIN = 3.6V VOUT = 1.8V ILOAD = 50mA FIGURE 3 CIRCUIT
3548A G02
Load Step
VOUT1 200mV/DIV VOUT2 100mV/DIV IL 500mA/DIV VIN 2V/DIV
Soft-Start
VOUT1 1V/DIV IL 500mA/DIV
ILOAD 500mA/DIV
20s/DIV VIN = 3.6V VOUT = 1.8V ILOAD = 50mA TO 600mA FIGURE 3 CIRCUIT
3548A G03
1ms/DIV VIN = 3.6V VOUT = 1.8V ILOAD = 500mA FIGURE 4 CIRCUIT
3548A G04
Efficiency vs Input Voltage
100 90 80 EFFICIENCY (%) 70 60 50 40 30 20 10 VOUT = 1.8V CIRCUIT OF FIGURE 3 0 4 2 3 5 INPUT VOLTAGE (V) 1mA 10mA FREQUENCY (MHz) 100mA 800mA 2.5
Oscillator Frequency vs Temperature
VIN = 3.6V FREQUENCY DEVIATION (%) 10 8 6 4 2 0 -2 -4 -6 -8 6
3548A G05
Oscillator Frequency vs Supply Voltage
2.4
2.3
2.2
2.1
2.0 -50 -25
-10 50 25 75 0 TEMPERATURE (C) 100 125 2 3
4 SUPPLY VOLTAGE (V)
5
6
3548A G07
3548A G06
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LTC3548A TYPICAL PERFORMANCE CHARACTERISTICS
Reference Voltage vs Temperature
0.615 0.610 REFERENCE VOLTAGE (V) 0.605 0.600 0.595 0.590 0.585 -50 -25 VIN = 3.6V 500 450 400 MAIN SWITCH 350 300 250 200 50 25 75 0 TEMPERATURE (C) 100 125 1 2 SYNCHRONOUS SWITCH
TA = 25C unless otherwise specified.
RDS(ON) vs Input Voltage
550 500 450 RDS(ON) (m) 400 350 300 250 200 150 3 4 VIN (V) 5 6 7
3548A G09
RDS(ON) vs Temperature
VIN = 2.7V VIN = 4.2V VIN = 3.6V
RDS(ON) (m)
100 -50 -25
MAIN SWITCH SYNCHRONOUS SWITCH 0 25 50 75 100 125 150 TEMPERATURE (C)
3548A G10
3548A G08
Efficiency vs Load Current
100 90 80 EFFICIENCY (%) 70 60 50 40 30 20 V OUT = 2.5V 10 Burst Mode OPERATION FIGURE 3 CIRCUIT 0 10 100 1 LOAD CURRENT (mA) 2.7V 4.2V 3.3V EFFICIENCY (%) 100 90 80
Efficiency vs Load Current
4 Burst Mode OPERATION 3 2 VOUT ERROR(%)
Load Regulation
70 60 50 40 30 20 10 0 VIN = 3.6V VOUT = 1.8V NO LOAD ON OTHER CHANNEL 1 10 100 LOAD CURRENT (mA) 1000
3548A G12
1 Burst Mode OPERATION 0 -1 -2 -3 -4 1 10 100 LOAD CURRENT (mA) 1000
3548A G13
PULSE-SKIPPING MODE
PULSE-SKIPPING MODE
VIN = 3.6V VOUT = 1.8V NO LOAD ON OTHER CHANNEL
1000
3548A G11
Efficiency vs Load Current
100 90 80 EFFICIENCY (%) 70 60 50 40 30 20 10 0 1 VOUT = 1.2V Burst Mode OPERATION 10 100 LOAD CURRENT (mA) 1000
3548A G14
Efficiency vs Load Current
100 0.5 3.3V 2.7V 4.2V VOUT ERROR (%) 90 80
Line Regulation
VOUT = 1.8V 0.4 IOUT = 200mA 0.3 0.2 0.1 0 -0.1 -0.2 -0.3 -0.4 -0.5 1000
3548A G15
3.3V 2.7V 4.2V EFFICIENCY (%)
70 60 50 40 30 20
10 VOUT = 1.5V Burst Mode OPERATION 0 10 100 1 LOAD CURRENT (mA)
2
3
4 VIN (V)
5
6
3548A G16
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LTC3548A PIN FUNCTIONS
VFB1 (Pin 1): Output Feedback. Receives the feedback voltage from the external resistive divider across the output. Nominal voltage for this pin is 0.6V. RUN/SS1 (Pin 2): Regulator 1 Enable and Soft-Start Input. Forcing this pin to VIN enables regulator 1, while forcing it to GND causes regulator 1 to shut down. Connect external RC network with desired time-constant to enable soft-start feature. This pin must be driven; do not float. VIN (Pin 3): Main Power Supply. Must be closely decoupled to GND. SW1 (Pin 4): Regulator 1 Switch Node Connection to the Inductor. This pin swings from VIN to GND. NC (Pin 5): No Connect. This pin is not connected internally. Connect to ground on PCB for shielding purposes. MODE/SYNC (Pin 6): Combination Mode Selection and Oscillator Synchronization. This pin controls the operation of the device. When tied to VIN or GND, Burst Mode operation or pulse-skipping mode is selected, respectively. Do not float this pin. The oscillation frequency can be synchronized to an external oscillator applied to this pin and pulse-skipping mode is automatically selected. SW2 (Pin 7): Regulator 2 Switch Node Connection to the Inductor. This pin swings from VIN to GND. POR (Pin 8): Power-On Reset. This common-drain logic output is pulled to GND when the output voltage is not within 8.5% of regulation and goes high after 216 clock cycles when both channels are within regulation. RUN/SS2 (Pin 9): Regulator 2 Enable and Soft-Start Input. Forcing this pin to VIN enables regulator 2, while forcing it to GND causes regulator 2 to shut down. Connect external RC network with desired time-constant to enable soft-start feature. This pin must be driven; do not float. VFB2 (Pin 10): Output Feedback. Receives the feedback voltage from the external resistive divider across the output. Nominal voltage for this pin is 0.6V. Exposed Pad (GND) (Pin 11): Power Ground. Connect to the (-) terminal of COUT, and (-) terminal of CIN. Must be soldered to electrical ground on PCB.
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LTC3548A BLOCK DIAGRAM
REGULATOR 1 MODE/SYNC 6 BURST CLAMP SLOPE COMP 0.6V VIN
+
EA
-
ITH 0.65V
EN SLEEP
-
ICOMP
+
5
VFB1
1
- -
UVDET
+
BURST S Q RS LATCH R Q SWITCHING LOGIC AND BLANKING CIRCUIT
0.55V
UV
+ +
OVDET 0.65V
ANTI SHOOTTHRU
4 SW1
OV
-
IRCMP
SHUTDOWN VIN 3 VIN 8 POR 0.6V REF RUN/SS2 9 OSC OSC POR COUNTER 5 GND PGOOD2 REGULATOR 2 (IDENTICAL TO REGULATOR 1) VFB2 10
3548A BD
PGOOD1 RUN/SS1 2
-
+
11 GND 7 SW2
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LTC3548A OPERATION
The LTC3548A uses a constant frequency, current mode architecture. The operating frequency is set at 2.25MHz and can be synchronized to an external oscillator. Both channels share the same clock and run in phase. To suit a variety of applications, the selectable MODE/SYNC pin allows the user to trade-off noise for efficiency. The output voltage is set by an external divider returned to the VFB pins. An error amplifier compares the divided output voltage with a reference voltage of 0.6V and adjusts the peak inductor current accordingly. Overvoltage and undervoltage comparators will pull the POR output low if the output voltage is not within 8.5%. The POR output will go high after 65,536 clock cycles (about 29ms in pulse-skipping mode) of achieving regulation. Main Control Loop During normal operation, the top power switch (P-channel MOSFET) is turned on at the beginning of a clock cycle when the VFB voltage is below the reference voltage. The current into the inductor and the load increases until the current limit is reached. The switch turns off and energy stored in the inductor flows through the bottom switch (N-channel MOSFET) into the load until the next clock cycle. The peak inductor current is controlled by the internally compensated ITH voltage, which is the output of the error amplifier. This amplifier compares the VFB pin to the 0.6V reference. When the load current increases, the VFB voltage decreases slightly below the reference. This decrease causes the error amplifier to increase the ITH voltage until the average inductor current matches the new load current. The main control loop is shut down by pulling the RUN/SS pin to ground. Low Current Operation Two modes are available to control the operation of the LTC3548A at low currents. Both modes automatically switch from continuous operation to the selected mode when the load current is low. To optimize efficiency, the Burst Mode operation can be selected. When the load is relatively light, the LTC3548A automatically switches into Burst Mode operation in which the PMOS switch operates intermittently based on load demand with a fixed peak inductor current. By running cycles periodically, the switching losses which are dominated by the gate charge losses of the power MOSFETs are minimized. The main control loop is interrupted when the output voltage reaches the desired regulated value. A voltage comparator trips when ITH is below 0.65V, shutting off the switch and reducing the power. The output capacitor and the inductor supply the power to the load until ITH exceeds 0.65V, turning on the switch and the main control loop which starts another cycle. For lower ripple noise at low currents, the pulse-skipping mode can be used. In this mode, the LTC3548A continues to switch at a constant frequency down to very low currents, where it will begin skipping pulses. Dropout Operation When the input supply voltage decreases toward the output voltage, the duty cycle increases to 100% which is the dropout condition. In dropout, the PMOS switch is turned on continuously with the output voltage being equal to the input voltage minus the voltage drops across the internal P-channel MOSFET and the inductor. An important design consideration is that the RDS(ON) of the P-channel switch increases with decreasing input supply voltage (See Typical Performance Characteristics). Therefore, the user should calculate the power dissipation when the LTC3548A is used at 100% duty cycle with low input voltage (See Thermal Considerations in the Applications Information Section). Low Supply Operation The LTC3548A incorporates an undervoltage lockout circuit which shuts down the part when the input voltage drops below about 1.65V to prevent unstable operation. A general LTC3548A application circuit is shown in Figure 1. External component selection is driven by the load requirement, and begins with the selection of the inductor L. Once the inductor is chosen, CIN and COUT can be selected.
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LTC3548A OPERATION
VIN = 2.5V TO 5.5V CIN R6 VIN MODE/SYNC RUN/SS2 C4 LTC3548A L2 VOUT2 C2 SW2 SW1 C1 L1 VOUT1 POR RUN/SS1 C3 R7 POWER-ON RESET BURST* PULSE SKIP* R5
COUT2
R4
VFB2 R3 GND
VFB1 R1 R2 COUT1
3548A F01
*MODE/SYNC = 0V: PULSE SKIP MODE/SYNC = VIN: Burst Mode OPERATION
Figure 1. LTC3548A General Schematic
APPLICATIONS INFORMATION
Inductor Selection Although the inductor does not influence the operating frequency, the inductor value has a direct effect on ripple current. The inductor ripple current IL decreases with higher inductance and increases with higher VIN or VOUT : IL = VOUT fO * L V * 1- OUT VIN The inductor value will also have an effect on Burst Mode operation. The transition from low current operation begins when the peak inductor current falls below a level set by the burst clamp. Lower inductor values result in higher ripple current which causes this transition to occur at lower load currents. This causes 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 LTC3548A requires to operate. Table 1 shows some typical surface mount inductors that work well in LTC3548A applications.
Accepting larger values of IL allows the use of low inductances, but results in higher output voltage ripple, greater core losses, and lower output current capability. A reasonable starting point for setting ripple current is IL = 0.3 * ILIM, where ILIM is the peak switch current limit. The largest ripple current IL occurs at the maximum input voltage. To guarantee that the ripple current stays below a specified maximum, the inductor value should be chosen according to the following equation: L VOUT V * 1- OUT fO * IL VIN(MAX)
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LTC3548A APPLICATIONS INFORMATION
Table 1. Representative Surface Mount Inductors
MANUFACTURER Taiyo Yuden Panasonic Sumida Murata Taiyo Yuden FDK PART NUMBER CB2016T2R2M CB2012T2R2M CB2016T3R3M ELT5KT4R7M CDRH2D18/LD LQH32CN4R7M23 NR30102R2M NR30104R7M FDKMIPF2520D FDKMIPF2520D FDKMIPF2520D VLF3010AT4R7MR70 VLF3010AT3R3MR87 VLF3010AT2R2M1R0 MAX DC VALUE CURRENT 2.2H 2.2H 3.3H 4.7H 4.7H 4.7H 2.2H 4.7H 4.7H 3.3H 2.2H 4.7H 3.3H 2.2H 510mA 530mA 410mA 950mA 630mA 450mA 1100mA 750mA 1100mA 1200mA 1300mA 700mA 870mA 1000mA DCR 0.13 0.33 0.27 0.2 0.086 0.2 0.1 0.19 0.11 0.1 0.08 0.28 0.17 0.12 HEIGHT 1.6mm 1.25mm 1.6mm 1.2mm 2mm 2mm 1mm 1mm 1mm 1mm 1mm 1mm 1mm 1mm
the size or height requirements of the design. An additional 0.1F to 1F ceramic capacitor is also recommended on VIN for high frequency decoupling, when not using an all ceramic capacitor solution. Output Capacitor (COUT) Selection The selection of COUT is driven by the required ESR to minimize voltage ripple and load step transients. Typically, once the ESR requirement is satisfied, the capacitance is adequate for filtering. The output ripple (VOUT) is determined by: 1 VOUT IL ESR + 8fO COUT where fO = operating frequency, COUT = output capacitance and IL = ripple current in the inductor. The output ripple is highest at maximum input voltage since IL increases with input voltage. With IL = 0.3 * ILIM the output ripple will be less than 100mV at maximum VIN and fO = 2.25MHz with: ESRCOUT < 150m Once the ESR requirements for COUT have been met, the RMS current rating generally far exceeds the IRIPPLE(P-P) requirement, except for an all ceramic solution. 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 electrolytic, special polymer, ceramic and dry tantulum capacitors are all available in surface mount packages. The OS-CON semiconductor dielectric capacitor available from Sanyo has the lowest ESR (size) product of any aluminum electrolytic at a somewhat higher price. Special polymer capacitors, such as Sanyo POSCAP of, fer very low ESR, but have a lower capacitance density than other types. Tantalum capacitors have the highest capacitance density. However, they also have a larger ESR and it is critical that they are surge tested for use in switching power supplies. An excellent choice is the
TDK
Input Capacitor (CIN) Selection In continuous mode, the input current of the converter is a square wave with a duty cycle of approximately VOUT/VIN. To prevent large voltage transients, a low equivalent series resistance (ESR) input capacitor sized for the maximum RMS current must be used. The maximum RMS capacitor current is given by: 1 VOUT IL ESR + 8fO COUT where the maximum average output current IMAX equals the peak current minus half the peak-to-peak ripple current, IMAX = ILIM - IL/2. This formula has a maximum at VIN = 2VOUT , where IRMS = IOUT/2. This simple worst-case is commonly used to 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 choose a capacitor rated at a higher temperature than required. Several capacitors may also be paralleled to meet
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LTC3548A APPLICATIONS INFORMATION
AVX TPS series of surface mount tantalums, available in case heights ranging from 2mm to 4mm. Aluminum electrolytic capacitors have a significantly larger ESR, and are often used in extremely cost-sensitive applications provided that consideration is given to ripple current ratings and long term reliability. Ceramic capacitors have the lowest ESR and cost, but also have the lowest capacitance density, a high voltage and temperature coefficient, and exhibit audible piezoelectric effects. In addition, the high Q of ceramic capacitors along with trace inductance can lead to significant ringing. Other capacitor types include the Panasonic Special Polymer (SP) capacitors. In most cases, 0.1F to 1F of ceramic capacitors should also be placed close to the LTC3548A in parallel with the main capacitors for high frequency decoupling. Ceramic Input and Output Capacitors Higher value, lower cost ceramic capacitors are now becoming available in smaller case sizes. These are tempting for switching regulator use because of their very low ESR. Unfortunately, the ESR is so low that it can cause loop stability problems. Solid tantalum capacitor ESR generates a loop zero at 5kHz to 50kHz that is instrumental in giving acceptable loop-phase margin. Ceramic capacitors remain capacitive to beyond 300kHz and usually resonate with their ESL before ESR becomes effective. Also, ceramic capacitors are prone to temperature effects which require the designer to check loop stability over the operating temperature range. To minimize their large temperature and voltage coefficients, only X5R or X7R ceramic capacitors should be used. A good selection of ceramic capacitors is available from Taiyo Yuden, TDK, and Murata. Great care must be taken when using only ceramic input and output capacitors. When a ceramic capacitor is used at the input and the power is being supplied through long wires, such as from a wall adapter, a load step at the output can induce ringing at the VIN pin. At best, this ringing can couple to the output and be mistaken as loop instability. At worst, the ringing at the input can be large enough to damage the part. Since the ESR of a ceramic capacitor is so low, the input and output capacitor must instead fulfill a charge storage requirement. During a load step, the output capacitor must instantaneously supply the current to support the load until the feedback loop raises the switch current enough to support the load. The time required for the feedback loop to respond is dependent on the compensation and the output capacitor size. Typically, 3-4 cycles are required to respond to a load step, but only in the first cycle does the output drop linearly. The output droop, VDROOP , is usually about 3 times the linear drop of the first cycle. Thus, a good place to start is with the output capacitor size of approximately: COUT 3 IOUT fO * VDROOP
More capacitance may be required depending on the duty cycle and load step requirements. In most applications, the input capacitor is merely required to supply high frequency bypassing, since the impedance to the supply is very low. A 10F ceramic capacitor is usually enough for these conditions. Setting the Output Voltage The LTC3548A develops a 0.6V reference voltage between the feedback pin, VFB, and ground as shown in Figure 1. The output voltage is set by a resistive divider according to the following formula: R2 VOUT = 0.6V 1+ R1 Keeping the current small (<5A) in these resistors maximizes efficiency, but making them too small may allow stray capacitance to cause noise problems and reduce the phase margin of the error amp loop. To improve the frequency response, a feedforward capacitor, CF , may also be used. Great care should be taken to route the VFB line away from noise sources, such as the inductor or the SW line.
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LTC3548A APPLICATIONS INFORMATION
Power-On Reset The POR pin is an open-drain output which pulls low when either regulator is out of regulation. When both output voltages are within 8.5% of regulation, a timer is started which releases POR after 216 clock cycles (about 29ms in pulse-skipping mode). This delay can be significantly longer in Burst Mode operation with low load currents, since the clock cycles only occur during a burst and there could be milliseconds of time between bursts. This can be bypassed by tying the POR output to the MODE/SYNC input, to force pulse-skipping mode during a reset. In addition, if the output voltage faults during Burst Mode sleep, POR could have a slight delay for an undervoltage output condition and may not respond to an overvoltage output. This can be avoided by using pulse-skipping mode instead. When either channel is shut down, the POR output is pulled low, since one or both of the channels are not in regulation. Mode Selection and Frequency Synchronization The MODE/SYNC pin is a multipurpose pin which provides mode selection and frequency synchronization. Connecting this pin to VIN enables Burst Mode operation, which provides the best low current efficiency at the cost of a higher output voltage ripple. When this pin is connected to ground, pulse-skipping operation is selected which provides the lowest output ripple, at the cost of low current efficiency. The LTC3548A can also be synchronized to another LTC3548A by the MODE/SYNC pin. During synchronization, the mode is set to pulse-skipping and the top switch turn-on is synchronized to the rising edge of the external clock. 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 generating a feedback error signal used by the regulator to return VOUT to its steady-state value. During this recovery time, VOUT can be monitored for overshoot or ringing that would indicate a stability problem. The initial output voltage step may not be within the bandwidth of the feedback loop, so the standard secondorder overshoot/DC ratio cannot be used to determine phase margin. In addition, a feedforward capacitor can be added to improve the high frequency response, as shown in Figure 1. Capacitors C1 and C2 provide phase lead by creating high frequency zeros with R2 and R4 respectively, which improve the phase margin. The output voltage settling behavior is related to the stability of the closed-loop 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. In some applications, a more severe transient can be caused by switching in loads with large (>1F) input capacitors. The discharged input 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 switch connecting the load has low resistance and is driven quickly. The solution is to limit the turn-on speed of the load switch driver. A Hot SwapTM controller is designed specifically for this purpose and usually incorporates current limiting, short-circuit protection, and soft-starting. Soft-Start The RUN/SS pins provide a means to separately run or shut down the two regulators. In addition, they can optionally be used to externally control the rate at which each regulator starts up and shuts down. Pulling the RUN/SS1 pin below 1V shuts down regulator 1 on the LTC3548A. Forcing this pin to VIN enables regulator 1. In order to control the rate at which each regulator turns on and off, connect a resistor and capacitor to the RUN/SS pins as shown in Figure 1. The soft-start duration can be calculated by using the following formula: V -1 t SS = RSSCSSIn IN (s) VIN - 1.6
Hot Swap is a registered trademark of Linear Technology Corporation.
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LTC3548A APPLICATIONS INFORMATION
For approximately a 1ms ramp time, use RSS = 4.7M and CSS = 680pF at VIN = 3.3V. Efficiency Considerations The percent 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. Percent 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, 4 main sources usually account for most of the losses in LTC3548A circuits: 1. VIN quiescent current, 2. switching losses, 3. I2R losses, 4. other losses. 1. The VIN current is the DC supply current given in the Electrical Characteristics section which excludes MOSFET driver and control currents. VIN current results in a small (<0.1%) loss that increases with VIN, even at no load. 2. The switching current is the sum of the MOSFET driver and control currents. The MOSFET driver current results from switching the gate capacitance of the power MOSFETs. Each time a MOSFET gate is switched from low to high to low again, a packet of charge dQ moves from VIN to ground. The resulting dQ/dt is a current out of VIN that is typically much larger than the DC bias current. In continuous mode, IGATECHG = fO(QT + QB), where QT and QB are the gate charges of the internal top and bottom MOSFET switches. The gate charge losses are proportional to VIN and thus their effects will be more pronounced at higher supply voltages. 3. I2R losses are calculated from the DC resistances of the internal switches, RSW, and external inductor, RL. In continuous mode, the average output current flows through inductor L, but is chopped between the internal top and bottom switches. Thus, the series resistance looking into the SW pin is a function of both top and bottom MOSFET RDS(ON) and the duty cycle (D) as follows: RSW = (RDS(ON)TOP)(D) + (RDS(ON)BOT)(1 - D) The RDS(ON) for both the top and bottom MOSFETs can be obtained from the Typical Performance Characteristics curves. Thus, to obtain I2R losses: I2R losses = IOUT2(RSW + RL) 4. Other hidden losses such as copper trace and internal battery resistances can account for additional efficiency degradations in portable systems. It is very important to include these system level losses in the design of a system. The internal battery and fuse resistance losses can be minimized by making sure that CIN has adequate charge storage and very low ESR at the switching frequency. Other losses including diode conduction losses during dead time and inductor core losses generally account for less than 2% total additional loss. Thermal Considerations In a majority of applications, the LTC3548A does not dissipate much heat due to its high efficiency. However, in applications where the LTC3548A is running at high ambient temperature with low supply voltage and high duty cycles, such as in dropout, 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.
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LTC3548A APPLICATIONS INFORMATION
To prevent the LTC3548A 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: TRISE = PD * JA 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 = TRISE + TAMBIENT As an example, consider the case when the LTC3548A is in dropout on both channels at an input voltage of 2.7V with a load current of 400mA and 800mA and an ambient temperature of 70C. From the Typical Performance Characteristics graph of Switch Resistance, the RDS(ON) resistance of the main switch is 0.425. Therefore, power dissipated by each channel is: PD = I2 * RDS(ON) = 272mW and 68mW The MS package junction-to-ambient thermal resistance, JA, is 45C/W. Therefore, the junction temperature of the regulator operating in a 70C ambient temperature is approximately: TJ = (0.272 + 0.068) * 45 + 70 = 85.3C which is below the absolute maximum junction temperature of 125C. Design Example As a design example, consider using the LTC3548A in a portable application with a Li-Ion battery. The battery provides a VIN = 2.8V to 4.2V. The load requires a maximum of 800mA in active mode and 2mA in standby mode. The output voltage is VOUT = 2.5V. Since the load still needs power in standby, Burst Mode operation is selected for good low load efficiency. First, calculate the inductor value for about 30% ripple current at maximum VIN: L 2.5V 2.5V * 1- = 1.25H 2.25MHz * 360mA 4.2V
Choosing the next highest standardized inductor value of 2.2H, results in a maximum ripple current of: IL = 2.5V 2.5V * 1- = 204mA 2.25MHz * 2.2H 4.2V
For cost reasons, a ceramic capacitor will be used. COUT selection is then based on load step droop instead of ESR requirements. For a 5% output droop: COUT 2.5 800mA = 7.1F 2.25MHz * (5% * 2.5V)
The closest standard value is 10F Since the output imped. . ance of a Li-Ion battery is very low, CIN is typically 10F The output voltage can now be programmed by choosing the values of R1 and R2. To maintain high efficiency, the current in these resistors should be kept small. Choosing 2A with the 0.6V feedback voltage makes R1~300k. A close standard 1% resistor is 280k, and R2 is then 887k. The POR pin is a common drain output and requires a pullup resistor. A 100k resistor is used for adequate speed. Figure 3 shows the complete schematic for this design example. The specific passive components chosen allow for a 1mm height power supply that maintains a high efficiency across load.
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LTC3548A APPLICATIONS INFORMATION
Board Layout Considerations When laying out the printed circuit board, the following checklist should be used to ensure proper operation of the LTC3548A. These items are also illustrated graphically in the layout diagram of Figure 2. Check the following in your layout: 1. Does the capacitor CIN connect to the power VIN (Pin 3) and GND (Exposed Pad) as closely as possible? This capacitor provides the AC current to the internal power MOSFETs and their drivers. 2. Are COUT and L1 closely connected? The (-) plate of COUT returns current to GND and the (-) plate of CIN. 3. The resistor divider formed by R1 and R2 must be connected between the (+) plate of COUT and a ground sense line terminated near GND (Exposed Pad). The feedback signals VFB1 and VFB2 should be routed away from noisy components and traces, such as the SW lines (Pins 4 and 7), and their traces should be minimized.
VIN CIN L2 VOUT2 C5 RUN/SS2 VIN RUN/SS1 MODE/SYNC SW2 LTC3548A POR SW1 C4 L1 VOUT1
4. Keep sensitive components away from the SW pins. The input capacitor, CIN, and the resistors R1 to R4 should be routed away from the SW traces and the inductors. 5. A ground plane is preferred, but if not available keep the signal and power grounds segregated with smallsignal components returning to the GND pin at one point. Additionally the two grounds should not share the high current paths of CIN or COUT . 6. Flood all unused areas on all layers with copper. Flooding with copper will reduce the temperature rise of power components. These copper areas should be connected to VIN or GND.
R4 COUT2 R3
VFB2 GND
VFB1 R2 R1 COUT1
3548A F02
BOLD LINES INDICATE HIGH CURRENT PATHS
Figure 2. LTC3548A Layout Diagram (See Board Layout Checklist)
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LTC3548A TYPICAL APPLICATIONS
VIN 2.5V TO 5.5V C1 10F L2 4.7H SW2 C5 22pF LTC3548A SW1 C4 22pF RUN/SS2 VIN RUN/SS1 POR L1 2.2H R5 100k POWER-ON RESET VOUT1 1.8V 800mA MODE/SYNC VOUT2 2.5V 400mA
C3 4.7F
R4 887k
VFB2 R3 280k GND
VFB1 R1 301k R2 604k C2 10F
3548 F03a
C1, C2, C3: TAIYO YUDEN JMK316BJ106MD L1: TDK VLF3010AT-2R2M1R0-1 L2: TDK VLF3010AT-4R7MR70-1
Figure 3. 1mm Height Core Supply
Efficiency vs Load Current
100 90 80 EFFICIENCY (%) 70 60 50 40 30 20 10 0 1 VIN = 3.3V Burst Mode OPERATION NO LOAD ON OTHER CHANNEL 10 100 LOAD CURRENT (mA) 1000
3548A F03b
2.5V 1.8V
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LTC3548A TYPICAL APPLICATIONS
VIN = 2.5V TO 5.5V R6 4.7M CIN 10F VIN POR RUN/SS2 C4 680pF V OUT2 2.5V 400mA L2 10H SW2 C2 22pF RUN/SS1 L1 4.7H SW1 C1 22pF C3 VOUT1 680pF 1.2V 800mA R7 100k R5 4.7M POWER-ON RESET
LTC3548A
COUT2 10F
R4 887k
VFB2 R3 280k MODE/SYNC
VFB1 GND R2 R1 604k 604k COUT1 10F
3548A F04a
CIN, COUT1, COUT2: TAIYO YUDEN JMK316BJ106ML
L1: TDK VLF3012ST-4R7MR91 L2: TDK VLF3012ST-100MR59
Figure 4. Low Ripple Buck Regulators with Soft-Start
Efficiency vs Load Current
100 90 80 EFFICIENCY (%) 70 60 50 40 30 20 10 0 1 10 100 LOAD CURRENT (mA) 1000
3548A F04b
2.5V 1.2V
VIN = 3.3V Burst Mode OPERATION NO LOAD ON OTHER CHANNEL
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17
LTC3548A PACKAGE DESCRIPTION
DD Package 10-Lead Plastic DFN (3mm x 3mm)
(Reference LTC DWG # 05-08-1699 Rev B)
0.70 0.05
3.55 0.05 1.65 0.05 2.15 0.05 (2 SIDES) PACKAGE OUTLINE 0.25 0.05 0.50 BSC 2.38 0.05 (2 SIDES) RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS R = 0.125 TYP 6 0.40 10 0.10
3.00 0.10 (4 SIDES) PIN 1 TOP MARK (SEE NOTE 6)
1.65 0.10 (2 SIDES)
(DD) DFN REV B 0309
5 0.200 REF 0.75 0.05 2.38 0.10 (2 SIDES)
1 0.25 0.05 0.50 BSC
0.00 - 0.05
BOTTOM VIEW--EXPOSED PAD NOTE: 1. DRAWING TO BE MADE A JEDEC PACKAGE OUTLINE M0-229 VARIATION OF (WEED-2). CHECK THE LTC WEBSITE DATA SHEET FOR CURRENT STATUS OF VARIATION ASSIGNMENT 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 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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LTC3548A PACKAGE DESCRIPTION
MSE Package 10-Lead Plastic MSOP Exposed Die Pad ,
(Reference LTC DWG # 05-08-1664 Rev C)
BOTTOM VIEW OF EXPOSED PAD OPTION
2.794 (.110
0.102 .004)
0.889 (.035
0.127 .005)
1
2.06 0.102 (.081 .004) 1.83 0.102 (.072 .004)
0.29 REF
5.23 (.206) MIN
2.083 (.082
0.102 3.20 - 3.45 .004) (.126 - .136)
0.05 REF DETAIL "B" CORNER TAIL IS PART OF DETAIL "B" THE LEADFRAME FEATURE. FOR REFERENCE ONLY NO MEASUREMENT PURPOSE 0.497 0.076 (.0196 .003) REF
10
0.50 0.305 0.038 (.0197) (.0120 .0015) BSC TYP RECOMMENDED SOLDER PAD LAYOUT
3.00 0.102 (.118 .004) (NOTE 3)
10 9 8 7 6
4.90 0.152 (.193 .006) 0.254 (.010) GAUGE PLANE 0.53 0.152 (.021 .006) DETAIL "A" 0.18 (.007) SEATING PLANE 1.10 (.043) MAX DETAIL "A" 0 - 6 TYP 12345
3.00 0.102 (.118 .004) (NOTE 4)
0.86 (.034) REF
NOTE: 1. DIMENSIONS IN MILLIMETER/(INCH) 2. DRAWING NOT TO SCALE 3. DIMENSION DOES NOT INCLUDE MOLD FLASH, PROTRUSIONS OR GATE BURRS. MOLD FLASH, PROTRUSIONS OR GATE BURRS SHALL NOT EXCEED 0.152mm (.006") PER SIDE 4. DIMENSION DOES NOT INCLUDE INTERLEAD FLASH OR PROTRUSIONS. INTERLEAD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.152mm (.006") PER SIDE 5. LEAD COPLANARITY (BOTTOM OF LEADS AFTER FORMING) SHALL BE 0.102mm (.004") MAX
0.17 - 0.27 (.007 - .011) TYP
0.50 (.0197) BSC
0.1016 (.004
0.0508 .002)
MSOP (MSE) 0908 REV C
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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.
19
LTC3548A TYPICAL APPLICATION
2mm Height Lithium-Ion Single Inductor Buck-Boost Regulator and a Buck Regulator
VIN 2.8V TO 4.2V C1 10F RUN/SS2 VIN RUN/SS1 MODE/SYNC VOUT2 3.3V 200mA D1 L2 15H SW2 C6 22F M1 VFB2 R3 196k GND VFB1 R2 R1 442k 887k C2 10F
3548A TA03a
R5 100k POWER-ON RESET L1 2.2H C4 22pF
POR
LTC3548A
+
SW1
VOUT1 1.8V 800mA
C3 4.7F
R4 887k
C1, C2, C3: TAIYO YUDEN JMK316BJ106ML L1: TDK VLF3012ST-2R2M1R4 L2: TDK VLF3012ST-150MR49 C6: SANYO 6TPB47M M1: SILICONIX Si2302 D1: B0540W SCHOTTKY DIODE
Efficiency vs Load Current
80 4.2V 70 60 EFFICIENCY (%) 50 40 30 20 10 0 1 VOUT = 3.3V Burst Mode OPERATION NO LOAD ON OTHER CHANNEL 10 100 LOAD CURRENT (mA) 1000
3548A TA03b
Efficiency vs Load Current
100 90 80 EFFICIENCY (%) 70 60 50 40 30 20 10 0 1 10 100 LOAD CURRENT (mA) 1000
3548A TA03c
2.8V 4.2V 3.6V
2.8V
3.6V
VOUT = 1.8V Burst Mode OPERATION NO LOAD ON OTHER CHANNEL
RELATED PARTS
PART NUMBER LTC3405/LTC3405A LTC3406/LTC3406B LTC3407/LTC3407-2 LTC3407-3/LTC3407-4/ LTC3407A/LTC3407A-2 LTC3410/LTC3410B LTC3411 LTC3412/LTC3412A LTC3414 LTC3440/LTC3441 LTC3548/ LTC3548-1/LTC3548-2 DESCRIPTION 300mA (IOUT), 1.5MHz, Synchronous Step-Down DC/DC Converter 600mA (IOUT), 1.5MHz, Synchronous Step-Down DC/DC Converter 600mA/800mA (IOUT), 1.5MHz/2.25MHz, Dual Synchronous Step-Down DC/DC Converter 300mA (IOUT), 2.25MHz, Synchronous Step-Down DC/DC Converter in SC70 1.25A (IOUT), 4MHz, Synchronous Step-Down DC/DC Converter 2.5A (IOUT), 4MHz, Synchronous Step-Down DC/DC Converter 4A (IOUT), 4MHz, Synchronous Step-Down DC/DC Converter 600mA/1.2A (IOUT), 2MHz/1MHz, Synchronous Buck-Boost DC/DC Converter 400mA/800mA (IOUT), 2.25MHz, Dual Synchronous Step-Down DC/DC Converter COMMENTS 96% Efficiency, VIN: 2.5V to 5.5V, VOUT(MIN) = 0.8V, IQ = 20A, ISD <1A, ThinSOT Package 96% Efficiency, VIN: 2.5V to 5.5V, VOUT(MIN) = 0.6V, IQ = 20A, ISD <1A, ThinSOT Package 96% Efficiency, VIN: 2.5V to 5.5V, VOUT(MIN) = 0.6V, IQ = 40A, ISD <1A, MS10E Package, DFN Package 96% Efficiency, VIN: 2.5V to 5.5V, VOUT(MIN) = 0.8V, IQ = 26A, ISD <1A, SC70 Package 95% Efficiency, VIN: 2.5V to 5.5V, VOUT(MIN) = 0.8V, IQ = 60A, ISD <1A, MSOP-10 Package 95% Efficiency, VIN: 2.5V to 5.5V, VOUT(MIN) = 0.8V, IQ = 60A, ISD <1A, TSSOP-16E Package 95% Efficiency, VIN: 2.25V to 5.5V, VOUT(MIN) = 0.8V, IQ = 64A, ISD <1A, TSSOP-28E Package 95% Efficiency, VIN: 2.5V to 5.5V, VOUT(MIN) = 2.5V, IQ = 25A, ISD <1A, MSOP-10 Package/DFN Package 95% Efficiency, VIN: 2.5V to 5.5V, VOUT(MIN) = 0.6V, IQ = 40A, ISD <1A, MS10E Package/DFN Package
3548af LT 0809 * PRINTED IN USA
20 Linear Technology Corporation
(408) 432-1900 FAX: (408) 434-0507
1630 McCarthy Blvd., Milpitas, CA 95035-7417
www.linear.com
(c) LINEAR TECHNOLOGY CORPORATION 2009

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