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LTC3523/LTC3523-2 Synchronous 600mA Step-Up and 400mA Step-Down DC/DC Converters FEATURES
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DESCRIPTION
The LTC(R)3523/LTC3523-2 combine a 600mA step-up DC/DC converter with a 400mA synchronous step-down DC/DC converter in a tiny 3mm x 3mm package. The 1.2MHz/2.4MHz switching frequencies minimize the solution footprint while maintaining high efficiency. Both converters feature soft-start and internal compensation, simplifying the design. Both the step-up and step-down converters are current mode controlled and utilize an internal synchronous rectifier for high efficiency. The step-up supports 0% duty cycle operation and the step-down converter supports 100% duty cycle operation to extend battery run time. If the MODE pin is held high, both converters automatically transition between Burst Mode operation and PWM operation improving light load efficiency. Fixed, low noise 1.2MHz/2.4MHz PWM operation is selected when MODE is grounded. The LTC3523/LTC3523-2 provide a sub-3A shutdown mode, overtemperature shutdown and current limit protection on both converters. The LTC3523/LTC3523-2 are housed in a 16-lead 3mm x 3mm x 0.75mm QFN package.
L, LT, LTC, LTM and Burst Mode are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners.
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Dual High Efficiency DC/DC Converters: Step-Up (VOUT = 1.8V to 5.25V, ISW = 600mA) Step-Down (VOUT = 0.615V to 5.5V, IOUT = 400mA) 1.8V to 5.5V Input Voltage Range Up to 94% Efficiency Pin Selectable Burst Mode(R) Operation 45A Quiescent Current in Burst Mode Operation 1.2MHz (LTC3523) or 2.4MHz (LTC3523-2) Switching Frequency Independent Power Good Indicator Outputs Integrated Soft-Start Thermal and Overcurrent Protection <3A Quiescent Current in Shutdown Small 16-Lead 3mm x 3mm x 0.75mm QFN Package
APPLICATIONS
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Digital Cameras Medical Instruments Industrial Handhelds GPS Navigators
TYPICAL APPLICATION
LTC3523 Efficiency and Power Loss vs Load Current
VIN 1.8V TO 3.2V 2-CELL ALKALINE
+
47F 4.7H VIN1 VIN2 VBAT SW2 10pF VOUT 10F 634k 365k FB1 PGOOD1 SHDN1 LTC3523 FB2 MODE PGOOD2 SHDN2 VIN 511k 511k 4.7H SW1 VOUT2 STEP-DOWN OUTPUT 10F 1.2V 200mA
100 90 80 EFFICIENCY (%) 70 60 50 40 30 20 10 OFF ON OFF ON
3523 TA01a
1000
EFFICIENCY
100 POWER LOSS (mW)
VOUT1 STEP-UP OUTPUT 3.3V 200mA
P0WER LOSS VIN = 2.4V VOUT1 = 3.3V VOUT2 = 1.2V fOSC = 1.2MHz STEP-UP STEP-DOWN 1 10 100 LOAD CURRENT (mA)
10
GND1 GND2 GND3
1
0 0.1
0.1 1000
3523 TA01b
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LTC3523/LTC3523-2 ABSOLUTE MAXIMUM RATINGS
(Note 1)
PIN CONFIGURATION
TOP VIEW SHDN1 SHDN2 12 FB2 17 11 PGOOD2 10 MODE 9 5 SW1 6 GND1 7 GND2 8 SW2 VIN2 GND3 VBAT
VIN1, VIN2, VBAT, VOUT Voltages .................... -0.3V to 6V SHDN1, PGOOD1, PGOOD2, FB1 Voltages .. -0.3V to 6V SHDN2, FB2, MODE Voltages ...... -0.3V to (VIN2 + 0.3V) SW1 Voltage DC.............................................................. 0.3V to 6V Pulse < 100ns .......................................... -0.3V to 7V SW2 Voltage Pulse < 100ns ......... -0.3V to (VIN2 + 0.3V) Operating Temperature Range (Notes 2, 3) .............................................. -40C to 85C Storage Temperature Range................... -65C to 125C
16 15 14 13 FB1 1 VIN1 2 PG00D1 3 VOUT 4
UD PACKAGE 16-LEAD (3mm 3mm) PLASTIC QFN TJMAX = 125C, JA = 68C/W EXPOSED PAD (PIN 17) IS GND, MUST BE SOLDERED TO PCB
ORDER INFORMATION
LEAD FREE FINISH LTC3523EUD#PBF LTC3523EUD-2#PBF TAPE AND REEL LTC3523EUD#TRPBF LTC3523EUD-2#TRPBF PART MARKING LCYC LDDR PACKAGE DESCRIPTION 16-Lead (3mm x 3mm) Plastic DFN 16-Lead (3mm x 3mm) Plastic DFN TEMPERATURE RANGE -40C to 85C -40C to 85C
Consult LTC Marketing for parts specified with wider operating temperature ranges. Consult LTC Marketing for information on non-standard lead based finish parts. 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/
ELECTRICAL CHARACTERISTICS
PARAMETER Minimum Start-Up Voltage Frequency Quiescent Current-Shutdown Quiescent Current -Sleep Quiescent Current VOUT - Sleep SHDN1, SHDN2 Input High SHDN1, SHDN2 Input Low SHDN1, SHDN2 Input Current PGOOD1, PGOOD2 Threshold PGOOD1, PGOOD2 Low Voltage PGOOD1, PGOOD2 Leakage MODE Input High MODE Input Low LTC3523 LTC3523-2
The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. VIN1 = VIN2 = VBAT = 2.4V, VOUT = 3.3V, unless otherwise specified.
CONDITIONS
l l l
MIN 0.9 1.8
TYP 1.6 1.2 2.4 0.5 45 15
MAX 1.8 1.5 2.65 3
UNITS V MHz MHz A A A V
VSHDN1 = VSHDN2 = 0V, VOUT = 0V, VIN1 = VIN2 = VBAT Measured from VSUPPLY, VIN1 = VIN2 = VBAT = 2.4V Measured from VOUT = 3.3V (Note 4) 1
0.35 VSHDN = 5.5V Referenced to the Feedback Voltage IPGOOD = 1mA VPGOOD = 5.25V 1.0 0.35 -6 1.4 -9 0.35 0.01 1 2 -14
V A % V A V V
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LTC3523/LTC3523-2 ELECTRICAL CHARACTERISTICS
PARAMETER MODE Leakage Current Soft-Start Time Step-Up Converter Input Voltage Range Output Voltage Adjust Range Feedback Voltage FB1 Feedback Input Current FB1 N-Channel Switch Leakage P-Channel Switch Leakage N-Channel Switch On Resistance P-Channel Switch On Resistance Peak Inductor Current Current Limit Delay to Output Maximum Duty Cycle Minimum Duty Cycle Step-Down Converter Input Voltage Range Output Voltage Range Feedback Voltage FB2 Feedback Input Current FB2 Reference Voltage Line Regulation Output Voltage Line Regulation Output Voltage Load Regulation Maximum Duty Cycle Peak Inductor Current N-Channel Switch On Resistance P-Channel Switch On Resistance SW Leakage (Note 7) VIN2 = 2.4V VIN2 = 2.4V VSHDN2 = 0V, VSW2 = 0V or 5V, VIN2 = 5.5V
l l l
The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. VIN1 = VIN2 = VBAT = 2.4V, VOUT = 3.3V, unless otherwise specified.
CONDITIONS VMODE = 5.5V MIN TYP 0.01 500 1.8 1.8 1.16 1.20 0 0.20 0.20 0.36 0.22 0.33 0.31
l l l
MAX 1
UNITS A s
5.25 5.25 1.23 50 2 2
V V V nA A A mA ns %
(Note 6) VFB1 = 1.25V VSW = 5.5V VSW = 5.5V, VOUT = 0V VOUT = 3.3V VOUT = 5V VOUT = 3.3V, ISW = 100mA VOUT = 5V, ISW = 100mA (Note 7) (Note 6) VFB = 1V VFB = 1.5V
l l
600 80
1000 40 87 0
% V V mV nA %/V %/V % % mA
1.8 0.615 585 600 0 0.04 0.04 1.0 100 400 650 0.33 0.58 0.20
5.5 5.5 615 50
(Note 6) VFB2 = 0.625V IOUT = 100mA (Notes 5, 6) IOUT = 100mA, 1.6V < VIN < 5.5V (Note 6) IOUT = 0mA to 600mA (Note 6)
l l
2
A
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 LTC3523/LTC3523-2 are guaranteed to meet performance specifications from 0C to 85C. Specifications over the -40C to 85C operating temperature range are assured by design, characterization and correlation with statistical process control. Note 3: The LTC3523/LTC3523-2 include an overtemperature shutdown that is intended to protect the device during momentary overload conditions. Junction temperature will exceed 125C when the overtemperature shutdown is active. Continuous operation above the specified maximum operating junction temperature may impair device reliability.
Note 4: Current is measured into the VOUT pin since the supply is bootstrapped to the output for the step-up. The current will reflect to the input supply by: (VOUT/VIN) * Efficiency. The outputs are not switching in sleep. Note 5: The LTC3523/LTC3523-2 are tested in a propriety test mode that connects FB2 to the output of the error amplifier. Note 6: Specification is guaranteed by design and not 100% tested in production. Note 7: Current measurements are performed when the LTC3523/ LTC3523-2 are not switching. The current limit values in operation will be somewhat higher due to the propagation delay of the comparator.
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LTC3523/LTC3523-2 TYPICAL PERFORMANCE CHARACTERISTICS (TA = 25C unless otherwise noted)
Normalized FBx Reference vs Temperature
1.00125 NORMALIZED FBx VOLTAGE (V) NORMALIZED FREQUENCY (Hz) 1.05 VOUT_BST 2V/DIV IL_BST 200mA/DIV 1.00 SHDN1 2V/DIV VOUT = 3.3V VIN = 2.4V COUT = 10F L1 = 4.7H 0.95 -45 200s/DIV
3523 G03
Normalized Oscillator Frequency vs Temperature
Inrush Current Control for the Step-Up Converter
1.00000
0.99875
0.99750
0.99625
0.99500 -45
-25
15 35 55 -5 TEMPERATURE (C)
75
3523 G01
-25
15 35 55 -5 TEMPERATURE (C)
75
3523 G02
Inrush Current Control for the Step-Down Converter
VOUT_BCK 1V/DIV IL_BCK 200mA/DIV SHDN2 2V/DIV VOUT = 1.2V VIN = 2.4V COUT = 10F L1 = 4.7H 200s/DIV
3523 G04
Load Transient Response Step-Up
OUTPUT RIPPLE 20mV/DIV LOAD CURRENT 20mA/DIV OUTPUT RIPPLE 20mV/DIV
Load Transient Response Step-Down
LOAD CURRENT 20mA/DIV VOUT = 3.3V 500s/DIV VIN = 2.4V COUT = 10F L1 = 4.7H 20mA TO 70mA STEP
3523 G05
VOUT = 1.2V 500s/DIV VIN = 2.4V COUT = 47F L1 = 4.7H CF = 68pF 10mA TO 30mA STEP
3523 G06
Current Limit vs Temperature
1.2 1.0 CURRENT LIMIT (A) 0.8 0.6 0.4 0.2 0 -45 BUCK CURRENT LIMIT BOOST CURRENT LIMIT 0.80 0.70 0.60 RDS(ON) () 0.50 0.40 0.30 0.20 0.10 -25 55 35 -5 15 TEMPERATURE (C) 75
3523 G07
RDS(ON) vs Input Voltage for the Step-Down Converter
0.50 0.45 0.40 PMOS RDS(0N) () 0.35 0.30 0.25 0.20 0.15 0.10 0.05 1 1.5 2 2.5 3 3.5 4 INPUT VOLTAGE (V) 4.5 5 0
RDS(ON) vs Output Voltage for the Step-Up Converter
PMOS
NMOS
NMOS
0
1
1.5
2
2.5 3 3.5 4 OUTPUT VOLTAGE (V)
4.5
5
3523 G08
3532 G09
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LTC3523/LTC3523-2 TYPICAL PERFORMANCE CHARACTERISTICS (TA = 25C unless otherwise noted)
Normalized RDS(ON) vs Temperature
1.3 1.2 NORMALIZED RDS(ON) () INPUT CURRENT (A) 1.1 1.0 0.9 NMOS 0.8 0.7 0.6 0.5 -45 -25 -5 55 35 15 TEMPERATURE (C) 75
3523 G10
Step-Up No-Load Input Current vs VIN
500 450 400 350 300 250 200 150 100 50 0 1.5 2 VOUT = 2.8V 3.5 3 2.5 4 INPUT VOLTAGE (V) 4.5 5 VOUT = 3.3V MODE 2V/DIV VOUT = 5V VOUT_BST 50mV/DIV VOUT_BCK 20mV/DIV
Mode Transition Response
PMOS
VOUT1 = 3.3V 200s/DIV VOUT2 = 1.2V VIN = 2.4V IOUT1 = 20mA IOUT2 = 25mA COUT1 = COUT2 = 10F L1 = L2 = 4.7H
3523 G12
3523 G11
Maximum IOUT vs VIN for the Step-Up Converter
500 MAXIMUM OUTPUT CURRENT (mA) 450 400 350 300 250 200 150 100 50 0 1 2 3 4 INPUT VOLTAGE (V) 5
3523 G13
Maximum IOUT vs VIN for the Step-Down Converter
450 MAXIMUM OUTPUT CURRENT (mA) 400 350 300 250 200 150 100 50 0 1 1.5 2 2.5 3 3.5 4 INPUT VOLTAGE (V) 4.5 5 VOUT = 1.8V VOUT = 2.5V VOUT = 1.2V
VOUT = 3.3V VOUT = 2.5V
VOUT = 5V
3523 G14
PIN FUNCTIONS
FB1 (Pin 1): Step-Up Converter Feedback Input to the Error Amplifier. Connect resistor divider tap to this pin. The output voltage can be adjusted from 1.8V to 5.25V by: R1 VOUT(STEP-UP) = 1.2V * 1+ R2 See Block Diagram. VIN1 (Pin 2): Step-Up Converter Power Voltage Input. This pin can be connected to a different supply than VIN2. This pin must be connected to a valid supply voltage. PGOOD1 (Pin 3): Step-Up Converter Power Good Comparator Output. This open-drain output is pulled low when VFB1 < -9% of its regulation voltage. VOUT (Pin 4): Step-Up Converter Output Voltage Sense Input and Drain of the Internal Synchronous Rectifier MOSFET. Driver bias is derived from VOUT. PCB trace length from VOUT to the output filter capacitor(s) should be as short and wide as possible.
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LTC3523/LTC3523-2 PIN FUNCTIONS
SW1 (Pin 5): Step-Up Converter Switch Pin. Connect the inductor between SW1 and VIN1. Keep these PCB trace lengths as short and wide as possible to reduce EMI and voltage overshoot. If the inductor current falls to zero or SHDN1 is low, an internal 150 anti-ringing resistor is connected from SW1 to VIN1 to minimize EMI. GND1 (Pin 6): Step-Up Converter Power Ground. Connect this pin to the ground plane. GND2 (Pin 7): Step-Down Converter Power Ground. Connect this pin to the ground plane. SW2 (Pin 8): Step-Down Converter Switch Pin. Connect one end of the inductor to SW2. Keep these PCB trace lengths as short and wide as possible to reduce EMI and voltage overshoot. VIN2 (Pin 9): Step-Down Converter Power Voltage Input. This pin can be connected to a different supply than VIN1. This pin must be connected to a valid supply voltage. MODE (Pin 10): Step-Up and Step-Down Converter Mode Selection Pin. Do not leave this pin floating. * MODE = Low: PWM mode * MODE = High: Automatic Burst Mode operation PGOOD2 (Pin 11): Step-Down Converter Power Good Comparator Output. This open-drain output is pulled low when VFB2 < -9% of its regulation voltage. FB2 (Pin 12): Step-Down Converter Feedback Input to the Error amplifier. Connect resistor divider tap to this pin. The output voltage can be adjusted from 0.6V to 5.5V by: R3 VOUT(STEP-DOWN) = 0.6 V * 1+ R4 See Block Diagram. If large feedback resistors, above 500k are used, then it will be necessary to use a lead capacitor connected to the output voltage and FB2. SHDN2 (Pin 13): Step-Down Converter Logic Controlled Shutdown Input. Do not leave this pin floating. * SHDN2 = High: Normal free-running operation, 1.2MHz/2.4MHz typical operating frequency. * SHDN2 = Low: Shutdown, quiescent current < 1A. This pin cannot exceed the voltage on VIN2. GND3 (Pin 14): Analog Ground. The feedback voltage dividers for each converter must be returned to GND3 for best performance. Note: When laying out your PCB provide a short direct path between GND1 and the (-) side of the step-up output capacitor(s) and GND2 and the step-down output capacitor.These pins are not connected together internally. VBAT (Pin 15): Analog Voltage Input. Connect this pin to the higher of VIN1 or VIN2. SHDN1 (Pin 16): Step-Up Converter Logic Controlled Shutdown Input. * SHDN1 = High: Normal free-running operation, 1.2MHz/2.4MHz typical operating frequency. * SHDN1 = Low: Shutdown, quiescent current < 1A. This pin cannot exceed the voltage on VIN1. Exposed Pad (Pin 17): Die attach pad must be soldered to PCB ground for electrical contact and optimum thermal performance.
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LTC3523/LTC3523-2 BLOCK DIAGRAM
VIN 1.8V TO 5.5V L1 4.7H
+
CIN 47F 16 SHDN1 2 VIN1 5 SW1 BULK CONTROL SIGNALS
SHUTDOWN AND VBIAS
ANTI-RING SHDN MODE OSC PWM LOGIC AND DRIVERS CURRENT SENSE
VOUT
4
VOUT STEP-UP 1.8V TO 5.25V
+-
IZERO COMP
SLOPE COMPENSATION 3 PGOOD1
- +
OSCILLATOR 15 VBAT
9
VIN2 SLOPE COMPENSATION
SLP ILIM REF PGOOD2
11
- +
13
SHDN2
SHUTDOWN AND VBIAS
MODE SHDN
MODE
CC1 RZ
gm ERROR AMPLIFIER
START-UP SOFT-START AND THERM REG GND2 7 GND3 14
3523 BD
GND1 6
+ -
-+- +
ILIM REF
PWM/ILIM COMP
MODE
MODE
R1 FB1 1.2V 1 R2 COUT 10F
+
CC1 RZ CC2
gm ERROR AMPLIFIER
+ -
FB1 1.2V -9%
START-UP SOFT-START AND THERM REG
SLP STEP-UP OSC SLP 0.6V 1.2V 1V
REFERENCE
MODE 10
THERMAL SHDN SHARED STEP-DOWN
+
+
ZERO CURRENT COMP PWM/ILIM COMP
+ -
0A PWM LOGIC AND DRIVERS VOUT LIMIT COMP L2 4.7H 8
+ - -
OSC
MODE
SW2
VOUT STEP-DOWN 0.615V TO 5.5V
FB2 0.6V -9% GND2 0.66V
R3 FB2 0.6V 12 R4 COUT 10F
- +
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LTC3523/LTC3523-2 OPERATION
The LTC3523 and LTC3523-2 are synchronous step-up and step-down converters housed in a 16-pin QFN package. Operating from inputs down to 1.8V, the devices feature fixed frequency, current mode PWM control for exceptional line and load regulation and transient response. With low RDS(ON) and internal MOSFET switches, the devices maintain high efficiency over a wide range of load current. Operation can be best understood by referring to the Block Diagram. Soft-Start Both the step-up and step-down converters on the LTC3523 /LTC3523-2 provide soft-start. The soft-start time is typically 500s. The soft-start function resets in the event of a commanded shutdown or thermal shutdown. Oscillator The frequency of operation is set by an internal oscillator to a nominal 1.2MHz for the LTC3523 and nominal 2.4MHz for the LTC3523-2. The oscillator is shared by both converters. Shutdown The step-up and the step-down converters have independent shutdown pins. To shut down a converter, pull SHDNx below 0.35V. To enable a converter, pull SHDNx above 1.0V. Error Amplifiers Power converter control loop compensation is provided internally for each converter. The noninverting input is internally connected to the 1.2V reference for the step-up and 0.6V for the step-down. The inverting input is connected to the respective FBx for both converters. Internal clamps limit the minimum and maximum error amp output voltage for improved large signal transient response. A voltage divider from VOUT to ground programs the output voltage via the respective FBx pins from 1.8V to 5.25V for the stepup and 0.615V to 5.5V for the step-down. From the Block Diagram the design equation for programming the output voltages is VOUT = 1.2V * [1 + (R1/R2)] for the step-up and VOUT = 0.6V * [1 + (R3/R4)] for the step-down. PWM Comparators The PWM comparators are used to compare the converters external inductor current to the current commanded by the error amplifiers. When the inductor current reaches the current commanded by the error amplifier the inductor charging cycle is terminated and the rectification cycle commences. Current Limit The current limit comparator shuts off the N-channel switch for the step-up and P-channel switch for the step-down once its current limit threshold is reached. The current limit comparator delay to output is typically 40ns. Peak switch current is limited to approximately 1000mA for the step-up and 650mA for the step-down independent of input or output voltage. Zero Current Comparator The zero current comparator monitors the inductor current to the output and shuts off the synchronous rectifier once this current reduces to approximately 20mA. This prevents the inductor current from reversing in polarity improving efficiency at light loads. Power Good Comparator Both converters have independent open drain power good comparators which monitor the output voltage via their respective FBx pins. The comparator output will allow the PGOODx to be pulled up high when the output voltage (VOUT) has exceeded 91% of it final value. If the output voltage decreases below 91%, the comparator will pull the PGOODx pin to ground. The step-up comparator has 3.3% of hysteresis and the step-down has 6.6% relative to FBx voltage for added noise immunity. Step-Down Overvoltage Comparator The step-down overvoltage comparator guards against transient overshoots greater than 10% of the output voltage by turning the P-channel switch off until the transient has subsided.
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LTC3523/LTC3523-2 OPERATION
Step-Up Anti-Ringing Control The anti-ring circuitry connects a resistor across the inductor to prevent high frequency ringing on the SW1 pin during discontinuous current mode operation. The ringing of the resonant circuit formed by L and CSW (capacitance on SW pin) is low energy, but can cause EMI radiation. Step-Up Output Disconnect The LTC3523/LTC3523-2 step-up is designed to provide true output disconnect by eliminating body diode conduction of the internal P-channel MOSFET rectifier. This allows for VOUT to go to zero volts during shutdown, drawing no current from the input source. Controlling the P-channel MOSFET body diode also enables inrush current limiting at turn-on, minimizing surge currents seen by the input supply. Note that to obtain the advantages of output disconnect, an external Schottky diode cannot be connected between SW1 and VOUT. Thermal Shutdown If the die temperature reaches 160C, the part will go into thermal shutdown. All switches will be turned off and the soft-start capacitor will be discharged. The device will be enabled again when the die temperature drops by approximately 15C.
APPLICATIONS INFORMATION
PCB LAYOUT GUIDELINES The high speed operation of the LTC3523/LTC3523-2 demands careful attention to board layout. You will not get advertised performance with careless layout. Figure 1 shows the recommended component placement. A large ground pin copper area will help to lower the chip temperature. A multilayer board with a separate ground plane is ideal, but not absolutely necessary. COMPONENT SELECTION Inductor Selection The LTC3523/LTC3523-2 can utilize small surface mount and chip inductors due to its fast 1.2MHz switching frequency and for the 2.4MHz version, the values are halved. The Inductor current ripple is typically set for 20% to 40% of the peak inductor current (IP). High
Figure 1. Recommended Component Placement for Double Layer Board
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LTC3523/LTC3523-2 APPLICATIONS INFORMATION
frequency ferrite core inductor materials reduce frequency dependent power losses compared to cheaper powdered iron types, improving efficiency. The inductor should have low ESR (series resistance of the windings) to reduce the I2R power losses, and must be able to handle the peak inductor current without saturating. Molded chokes and some chip inductors usually do not have enough core to support the peak inductor currents of 1000mA seen on the LTC3523/LTC3523-2. To minimize radiated noise, use a toroid, pot core or shielded bobbin inductor. See Table 1 for suggested inductors and suppliers. Step-Up: For the step-up converter a minimum inductance value of 3.3H is recommended for 3.6V and lower output voltage applications, and a 4.7H for output voltages greater than 3.6V. Larger values of inductance will allow greater output current capability by reducing the inductor ripple current. Increasing the inductance above 10H will increase size while providing little improvement in output current capability. Step-Down: For most applications, the value of the inductor will fall in the range of 3.3H to 10H, depending upon the amount of current ripple desired. A reasonable point to start is to set the current ripple at 30% of the output current. Note that larger values of inductance will allow greater output current capability by reducing the inductor ripple
Table 1. Recommended Inductors
PART ME3220 LPS3010 DO2010 SD3112 MIP3226D LQH32CN LQH2MC CDRH3D16 CDRH2D14 NR3010 NR3015 L (H) 4.7 to 15 4.7 to 10 4.7 to 15 4.7 to 15 4.7 to 10 4.7 to 15 4.7 to 15 4.7 to 15 4.7 to 12 4.7 to 15 4.7 to 15 MAXIMUM CURRENT (mA) 1200 to 700 720 to 510 800 to 510 740 to 405 600 to 200 650 to 300 300 to 200 900 to 450 680 to 420 750 to 400 1000 to 560 DCR () 0.19 to 0.52 0.3 to 0.54 0.8 to 1.84 0.25 to 0.65 0.1 to 0.16 0.15 to 0.58 0.8 to 1.6 0.11 to 0.29 0.12 to 0.32 0.19 to 0.74 0.12 to 0.36 DIMENSIONS (mm) (L x W x H) 3.2 x 2.5 x 2.0 3.0 x 3.0 x 1.0 2.0 x 2.0 x 1.0 3.1 x 3.1 x 1.2 3.2 x 2.6 x 1.0 3.2 x 2.5 x 1.5 2 x 1.6 x 0.9 3.8 x 3.8 x 1.8 3.2 x 3.2 x 1.5 3.0 x 3.0 x 1.0 3.0 x 3.0 x 1.5 Cooper www.cooperet.com FDK www.fdk.com Murata www.murata.com Sumida www.sumida.com Taiyo Yuden www.t-yuden.com
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current. Increasing the inductance above 10H will increase size while providing little improvement in output current capability. A 4.7H inductor will work well for most Li-Ion or 2-cell alkaline/NiMH cell applications Output and Input Capacitor Selection Low ESR (equivalent series resistance) capacitors should be used to minimize the output voltage ripple. Multilayer ceramic capacitors are an excellent choice as they have extremely low ESR and are available in small footprints. Step-Up: A 2.2F to 10F output capacitor is sufficient for most applications. Larger values up to 22F may be used to obtain extremely low output voltage ripple and improve transient response. An additional phase lead capacitor connected between VOUT and FB1 may be required with output capacitors larger than 10F to maintain acceptable phase margin. X5R and X7R dielectric materials are preferred for their ability to maintain capacitance over wide voltage and temperature ranges. Step-Down: Low ESR input capacitors reduce input switching noise and reduce the peak current drawn from the battery. It follows that ceramic capacitors are also a good choice for input decoupling and should be located as close as possible to the device. Table 2 shows the range of acceptable capacitors for a given programmed output voltage. Minimum capacitance values in the table
MANUFACTURER Coil Craft www.coilcraft.com
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LTC3523/LTC3523-2 APPLICATIONS INFORMATION
will increase loop bandwidth resulting in a faster transient response. Maximum capacitance values will produce lower ripple. Table 3 shows a list of several ceramic capacitor manufacturers. Consult the manufacturers directly for detailed information on their entire selection of ceramic parts.
Table 2. Step-Down Output Capacitor Range vs Programmed Output Voltage
VOUT 0.8 1.2 1.5 1.8 2.5 5 MINIMUM CAPACITANCE (F) MAXIMUM CAPACITANCE (F) 8.4 5.6 4.5 3.7 2.7 1.3 33.6 22.4 17.9 14.9 10.7 5.4
SHORT-CIRCUIT PROTECTION The LTC3523/LTC3523-2's step-up output disconnect feature allows output short circuit while maintaining a maximum internally set current limit. However, the LTC3523/LTC3523-2 also incorporate internal features such as current limit foldback and thermal shutdown for protection from an excessive overload or short circuit. During a prolonged short circuit of VOUT less than 950mV, the current limit folds back to 2/3 the normal current limit. This 2/3 current limit remains in effect until VOUT exceeds 1V, at which time the normal internal set current limit is restored. When the LTC3523/LTC3523-2 step-down converters output is shorted to ground, the step-down uses a comparator to limit the current through the synchronous rectifying N-channel switch to 650mA. If this limit is exceeded, the P-channel switch is inhibited from turning on until the current through the synchronous rectifying N-channel switch falls below 650mA. THERMAL CONSIDERATIONS To deliver the LTC3523/LTC3523-2's full-rated power, it is imperative that a good thermal path be provided to dissipate the heat generated within the package. This can be accomplished by taking advantage of the large thermal pad on the underside of the LTC3523/LTC3523-2. It is recommended that multiple vias in the printed circuit board be used to conduct heat away from the LTC3523/LTC3523-2 and into the copper plane with as much area as possible. In the event that the junction temperature gets too high, the LTC3523/LTC3523-2 will go into thermal shutdown and all switching will cease until the internal temperature drops to a safe level at which point the soft-start cycle will be initiated.
Table 3. Capacitor Vendor Information
SUPPLIER AVX Murata Taiyo-Yuden PHONE (803) 448-9411 (714) 852-2001 (408) 573-4150 WEBSITE www.avxcorp.com www.murata.com www.t-yuden.com
STEP-UP VIN > VOUT OPERATION The LTC3523/LTC3523-2 step-up converters will maintain voltage regulation when the input voltage is above the output voltage. Since this mode will dissipate more power, the maximum output current is limited in order to maintain an acceptable junction temperature and is given by: IOUT(MAX ) = 250 - TA T 136 * (VIN + 1.5) - VOUT
where TA = ambient temperature. For example, at VIN = 4.5V, VOUT = 3.3V and TA = 85C, the maximum output current is limited to 449mA.
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LTC3523/LTC3523-2 APPLICATIONS INFORMATION
DUAL BUCK-BOOST AND STEP-UP CONVERTER OPERATION The LTC3523/LTC3523-2 can be operated in a cascaded configuration as shown in Figure 2, allowing buck-boost and step-up converter operation. Supply rail sequencing is achieved by feeding the step-up converter PGOOD1
VIN 1.8V TO 5.25V 4.7F 10H VIN1 VOUT1 STEP-UP OUTPUT 5V 100mA SW1 VOUT 10F 768k 243k FB1 PGOOD1 SHDN1 VIN2 VBAT SW2 10pF LTC3523 FB2 MODE PGOOD2 SHDN2 VIN 182k 825k 10F 4.7H VOUT2 STEP-DOWN OUTPUT 3.3V 50mA
into the step-down's SHDN2 pin. Note that the overall 3.3V converter efficiency is the product of the individual efficiencies.
GND1 GND2 GND3
100k VIN OFF ON
3523 F02a
100 90 80 EFFICIENCY (%) 70 60 50 40 30 20 10 0 0.1 VIN = 2.4V VOUT1 = 5V VOUT2 = 3.3V fOSC = 1.2MHz BURST ENABLED 10 1 100 OUTPUT CURRENT (mA) 1000
3523 F02b
5V OUTPUT
3.3V OUTPUT
Figure 2. Dual Converter Efficiency (Load Applied to Step-Down Output)
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12
LTC3523/LTC3523-2 TYPICAL APPLICATIONS
Power Sequence Operation
VIN 1.8V TO 3.2V
2-CELL ALKALINE
+
4.7F 4.7H VIN1 VIN2 VBAT SW2 10pF VOUT 4.7F 634k 365k FB1 PGOOD1 SHDN1 LTC3523 FB2 MODE PGOOD2 SHDN2 511k 511k 10F 4.7H SW1 VOUT2 STEP-DOWN OUTPUT 1.2V 200mA
VOUT1 STEP-UP OUTPUT 3.3V 200mA
GND1 GND2 GND3
100k VIN OFF ON
3523 TA02a
VOUT1 2V/DIV PGOOD2 VOUT2 1V/DIV SHDN2 500s/DIV
3523 TA02b
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13
LTC3523/LTC3523-2 TYPICAL APPLICATIONS
Li-Ion to 5V/150mA, 2.5V/200mA
VIN 2.5V TO 4.2V Li-Ion
+
4.7F
10H VIN1 VIN2 VBAT SW2 10pF VOUT 768k 243k 10F FB2 MODE PGOOD2 SHDN2 VIN 4.7H SW1
VOUT1 STEP-UP OUTPUT 5V 150mA
VOUT2 STEP-DOWN OUTPUT 2.5V 200mA
10F
768k 243k
FB1 PGOOD1 SHDN1
LTC3523
GND1 GND2 GND3 OFF ON OFF ON
3523 TA03
Efficiency and Power Loss vs Load Current
100 90 80 EFFICIENCY (%) 70 60 50 40 30 20 10 0 0 P0WER LOSS VIN = 3.6V VOUT1 = 5V VOUT2 = 2.5V fOSC = 1.2MHz STEP-UP STEP-DOWN 10 1 100 LOAD CURRENT (mA) 0 1000
3523 TA03b
1000 EFFICIENCY 100 POWER LOSS (mW)
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1
14
LTC3523/LTC3523-2 PACKAGE DESCRIPTION
UD Package 16-Lead Plastic QFN (3mm x 3mm)
(Reference LTC DWG # 05-08-1691)
0.70 0.05
3.50
0.05 2.10
1.45 0.05 0.05 (4 SIDES)
PACKAGE OUTLINE 0.25 0.05 0.50 BSC RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS 0.75 0.05 BOTTOM VIEW--EXPOSED PAD R = 0.115 TYP 15 16 0.40 1 1.45 0.10 (4-SIDES) 2 0.10 PIN 1 NOTCH R = 0.20 TYP OR 0.25 45 CHAMFER
3.00 0.10 (4 SIDES) PIN 1 TOP MARK (NOTE 6)
(UD16) QFN 0904
0.200 REF 0.00 - 0.05 NOTE: 1. DRAWING CONFORMS TO JEDEC PACKAGE OUTLINE MO-220 VARIATION (WEED-2) 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
0.25
0.05
0.50 BSC
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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.
15
LTC3523/LTC3523-2 RELATED PARTS
PART NUMBER LTC3400/LTC3400B LTC3401 LTC3402 LTC3421 LTC3422 LTC3426 LTC3427 LTC3429/LTC3429B LTC3459 LTC3525-3 LTC3525-3.3 LTC3525-5 LTC3526/LTC3526L LTC3526B LTC3528/LTC3528B DESCRIPTION 600mA (ISW), 1.2MHz, Synchronous Step-Up DC/DC Converters 1A (ISW), 3MHz, Synchronous Step-Up DC/DC Converter 2A (ISW), 3MHz, Synchronous Step-Up DC/DC Converter 3A (ISW), 3MHz, Synchronous Step-Up DC/DC Converter with Output Disconnect Converter 1.5A (ISW), 3MHz, Synchronous Step-Up DC/DC with Output Disconnect Converter 2A (ISW), 1.5MHz, Step-Up DC/DC Converter 500mA (ISW), 1.25MHz, Synchronous Step-Up DC/DC with Output Disconnect Converter 600mA (ISW), 550kHz, Synchronous Step-Up DC/DC Converters Soft-Start/Output Disconnect 80mA (ISW), Synchronous Step-Up DC/DC Converter COMMENTS 92% Efficiency, VIN: 0.85V to 5V, VOUT(MAX) = 5V, IQ = 19A/300A, ISD < 1A, ThinSOTTM Package 97% Efficiency, VIN: 0.85V to 5V, VOUT(MAX) = 5.5V, IQ = 38A, ISD < 1A, 10-Pin MS Package 97% Efficiency, VIN: 0.85V to 5V, VOUT(MAX) = 5.5V, IQ = 38A, ISD < 1A, 10-Pin MS Package 94% Efficiency, VIN: 0.85V to 4.5V, VOUT(MAX) = 5.25V, IQ = 12A, ISD < 1A, 24-Pin (4mm x 4mm) QFN Package 94% Efficiency, VIN: 0.85V to 4.5V, VOUT(MAX) = 5.25V, IQ = 25A, ISD < 1A, 10-Pin (3mm x 3mm) DFN Package 92% Efficiency, VIN: 1.6V to 5.5V, VOUT(MAX) = 5V, IQ = 600A, ISD < 1A, ThinSOT Package 94% Efficiency, VIN: 1.8V to 5V, VOUT(MAX) = 5.25V, IQ = 350A, ISD < 1A, 6-Pin (2mm x 2mm) DFN Package 96% Efficiency, VIN: 0.85V to 4.3V, VOUT(MAX) = 5V, IQ = 20A, ISD < 1A, ThinSOT Package 92% Efficiency, VIN: 1.5V to 5.5V, VOUT(MAX) = 10V, IQ = 10A, ISD < 1A, ThinSOT Package
400mA (ISW), Synchronous Step-Up DC/DC Converters with Output 94% Efficiency, VIN: 0.85V to 4V, VOUT(MAX) = 5V, Disconnect IQ = 7A, ISD < 1A, SC-70 Package 500mA (ISW), 1MHz Synchronous Step-Up DC/DC Converters with Output Disconnect 1A (ISW), 1MHz Synchronous Step-Up DC/DC Converters with Output Disconnect 94% Efficiency, VIN: 0.85V to 5V, VOUT(MAX) = 5.25V, IQ = 9A, ISD < 1A, 6-Pin (2mm x 2mm) DFN Package 94% Efficiency, VIN: 0.85V to 5V, VOUT(MAX) = 5.25V, IQ = 10A, ISD < 1A, 8-Pin (2mm x 3mm) DFN Package
ThinSOT is a trademark of Linear Technology Corporation.
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16 Linear Technology Corporation
(408) 432-1900 FAX: (408) 434-0507
LT 1108 REV B * PRINTED IN USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
www.linear.com
(c) LINEAR TECHNOLOGY CORPORATION 2008

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