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FEATURES
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LTC3535 Dual Channel 550mA 1MHz Synchronous Step-Up DC/DC Converter DESCRIPTION
The LTC(R)3535 is a dual channel, synchronous, fixed frequency step-up DC/DC converter with output disconnect. Extended battery life in single AA/AAA powered products is realized with a 680mV start-up voltage and operation down to 500mV once started. A switching frequency of 1MHz minimizes solution footprint by allowing the use of tiny, low profile inductors and ceramic capacitors. The current mode PWM design is internally compensated, reducing external parts count. The LTC3535 features Burst Mode operation at light load conditions allowing it to maintain high efficiency over a wide range of load. Anti-ring circuitry reduces EMI by damping the inductor in discontinuous mode. Additional features include a low shutdown current of under 1A and thermal shutdown. The LTC3535 is housed in a 3mm x 3mm x 0.75mm DFN package.
, LT LTC, LTM and Burst Mode are registered trademarks of Linear Technology , Corporation. All other trademarks are the property of their respective owners.
Two Independent Step-Up Converters Each Channel Delivers 3.3V at 100mA from a Single Alkaline/NiMH Cell or 3.3V at 200mA from Two Cells VIN Start-Up Voltage: 680mV 1.5V to 5.25V VOUT Range Up to 94% Efficiency Output Disconnect 1MHz Fixed Frequency Operation VIN > VOUT Operation Integrated Soft-Start Current Mode Control with Internal Compensation Burst Mode(R) Operation with 9A IQ Each Channel Internal Synchronous Rectifier Logic Controlled Shutdown (IQ < 1A) Anti-Ring Control Low Profile (3mm x 3mm x 0.75mm) 12-Lead DFN Package
APPLICATIONS
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Medical Instruments Noise Canceling Headphones Wireless Mice Bluetooth Headsets
TYPICAL APPLICATION
4.7H 100 90 80 EFFICIENCY (%) 70 60 50 40 30 1M 4.7H
3535 TA01
Efficiency vs Load Current
SW1 VOUT1 1.8V 100mA 511k VOUT = 1.8V
VIN1 VIN 0.8V TO 1.5V 2.2F OFF ON OFF ON SHDN1
VOUT1 VOUT2 10F FB1 FB2 1.78M 10F VOUT2 3.3V 50mA
VOUT = 3.3V
LTC3535 VIN2 SHDN2
1M
GND SW2 GND
20 10 VIN = 1.2V 0 0.01 0.1 10 100 1 LOAD CURRENT (mA) 1000
3535 TA01b
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LTC3535 ABSOLUTE MAXIMUM RATINGS
(Note 1)
PIN CONFIGURATION
TOP VIEW VOUT1 SW1 GND VOUT2 SW2 GND 1 2 3 4 5 6 13 12 FB1 11 SHDN1 10 VIN1 9 FB2 8 SHDN2 7 VIN2
VIN1,2 Voltage ............................................... -0.3V to 6V SW1,2 Voltage DC............................................................ -0.3V to 6V Pulsed <100ns ......................................... -0.3V to 7V SHDN1,2, FB1,2 Voltage .............................. -0.3V to 6V VOUT1,2 ......................................................... -0.3V to 6V Operating Temperature Range (Notes 2, 5) .............................................. -40C to 85C Junction Temperature ........................................... 125C Storage Temperature Range................... -65C to 150C
DD PACKAGE 12-LEAD (3mm 3mm) PLASTIC DFN JA = 45C/W, JC(PAD) = 10C/W, EXPOSED PAD (PIN 13) IS GND, MUST BE SOLDERED TO PCB
ORDER INFORMATION
LEAD FREE FINISH LTC3535EDD#PBF TAPE AND REEL LTC3535EDD#TRPBF PART MARKING LDWV PACKAGE DESCRIPTION 12-Lead (3mm x 3mm) Plastic DFN TEMPERATURE RANGE -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 (For each channel) The l denotes the specifications which apply over the specified
operating temperature range of -40C to 85C, otherwise specifications are at TA = 25C. VIN = 1.2V VOUT = 3.3V unless otherwise noted. ,
PARAMETER Minimum Start-Up Input Voltage Input Voltage Range Output Voltage Adjust Range Feedback Pin Voltage Feedback Pin Input Current Quiescent Current--Shutdown Quiescent Current--Active Quiescent Current--Burst N-Channel MOSFET Switch Leakage Current P-Channel MOSFET Switch Leakage Current N-Channel MOSFET Switch On Resistance P-Channel MOSFET Switch On Resistance N-Channel MOSFET Current Limit Current Limit Delay to Output Maximum Duty Cycle (Note 3) VFB = 1.15V
l
CONDITIONS ILOAD = 1mA After Start-Up. (Minimum Voltage is Load Dependent)
l l l
MIN 0.5 1.5 1.165
TYP 0.68
MAX 0.8 5 5.25
UNITS V V V V nA A A A A A mA ns %
1.195 1 0.01 250 9 0.1 0.1 0.4 0.6
1.225 50 1 500 18 5 10
VFB = 1.30V VSHDN= 0V, Not Including Switch Leakage, VOUT = 0V Measured on VOUT, Non-Switching Measured on VOUT, FB > 1.230V VSW = 5V VSW = 5V, VOUT = 0V VOUT = 3.3V VOUT = 3.3V
l
550 87
750 60 90
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LTC3535 ELECTRICAL CHARACTERISTICS (For each channel) The l denotes the specifications which apply over the specified
, operating temperature range of -40C to 85C, otherwise specifications are at TA = 25C. VIN = 1.2V VOUT = 3.3V unless otherwise noted.
PARAMETER Minimum Duty Cycle Switching Frequency SHDN Pin Input High Voltage SHDN Pin Input Low Voltage 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 LTC3535 is 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 controls. Note 3: Specification is guaranteed by design and not 100% tested in production. CONDITIONS VFB = 1.3V
l l
MIN 0.75 0.8
TYP 1
MAX 0 1.25 0.3
UNITS % MHz V V
Note 4: Current measurements are made when the output is not switching. Note 5: This IC includes overtemperature protection that is intended to protect the device during momentary overload conditions. Junction temperature will exceed 125C when overtemperature protection is active. Continuous operation above the specified maximum operating junction temperature may result in device degradation or failure. Note 6: Failure to solder the exposed backside of the package to the PC board ground plane will result in a thermal resistance much higher than 60C/W.
TYPICAL PERFORMANCE CHARACTERISTICS (Each Channel) TA = 25C, unless otherwise noted.
Efficiency vs Load Current and VIN for VOUT = 1.8V
100 90 80 EFFICIENCY (%) 70 60 50 40 30 20 10 0 0.01 0.1 VIN = 1.0V VIN = 1.2V VIN = 1.5V 1 10 100 LOAD CURRENT (mA) 0.1 POWER LOSS 1 10 EFFICIENCY 100 POWER LOSS (mW) EFFICIENCY (%) 1000 100 90 80 70 60 50 POWER LOSS 40 30 20 10 0 0.01 0.1 VIN = 1.2V VIN = 1.8V VIN = 2.4V VIN = 3.0V 1 10 100 LOAD CURRENT (mA) 1 10 100 POWER LOSS (mW)
Efficiency vs Load Current and VIN for VOUT = 3.3V
EFFICIENCY 1000 100 90 80 70 IIN (A) 60 50 40 30 20
No-Load Input Current vs VIN
VOUT = 5V VOUT = 3.3V VOUT = 2.5V VOUT = 1.8V
0.1
0.01 1000
3535 G01
0.01 1000
3535 G02
10 0.5
1.0
1.5
2.0
2.5 VIN (V)
3.0
3.5
4.0
4.5
3535 G04
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LTC3535 TYPICAL PERFORMANCE CHARACTERISTICS (Each Channel) TA = 25C, unless otherwise noted.
Efficiency vs Load Current and VIN for VOUT = 5V
100 90 80 EFFICIENCY (%) 70 60 50 40 30 20 10 0 0.01 0.1 POWER LOSS 1 10 EFFICIENCY 350 100 POWER LOSS (mW) 300 IOUT (mA) 250 200 VOUT = 5V 150 100 0.1 50 0 0.5 L = 4.7H 1.0 1.5 2.0 2.5 VIN (V)
3535 G03 3535 G05
Maximum Output Current vs VIN
1000 400 VOUT = 3.3V VOUT = 2.5V VOUT = 1.8V 1000 LOAD () 10000
Minimum Load Resistance During Start-Up vs VIN
VOUT = 3.3V
100
VIN = 1.2V VIN = 2.4V VIN = 3.6V VIN = 4.2V 1 10 100 LOAD CURRENT (mA)
0.01 1000
3.0
3.5
4.0
4.5
10 0.65
0.75
0.85
0.95 VIN (V)
1.05
1.15
3526 G06
Start-Up Delay Time vs VIN
100 90 80 LOAD CURRENT (mA) 70 DELAY (s) 60 50 40 30 20 10 0 1.0 1.5 2.0 2.5 3.0 VIN (V) 3.5 4.0 4.5 0 25 20 15 10 5 30
Burst Mode Threshold Current vs VIN
VOUT = 1.8V L = 4.7H 40 LEAVE BURST LOAD CURRENT (mA) 35 30 25 20 15 10 5 1 1.25 VIN (V) 1.5
3535 G08a
Burst Mode Threshold Current vs VIN
VOUT = 2.5V L = 4.7H LEAVE BURST
ENTER BURST
ENTER BURST
0
1
1.25
1.5 VIN (V)
1.75
2
3535 G08b
3535 G07
Burst Mode Threshold Current vs VIN
40 35 LOAD CURRENT (mA) 30 25 20 15 10 5 0 1.0 1.5 2.0 VIN (V) 2.5 3.0
3535 G08c
Burst Mode Threshold Current vs VIN
50 3 VOUT = 5V L = 4.7H FREQUENCY CHANGE (%) 40 LOAD CURRENT (mA) LEAVE BURST 2 1 0 -1 -2
Oscillator Frequency Change vs VOUT
NORMALIZED TO VOUT = 3.3V
VOUT = 3.3V L = 4.7H
LEAVE BURST
30
ENTER BURST
ENTER BURST
20
10
0 1.0
1.5
2.0
2.5 3.0 VIN (V)
3.5
4.0
4.5
-3 1.5
2.0
2.5
3.0 3.5 VOUT (V)
4.0
4.5
5.0
3535 G08d
3535 G09
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LTC3535 TYPICAL PERFORMANCE CHARACTERISTICS (Each Channel) TA = 25C, unless otherwise noted.
RDS(ON) vs VOUT
0.90 0.85 0.80 0.75 0.70 RDS(ON) () 0.65 0.60 0.55 0.50 0.45 0.40 0.35 0.30 1.5 2.0 2.5 3.0 3.5 VOUT (V) 4.0 4.5 5.0 NMOS PMOS FREQUECNY CHANGE (%) 10 8 6 4 2 0 -2 -4 -6 -8 -10 -50 -30 -10 10 30 50 TEMPERATURE (C) 70 90 0.7 -50 -30 -10 10 30 50 TEMPERATURE (C) 70 90 NORMALIZED RDS(ON) 1.2 1.1 1.0 0.9 0.8
Oscillator Frequency Change vs Temperature
NORMALIZED TO 25C 1.3
RDS(ON) Change vs Temperature
NORMALIZED TO 25C
3535 G10
3535 G11
3535 G12
VFB vs Temperature
0.50 0.25 CHANGE IN VFB (%) 0 -0.25 -0.50 -0.75 -1.00 -60 -40 -20 0 20 40 60 TEMPERATURE (C) 0.80 NORMALIZED TO 25C 0.75
Start-Up Voltage vs Temperature
10.0
Burst Mode Quiescent Current vs VOUT
MEASURED ON VOUT 9.5
1mA LOAD 0.70 VIN (V) IQ (A) 75 100
3526 G14
9.0 8.5 8.0 7.5 7.0 1.5
0.65 0.60
NO LOAD
0.55 0.50 -50
80
100
25
0 25 50 TEMPERATURE (C)
2.0
2.5
3.0 3.5 VOUT (V)
4.0
4.5
5.0
3535 G13
3535 G15
Fixed Frequency Switching Waveform and VOUT Ripple
SW PIN 2V/DIV VOUT 10mV/DIV AC COUPLED VIN = 1.2V 500ns/DIV VOUT = 3.3V AT 100mA COUT = 10F
3535 G16
Burst Mode Waveforms
SW PIN 2V/DIV VOUT 20mV/DIV AC COUPLED INDUCTOR CURRENT 0.2A/DIV VIN = 1.2V VOUT = 3.3V COUT = 10F 10s/DIV
3535 G17
VOUT and IIN During Soft-Start
VOUT 1V/DIV INPUT CURRENT 0.2A/DIV SHDN PIN 1V/DIV VOUT = 3.3V COUT = 10F 200s/DIV
3535 G18
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LTC3535 TYPICAL PERFORMANCE CHARACTERISTICS (Each Channel) TA = 25C, unless otherwise noted.
Load Step Response (from Burst Mode Operation)
VOUT 100mV/DIV AC COUPLED LOAD CURRENT 50mA/DIV VIN = 3.6V 100s/DIV VOUT = 5V 20mA TO 170mA STEP COUT = 10F
3535 G19
Load Step Response (Fixed Frequency)
VOUT 100mV/DIV AC COUPLED LOAD CURRENT 50mA/DIV VIN = 3.6V 100s/DIV VOUT = 5V 50mA TO 150mA STEP COUT = 10F
3535 G20
Load Step Response (Fixed Frequency)
VOUT 100mV/DIV AC COUPLED LOAD CURRENT 50mA/DIV VIN = 1.2V 100s/DIV VOUT = 3.3V 50mA TO 100mA STEP COUT = 10F
3535 G21
Load Step Response (from Burst Mode Operation)
VOUT 100mV/DIV AC COUPLED LOAD CURRENT 50mA/DIV VIN = 1.2V 50s/DIV VOUT = 3.3V 5mA TO 100mA STEP COUT = 10F
3535 G22
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LTC3535 PIN FUNCTIONS
VOUT1 (Pin 1): Output Voltage Sense and Drain of the Internal Synchronous Rectifier for Channel 1. PCB trace length from VOUT1 to the output filter capacitor (4.7F minimum) should be as short and wide as possible. SW1 (Pin 2): Switch Pin for Channel 1. Connect inductor between SW1 and VIN1. Keep PCB trace lengths as short and wide as possible to reduce EMI. If the inductor current falls to zero, or SHDN1 is low, an internal anti-ringing switch is connected from SW1 to VIN1 to minimize EMI. GND (Pins 3, 6): Signal and Power Ground. Provide a short direct PCB path between GND and the (-) side of the input and output capacitors. VOUT2 (Pin 4): Output Voltage Sense and Drain of the Internal Synchronous Rectifier for Channel 2. PCB trace length from VOUT2 to the output filter capacitor (4.7F minimum) should be as short and wide as possible. SW2 (Pin 5): Switch Pin for Channel 2. Connect inductor between SW2 and VIN2. Keep PCB trace lengths as short and wide as possible to reduce EMI. If the inductor current falls to zero, or SHDN2 is low, an internal anti-ringing switch is connected from SW2 to VIN2 to minimize EMI. VIN2 (Pin 7): Battery Input Voltage for Channel 2. Connect a minimum of 1F ceramic decoupling capacitor from this pin to ground. SHDN2 (Pin 8): Logic Controlled Shutdown Input for Channel 2. There is an internal 4Meg pull-down on this pin. SHDN = High: Normal operation. SHDN = Low: Shutdown, quiescent current < 1A. FB2 (Pin 9): Feedback Input to the gm Error Amplifier of Channel 2. Connect resistor divider tap to this pin. The output voltage can be adjusted from 1.5V to 5.25V by: VOUT = 1.195V x [1 + (R4/R3)] VIN1 (Pin 10): Battery Input Voltage for Channel 1. Connect a minimum of 1F ceramic decoupling capacitor from this pin to ground. SHDN1 (Pin 11): Logic Controlled Shutdown Input for Channel 1. There is an internal 4Meg pull-down on this pin. SHDN = High: Normal operation. SHDN = Low: Shutdown, quiescent current < 1A. FB1 (Pin 12): Feedback Input to the gm Error Amplifier of Channel 1. Connect resistor divider tap to this pin. The output voltage can be adjusted from 1.5V to 5.25V by: VOUT = 1.195V x [1 + (R2/R1)] Exposed Pad (Pin 13): The Exposed Pad must be soldered to the PCB ground plane. It serves as another ground connection and as a means of conducting heat away from the die.
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LTC3535 BLOCK DIAGRAM
VIN1 0.8V TO 5V L1 4.7H CIN 2.2F VOUT VSEL VBEST VB VOUT1 ANTI-RING GATE DRIVERS AND ANTI-CROSS CONDUCTION R2 FB1 1 VOUT1 1.5V TO 5.25V WELL SWITCH 10 VIN1 2 SW1
11
SHDN1 4M
SHUTDOWN
SHUTDOWN
-+
IPK COMP IPK IZERO SLOPE COMP
12 R1
COUT1 10F
IZERO COMP
UVLO
START-UP LOGIC 1MHz OSC CLK1 MODE CONTROL CLAMP THERMAL SHUTDOWN 5 VIN2 0.8V TO 5V 7 CIN2 2.2F SW2 VIN2 VIN2 VOUT2 VSEL VBEST VB WELL SWITCH TSD WAKE CSS
L2 4.7H
ANTI-RING GATE DRIVERS AND ANTI-CROSS CONDUCTION R4 FB2
8
SHDN2 4M
SHUTDOWN
SHUTDOWN
UVLO
IPK IZERO
START-UP LOGIC 1MHz OSC CLK2 MODE CONTROL CLAMP THERMAL SHUTDOWN TSD WAKE CSS GND 3 EXPOSED PAD 13 GND 6
8
+ -
VREF
VREF2
IPK COMP
+ - -+
SLOPE COMP
VREF
VREF1
ERROR AMP SLEEP COMP
+ -
VREF
VOUT2
4
VOUT2 1.5V TO 5.25V
9 R3
COUT2 10F
IZERO COMP
ERROR AMP SLEEP COMP
+ -
VREF
3535 BD
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LTC3535 OPERATION
(Refer to Block Diagram)
The LTC3535 is a dual channel 1MHz synchronous boost converter housed in a 12-lead 3mm x 3mm DFN package. Each channel is identical and fully independent. They can operate from the same source, or from different voltage sources. In addition, their output voltages can be tied together to allow operation of a single output from two different input sources. However, note that the two channels are not designed to current share, so if both input voltages are present either one may be supplying the load. The following description of operation applies to each channel. Note that references to VIN or VOUT apply to the corresponding channel. With a guaranteed ability to start up and operate from inputs less than 0.8V, each channel features fixed frequency, current mode PWM control for exceptional line and load regulation. The current mode architecture with adaptive slope compensation provides excellent transient load response, requiring minimal output filtering. Internal soft-start and internal loop compensation simplifies the design process while minimizing the number of external components. With its low RDS(ON) and low gate charge internal N-channel MOSFET switch and P-channel MOSFET synchronous rectifier, the LTC3535 achieves high efficiency over a wide range of load currents. Burst Mode operation maintains high efficiency at very light loads, reducing the quiescent current to just 9A per channel. Operation can be best understood by referring to the Block Diagram.
LOW VOLTAGE START-UP The LTC3535 includes an independent start-up oscillator designed to start up at an input voltage of 0.68V (typical). Soft-start and inrush current limiting are provided during start-up, as well as normal mode. When either VIN or VOUT for a given channel exceeds 1.3V typical, the channel enters normal operating mode. When the output voltage exceeds the input by 0.24V, the channel powers itself from VOUT instead of VIN. At this point the internal circuitry has no dependency on the VIN input voltage, eliminating the requirement for a large input capacitor. The input voltage can drop as low as 0.5V. The limiting factor for the application becomes the availability of the power source to supply sufficient energy to the output at low voltages, and maximum duty cycle, which is clamped at 90% typical. Note that at low input voltages, small voltage drops due to series resistance become critical, and greatly limit the power delivery capability of the converter. LOW NOISE FIXED FREQUENCY OPERATION Soft-Start The LTC3535 contains internal circuitry to provide softstart operation. The soft-start circuitry slowly ramps the peak inductor current from zero to its peak value of 750mA (typical) in approximately 0.5ms, allowing start-up into heavy loads. The soft-start circuitry is reset in the event of a shutdown command or a thermal shutdown.
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LTC3535 OPERATION
Oscillator An internal oscillator (independent for each channel) sets the switching frequency to 1MHz. Shutdown Shutdown is accomplished by pulling the SHDN pin below 0.3V and enabled by pulling the SHDN pin above 0.8V. Note that SHDN can be driven above VIN or VOUT, as long as it is limited to less than the absolute maximum rating. Error Amplifier The positive input of the transconductance error amplifier is internally connected to the 1.195V reference and the negative input is connected to FB. Clamps limit the minimum and maximum error amp output voltage for improved large-signal transient response. Power converter control loop compensation is provided internally. An external resistive voltage divider from VOUT to ground programs the output voltage via FB from 1.5V to 5.25V. R2 VOUT = 1.195V * 1+ R1 Current Sensing Lossless current sensing converts the peak current signal of the N-channel MOSFET switch into a voltage that is summed with the internal slope compensation. The summed signal is compared to the error amplifier output to provide a peak current control command for the PWM.
(Refer to Block Diagram)
Current Limit The current limit comparator shuts off the N-channel MOSFET switch once its threshold is reached. The current limit comparator delay to output is typically 60ns. Peak switch current is limited to approximately 750mA, independent of input or output voltage, unless VOUT falls below 0.7V, in which case the current limit is cut in half. Zero Current Comparator The zero current comparator monitors the inductor current to the output and shuts off the synchronous rectifier when this current reduces to approximately 30mA. This prevents the inductor current from reversing in polarity, improving efficiency at light loads. Synchronous Rectifier To control inrush current and to prevent the inductor current from running away when VOUT is close to VIN, the P-channel MOSFET synchronous rectifier is only enabled when VOUT > (VIN + 0.24V). Anti-Ringing Control The anti-ring circuit connects a resistor across the inductor to prevent high frequency ringing on the SW pin during discontinuous current mode operation. Although the ringing of the resonant circuit formed by L and CSW (capacitance on SW pin) is low energy, it can cause EMI radiation.
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LTC3535 OPERATION
Output Disconnect The LTC3535 is designed to allow true output disconnect by eliminating body diode conduction of the internal Pchannel MOSFET rectifier. This allows for VOUT to go to zero volts during shutdown, drawing no current from the input source. It also allows for inrush current limiting at turnon, minimizing surge currents seen by the input supply. Note that to obtain the advantages of output disconnect, there must not be an external Schottky diode connected between SW and VOUT. The output disconnect feature also allows VOUT to be pulled high, without any reverse current into a battery connected to VIN. Thermal Shutdown If the die temperature exceeds 160C, the LTC3535 will go into thermal shutdown. All switches will be off and the soft-start capacitor will be discharged. The device will be enabled again when the die temperature drops by about 15C. Burst Mode OPERATION Each channel of the LTC3535 will enter Burst Mode operation at light load current and return to fixed frequency PWM mode when the load increases. Refer to the Typical Performance Characteristics to see the output load Burst Mode threshold current vs VIN. The load current at which Burst Mode operation is entered can be changed by adjusting the inductor value. Raising the inductor value will lower the load current at which Burst Mode operation is entered.
(Refer to Block Diagram)
In Burst Mode operation, the LTC3535 still switches at a fixed frequency of 1MHz, using the same error amplifier and loop compensation for peak current mode control. This control method eliminates any output transient when switching between modes. In Burst Mode operation, energy is delivered to the output until it reaches the nominal regulation value, then the LTC3535 transitions to sleep mode where the outputs are off and the LTC3535 consumes only 9A of quiescent current from VOUT for each channel. When the output voltage droops slightly, switching resumes. This maximizes efficiency at very light loads by minimizing switching and quiescent losses. Burst Mode output voltage ripple, which is typically 1% peak-topeak, can be reduced by using more output capacitance (10F or greater), or with a small capacitor (10pF to 50pF) connected between VOUT and FB. As the load current increases, the LTC3535 will automatically leave Burst Mode operation. Note that larger output capacitor values may cause this transition to occur at lighter loads. Once the LTC3535 has left Burst Mode operation and returned to normal operation, it will remain there until the output load is reduced below the burst threshold current. Burst Mode operation is inhibited during start-up and soft-start and until VOUT is at least 0.24V greater than VIN. Note that each channel can enter or leave Burst Mode operation independent of the other channel.
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LTC3535 APPLICATIONS INFORMATION
VIN > VOUT OPERATION The LTC3535 will maintain voltage regulation even when the input voltage is above the desired output voltage. Note that the efficiency is much lower in this mode, and the maximum output current capability will be less. Refer to the Typical Performance Characteristics. SHORT-CIRCUIT PROTECTION The LTC3535 output disconnect feature allows output short circuit while maintaining a maximum internally set current limit. To reduce power dissipation under shortcircuit conditions, the peak switch current limit is reduced to 400mA (typical per channel). SCHOTTKY DIODE Although not recommended, adding a Schottky diode from SW to VOUT will improve efficiency by about 2%. Note that this defeats the output disconnect and short-circuit protection features. PCB LAYOUT GUIDELINES The high speed operation of the LTC3535 demands careful attention to board layout. A careless layout will result in reduced performance. Figure 1 shows the recommended component placement. A large ground pin copper area will help to lower the die temperature. A multilayer board with a separate ground plane is ideal, but not absolutely necessary. COMPONENT SELECTION Inductor Selection The LTC3535 can utilize small surface mount chip inductors due to their fast 1MHz switching frequency. Inductor values between 3.3H and 6.8H are suitable for most applications. Larger values of inductance will allow slightly greater output current capability (and lower the Burst Mode threshold) by reducing the inductor ripple current. Increasing the inductance above 10H will increase component size while providing little improvement in output current capability. The minimum inductance value is given by: L> where: Ripple = Allowable inductor current ripple (amps peakpeak) VIN(MIN) = Minimum input voltage VOUT(MAX) = Maximum output voltage The inductor current ripple is typically set for 20% to 40% of the maximum inductor current. High 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 support the peak inductor current without saturating. Molded chokes and some chip inductors usually do not have enough core area to support the peak inductor current of 750mA seen on the LTC3535. To minimize radiated noise, use a shielded inductor. See Table 1 for suggested components and suppliers. VIN(MIN) * VOUT(MAX ) - VIN(MIN) Ripple * VOUT(MAX)
(
)
SHDN VOUT1 VIN1
GND
GND
VOUT2
SHDN
VIN2
Figure 1. Recommended Component Placement
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LTC3535 APPLICATIONS INFORMATION
Table 1. Recommended Inductors
VENDOR Coilcraft (847) 639-6400 www.coilcraft.com PART/STYLE LPO4815 LPS4012, LPS4018 MSS5131 MSS4020 MOS6020 ME3220 DS1605, DO1608 SD10, SD12, SD14, SD18, SD20, SD52, SD3114, SD3118 MIP3226D4R7M, MIP3226D3R3M MIPF2520D4R7 MIPWT3226D3R0 LQH43C LQH32C (-53 series) 301015 CDRH5D18 CDRH2D14 CDRH3D16 CDRH3D11 CR43 CMD4D06-4R7MC CMD4D06-3R3MC NP03SB NR3015T NR3012T VLP VLF VLCF , D412C D518LC D52LC D62LCB WE-TPC type S, M
4.7F to 10F output capacitor is sufficient for most applications. Larger values may be used to obtain extremely low output voltage ripple and improve transient response. X5R and X7R dielectric materials are preferred for their ability to maintain capacitance over wide voltage and temperature ranges. Y5V types should not be used. The internal loop compensation of the LTC3535 is designed to be stable with output capacitor values of 4.7F or greater (without the need for any external series resistor). Although ceramic capacitors are recommended, low ESR tantalum capacitors may be used as well. A small ceramic capacitor in parallel with a larger tantalum capacitor may be used in demanding applications that have large load transients. Another method of improving the transient response is to add a small feed-forward capacitor across the top resistor of the feedback divider (from VOUT to FB). A typical value of 22pF will generally suffice. 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. A 2.2F input capacitor is sufficient for most applications, although larger values may be used without limitations. Table 2 shows a list of several ceramic capacitor manufacturers. Consult the manufacturers directly for detailed information on their selection of ceramic capacitors.
Table 2. Capacitor Vendor Information
SUPPLIER PHONE (803) 448-9411 (714) 852-2001 (408) 573-4150 (847) 803-6100 (408) 544-5200 WEBSITE www.avxcorp.com www.murata.com www.t-yuden.com www.component.tdk.com www.sem.samsung.com AVX Murata Taiyo-Yuden TDK Samsung
Coiltronics www.cooperet.com FDK (408) 432-8331 www.fdk.com Murata (714) 852-2001 www.murata.com Sumida (847) 956-0666 www.sumida.com
Taiyo-Yuden www.t-yuden.com TDK (847) 803-6100 www.component.tdk.com Toko (408) 432-8282 www.tokoam.com Wurth (201) 785-8800 www.we-online.com
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. A
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13
LTC3535 TYPICAL APPLICATION
Single Cell to 3.3V Converter with 20 Seconds of Holdup with 30mA Load
4.7H 499k VIN 0.8V TO 1.5V 2.2F SW VIN1 SHDN1 VOUT1 VOUT2 10F 1.78M FB1 FB2 1M 392k VOUT 3.3V 30mA
+
VHOLDUP CHOLD* 0.47F
4.25V
1M
LTC3535 VIN2 2.2F 1.5M 4.7H SHDN2
GND SW2 GND
3535 TA02
*POWERSTOR PA-5R0H474-R
VIN 1V/DIV VHOLDUP 2V/DIV VOUT 2V/DIV 5s/DIV
3535 TA02b
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14
LTC3535 PACKAGE DESCRIPTION
DC Package 12-Lead Plastic DFN (3mm x 3mm)
(Reference LTC DWG # 05-08-1725 Rev A)
0.70
0.05
3.50
0.05 2.10 0.05
2.38 0.05 1.65 0.05 PACKAGE OUTLINE
0.25
0.05 0.45 BSC 2.25 REF
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED
R = 0.115 TYP 7
0.40 12
0.10
3.00 0.10 (4 SIDES) PIN 1 TOP MARK (SEE NOTE 6)
2.38 0.10 1.65 0.10 PIN 1 NOTCH R = 0.20 OR 0.25 45 CHAMFER 0.05
6 0.200 REF 0.75 0.05 2.25 REF 0.00 - 0.05
1 0.23 0.45 BSC
(DD12) DFN 0106 REV A
BOTTOM VIEW--EXPOSED PAD NOTE: 1. DRAWING IS NOT A JEDEC PACKAGE OUTLINE 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 AND TIE BARS SHALL BE SOLDER PLATED 6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION ON THE TOP AND BOTTOM OF PACKAGE
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Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
15
LTC3535 RELATED PARTS
PART NUMBER LTC3400/LTC3400B LTC3421 LTC3422 LTC3426 LTC3427 LTC3428 LTC3429 LTC3458/LTC3458L LTC3459 LTC3499 LTC3525-3 LTC3525-3.3 LTC3525-5 LTC3525L-3 LTC3526/LTC3526B LTC3526-2 LTC3526B-2 LTC3526L LTC3526LB LTC3526/LTC3526B LTC3527/LTC3527-1 LTC3528 LTC3528-2 LTC3537 LTC3539 LTC3539-2 DESCRIPTION 600mA ISW, 1.2MHz, Synchronous Step-Up DC/DC Converters 3A ISW, 3MHz, Synchronous Step-Up DC/DC Converter with Output Disconnect 1.5A ISW, 3MHz Synchronous Step-Up DC/DC Converter with Output Disconnect 2A ISW, 1.2MHz, Step-Up DC/DC Converter 500mA ISW, 1.2MHZ, Synchronous Step-Up DC/DC Converter with Output Disconnect 500mA ISW, 1.25MHz/2.5MHz, Synchronous Step-Up DC/DC Converters with Output Disconnect 600mA ISW, 500kHz, Synchronous Step-Up DC/DC Converter with Output Disconnect and Soft-Start 1.4A ISW, 1.5MHz, Synchronous Step-Up DC/DC Converter/Output Disconnect/Burst Mode Operation 70mA ISW, 10V Micropower Synchronous Boost Converter/Output Disconnect/Burst Mode Operation 750mA (ISW), 1.2MHz, Step-Up DC/DC Converter with Reverse Battery Protection and Output Disconnect 400mA Micropower Synchronous Step-Up DC/DC Converter with Output Disconnect 400mA Micropower Synchronous Step-Up DC/DC Converter with Output Disconnect 500mA, 1MHz/2.2MHz, Synchronous Step-Up DC/DC Converters with Output Disconnect 550mA, 1MHz, Synchronous Step-Up DC/DC Converters with Output Disconnect 500mA (ISW), 1MHz Synchronous Step-Up DC/DC Converter with Output Disconnect Dual 800mA and 400mA (ISW), 2.2MHz, Synchronous Step-Up DC/DC Converter with Output Disconnect 1A (ISW), 1MHz Synchronous Step-Up DC/DC with Output DisconnectConverter 600mA , 2.2MHz, Synchronous Step-Up DC/DC Converter with Output Disconnect and 100mA LDO 2A (ISW), 1/2MHz, Synchronous Step-Up DC/DC Converter with Output Disconnect COMMENTS 92% Efficiency VIN: 1V to 5V, VOUT(MAX) = 5V, IQ = 19A/300A, ISD < 1A, ThinSOT Package 95% Efficiency VIN: 1V to 4.5V, VOUT(MAX) = 5.25V, IQ = 12A, ISD < 1A, QFN24 Package 95% Efficiency VIN: 1V to 4.5V, VOUT(MAX) = 5.25V, IQ = 25A, ISD < 1A, 3mm x 3mm DFN Package 92% Efficiency VIN: 1.6V to 4.3V, VOUT(MAX) = 5V, ISD < 1A, SOT-23 Package 93% Efficiency VIN: 1.8V to 4.5V, VOUT(MAX) = 5V, 2mm x 2mm DFN Package 92% Efficiency VIN: 1.8V to 5V, VOUT(MAX) = 5.25V, ISD < 1A, 2mm x 2mm DFN Package 96% Efficiency VIN: 1V to 4.4V, VOUT(MAX) = 5V, IQ = 20A/300A, ISD < 1A, ThinSOT Package 93% Efficiency VIN: 1.5V to 6V, VOUT(MAX) = 7.5V, IQ = 15A, ISD < 1A, DFN12 Package VIN: 1.5V to 5.5V, VOUT(MAX) = 10V, IQ = 10A, ISD < 1A, ThinSOT Package 92% Efficiency VIN: 1.8V to 5.5V, VOUT(MAX) = 6V, IQ = 20A, ISD < 1A, 3mm x 3mm DFN-8 Package, MSOP-8 Package 95% Efficiency VIN: 1V to 4.5V, VOUT(MAX) = 3.3V or 5V, IQ = 7A, ISD < 1A, SC-70 Package 93% Efficiency VIN: 0.88V to 4.5V, VOUT = 3V, IQ = 7A, ISD < 1A, SC-70 Package 94% Efficiency VIN: 0.85V to 5V, VOUT(MAX) = 5.25V, IQ = 9A, ISD < 1A, 2mm x 2mm DFN-6 Package 94% Efficiency VIN: 0.7V to 5V, VOUT(MAX) = 5.25V, IQ = 9A, ISD < 1A, 2mm x 2mm DFN-6 Package 94% Efficiency VIN: 0.8V to 5V, VOUT(MAX) = 5.25V, IQ = 9A, ISD < 1A, 2mm x 2mm DFN-6 Package 94% Efficiency VIN: 0.7V to 5V, VOUT(MAX) = 5.25V, IQ = 12A, ISD < 1A, 3mm x 3mm QFN-16 Package 94% Efficiency VIN: 0.7V to 5V, VOUT(MAX) = 5.25V, IQ = 12A, ISD < 1A, 2mm x 3mm DFN-8 Package 94% Efficiency VIN: 0.7V to 5V, VOUT(MAX) = 5.25V, IQ = 30A, ISD < 1A, 3mm x 3mm QFN-16 Package 94% Efficiency VIN: 0.7V to 5V, VOUT(MAX) = 5.25V, IQ = 10A, ISD < 1A, 2mm x 3mm DFN-8 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 0109 * PRINTED IN USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
www.linear.com
(c) LINEAR TECHNOLOGY CORPORATION 2009

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