LTC3535 Dual Channel 550mA 1MHz Synchronous Step-Up DC/DC Converter DESCRIPTION FEATURES n n n n n n n n n n n n n n n 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® Operation with 9μA IQ Each Channel Internal Synchronous Rectifier Logic Controlled Shutdown (IQ < 1μA) Anti-Ring Control Low Profile (3mm × 3mm × 0.75mm) 12-Lead DFN Package The LTC®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 1μA and thermal shutdown. The LTC3535 is housed in a 3mm × 3mm × 0.75mm DFN package. APPLICATIONS n n n n , LT, LTC, LTM and Burst Mode are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. Medical Instruments Noise Canceling Headphones Wireless Mice Bluetooth Headsets TYPICAL APPLICATION Efficiency vs Load Current 4.7μH SHDN1 VOUT2 OFF ON VOUT2 3.3V 50mA 10μF LTC3535 2.2μF 10μF VIN2 FB1 SHDN2 FB2 90 1.78M 1M GND SW2 GND VOUT = 1.8V 80 511k EFFICIENCY (%) OFF ON VIN 0.8V TO 1.5V VOUT1 100 VOUT1 1.8V 100mA SW1 VIN1 70 VOUT = 3.3V 60 50 40 30 1M 20 4.7μH 3535 TA01 10 VIN = 1.2V 0 0.01 0.1 10 100 1 LOAD CURRENT (mA) 1000 3535 TA01b 3535f 1 LTC3535 ABSOLUTE MAXIMUM RATINGS PIN CONFIGURATION (Note 1) TOP VIEW 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) .............................................. –40°C to 85°C Junction Temperature ........................................... 125°C Storage Temperature Range................... –65°C to 150°C 12 FB1 VOUT1 1 SW1 2 GND 3 VOUT2 4 9 FB2 SW2 5 8 SHDN2 GND 6 7 VIN2 11 SHDN1 13 10 VIN1 DD PACKAGE 12-LEAD (3mm s 3mm) PLASTIC DFN θJA = 45°C/W, θJC(PAD) = 10°C/W, EXPOSED PAD (PIN 13) IS GND, MUST BE SOLDERED TO PCB ORDER INFORMATION LEAD FREE FINISH TAPE AND REEL PART MARKING PACKAGE DESCRIPTION TEMPERATURE RANGE LTC3535EDD#PBF LTC3535EDD#TRPBF LDWV 12-Lead (3mm × 3mm) Plastic DFN –40°C to 85°C 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 –40°C to 85°C, otherwise specifications are at TA = 25°C. VIN = 1.2V, VOUT = 3.3V unless otherwise noted. PARAMETER CONDITIONS Minimum Start-Up Input Voltage ILOAD = 1mA Input Voltage Range After Start-Up. (Minimum Voltage is Load Dependent) Output Voltage Adjust Range Feedback Pin Voltage Feedback Pin Input Current VFB = 1.30V Quiescent Current—Shutdown VSHDN= 0V, Not Including Switch Leakage, VOUT = 0V Quiescent Current—Active Measured on VOUT, Non-Switching Quiescent Current—Burst Measured on VOUT, FB > 1.230V N-Channel MOSFET Switch Leakage Current VSW = 5V MIN TYP MAX 0.68 0.8 UNITS V l 0.5 5 V l 1.5 5.25 V l 1.165 1.195 1.225 V 1 50 nA 0.01 1 μA 250 500 μA 9 18 μA 0.1 5 μA 10 μA P-Channel MOSFET Switch Leakage Current VSW = 5V, VOUT = 0V 0.1 N-Channel MOSFET Switch On Resistance VOUT = 3.3V 0.4 Ω P-Channel MOSFET Switch On Resistance VOUT = 3.3V 0.6 Ω 750 mA 60 ns 90 % l N-Channel MOSFET Current Limit Current Limit Delay to Output (Note 3) Maximum Duty Cycle VFB = 1.15V l 550 87 3535f 2 LTC3535 ELECTRICAL CHARACTERISTICS (For each channel) The l denotes the specifications which apply over the specified operating temperature range of –40°C to 85°C, otherwise specifications are at TA = 25°C. VIN = 1.2V, VOUT = 3.3V unless otherwise noted. PARAMETER CONDITIONS Minimum Duty Cycle VFB = 1.3V MIN TYP MAX 0.75 1 1.25 l 0 l Switching Frequency UNITS SHDN Pin Input High Voltage % MHz 0.8 V SHDN Pin Input Low Voltage 0.3 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 0°C to 85°C. Specifications over the –40°C to 85°C 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. 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 125°C 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 60°C/W. TYPICAL PERFORMANCE CHARACTERISTICS (Each Channel) TA = 25°C, unless otherwise noted. Efficiency vs Load Current and VIN for VOUT = 1.8V Efficiency vs Load Current and VIN for VOUT = 3.3V 100 EFFICIENCY EFFICIENCY (%) 50 POWER LOSS 1 30 VIN = 1.0V VIN = 1.2V VIN = 1.5V 10 0.1 100 80 1 10 100 LOAD CURRENT (mA) 0.1 0.01 1000 3535 G01 90 80 70 10 60 50 POWER LOSS 1 40 30 VIN = 1.2V VIN = 1.8V VIN = 2.4V VIN = 3.0V 20 10 0 0.01 0.1 1 10 100 LOAD CURRENT (mA) POWER LOSS (mW) 10 60 POWER LOSS (mW) 70 20 EFFICIENCY 100 0.1 0.01 1000 VOUT = 5V VOUT = 3.3V 70 IIN (μA) 100 40 No-Load Input Current vs VIN 1000 90 80 0 0.01 100 EFFICIENCY (%) 90 1000 VOUT = 2.5V 60 50 VOUT = 1.8V 40 30 20 10 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 VIN (V) 3535 G02 3535 G04 3535f 3 LTC3535 TYPICAL PERFORMANCE CHARACTERISTICS (Each Channel) TA = 25°C, unless otherwise noted. Efficiency vs Load Current and VIN for VOUT = 5V 100 50 POWER LOSS 40 30 1 VIN = 1.2V VIN = 2.4V VIN = 3.6V VIN = 4.2V 20 10 0 0.01 0.1 VOUT = 1.8V 250 200 VOUT = 5V 150 100 100 0.1 50 0 0.5 0.01 1000 1 10 100 LOAD CURRENT (mA) L = 4.7μH 1.0 1.5 2.0 2.5 3.0 3.5 4.0 0.75 0.85 0.95 1.05 1.15 VIN (V) 3526 G06 3535 G05 Burst Mode Threshold Current vs VIN Burst Mode Threshold Current vs VIN Start-Up Delay Time vs VIN 100 30 25 LOAD CURRENT (mA) 80 70 60 50 40 30 40 LEAVE BURST 20 ENTER BURST 15 VOUT = 2.5V L = 4.7μH 35 LOAD CURRENT (mA) VOUT = 1.8V L = 4.7μH 90 DELAY (μs) 10 0.65 4.5 VIN (V) 3535 G03 10 LEAVE BURST 30 25 ENTER BURST 20 15 10 20 5 5 10 0 1.0 1.5 2.0 2.5 3.0 VIN (V) 3.5 4.0 0 4.5 1 1.25 VIN (V) 3 LOAD CURRENT (mA) ENTER BURST 15 10 30 ENTER BURST 20 10 1.0 1.5 2.0 VIN (V) 2.5 3.0 3535 G08c 0 1.0 1 0 –1 –2 5 0 NORMALIZED TO VOUT = 3.3V 2 LEAVE BURST FREQUENCY CHANGE (%) LEAVE BURST 40 20 2 Oscillator Frequency Change vs VOUT VOUT = 5V L = 4.7μH 25 1.75 3535 G08b 50 30 1.5 VIN (V) Burst Mode Threshold Current vs VIN 40 VOUT = 3.3V L = 4.7μH 1.25 1 3535 G08a Burst Mode Threshold Current vs VIN 35 0 1.5 3535 G07 LOAD CURRENT (mA) VOUT = 3.3V VOUT = 2.5V 1000 IOUT (mA) 10 60 300 POWER LOSS (mW) 70 10000 VOUT = 3.3V 350 80 EFFICIENCY (%) 400 1000 EFFICIENCY LOAD (Ω) 100 90 Minimum Load Resistance During Start-Up vs VIN Maximum Output Current vs VIN 1.5 2.0 2.5 3.0 VIN (V) 3.5 4.0 4.5 3535 G08d –3 1.5 2.0 2.5 3.0 3.5 VOUT (V) 4.0 4.5 5.0 3535 G09 3535f 4 LTC3535 TYPICAL PERFORMANCE CHARACTERISTICS (Each Channel) TA = 25°C, unless otherwise noted. Oscillator Frequency Change vs Temperature RDS(ON) vs VOUT 0.90 10 0.85 8 0.80 0.65 PMOS 0.60 0.55 0.50 0.45 4 2 0 –2 –4 NMOS 0.40 –6 1.1 1.0 0.9 0.8 –8 0.35 0.30 1.5 2.0 2.5 3.0 3.5 VOUT (V) 4.0 5.0 4.5 –10 –50 –30 –10 10 30 50 TEMPERATURE (°C) 0.7 –50 90 –30 –10 10 30 50 TEMPERATURE (°C) 70 90 3535 G12 Burst Mode Quiescent Current vs VOUT Start-Up Voltage vs Temperature VFB vs Temperature 0.50 70 3535 G11 3535 G10 0.80 10.0 NORMALIZED TO 25°C MEASURED ON VOUT 0.75 0.25 9.5 1mA LOAD 9.0 –0.25 0.65 IQ (μA) 0.70 0 VIN (V) CHANGE IN VFB (%) NORMALIZED TO 25°C 1.2 NORMALIZED RDS(ON) RDS(ON) (Ω) 0.70 NORMALIZED TO 25°C 6 FREQUECNY CHANGE (%) 0.75 RDS(ON) Change vs Temperature 1.3 NO LOAD 8.5 –0.50 0.60 8.0 –0.75 0.55 7.5 –1.00 –60 –40 –20 0 20 40 60 TEMPERATURE (°C) 80 100 0.50 –50 25 0 25 50 TEMPERATURE (°C) 75 100 Fixed Frequency Switching Waveform and VOUT Ripple 3535 G16 3.0 3.5 VOUT (V) 4.0 5.0 4.5 VOUT and IIN During Soft-Start VOUT 1V/DIV SW PIN 2V/DIV VOUT 20mV/DIV AC COUPLED INDUCTOR CURRENT 0.2A/DIV VOUT 10mV/DIV AC COUPLED 2.5 3535 G15 Burst Mode Waveforms SW PIN 2V/DIV 2.0 3526 G14 3535 G13 VIN = 1.2V 500ns/DIV VOUT = 3.3V AT 100mA COUT = 10μF 7.0 1.5 INPUT CURRENT 0.2A/DIV SHDN PIN 1V/DIV VIN = 1.2V VOUT = 3.3V COUT = 10μF 10μs/DIV 3535 G17 VOUT = 3.3V COUT = 10μF 200μs/DIV 3535 G18 3535f 5 LTC3535 TYPICAL PERFORMANCE CHARACTERISTICS (Each Channel) TA = 25°C, unless otherwise noted. Load Step Response (from Burst Mode Operation) Load Step Response (Fixed Frequency) VOUT 100mV/DIV AC COUPLED VOUT 100mV/DIV AC COUPLED LOAD CURRENT 50mA/DIV LOAD CURRENT 50mA/DIV VIN = 3.6V 100μs/DIV VOUT = 5V 20mA TO 170mA STEP COUT = 10μF 3535 G19 VIN = 3.6V 100μs/DIV VOUT = 5V 50mA TO 150mA STEP COUT = 10μF Load Step Response (Fixed Frequency) 3535 G20 Load Step Response (from Burst Mode Operation) VOUT 100mV/DIV AC COUPLED VOUT 100mV/DIV AC COUPLED LOAD CURRENT 50mA/DIV LOAD CURRENT 50mA/DIV VIN = 1.2V 100μs/DIV VOUT = 3.3V 50mA TO 100mA STEP COUT = 10μF 3535 G21 VIN = 1.2V 50μs/DIV VOUT = 3.3V 5mA TO 100mA STEP COUT = 10μF 3535 G22 3535f 6 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 (4.7μF minimum) should be as short and wide as possible. SHDN2 (Pin 8): Logic Controlled Shutdown Input for Channel 2. There is an internal 4MegΩ pull-down on this pin. SHDN = High: Normal operation. SHDN = Low: Shutdown, quiescent current < 1μA. 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. 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 × [1 + (R4/R3)] 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. VIN1 (Pin 10): Battery Input Voltage for Channel 1. Connect a minimum of 1μF ceramic decoupling capacitor from this pin to ground. 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 (4.7μF minimum) should be as short and wide as possible. SHDN1 (Pin 11): Logic Controlled Shutdown Input for Channel 1. There is an internal 4MegΩ pull-down on this pin. SHDN = High: Normal operation. SHDN = Low: Shutdown, quiescent current < 1μA. 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. 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 × [1 + (R2/R1)] VIN2 (Pin 7): Battery Input Voltage for Channel 2. Connect a minimum of 1μF ceramic decoupling capacitor from this pin to ground. 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. 3535f 7 LTC3535 BLOCK DIAGRAM VIN1 0.8V TO 5V L1 4.7μH CIN 2.2μF 10 2 VIN1 SW1 VOUT VSEL VBEST WELL SWITCH VB VOUT1 VOUT1 1.5V TO 5.25V 1 ANTI-RING 11 SHDN1 SHUTDOWN SHUTDOWN GATE DRIVERS AND ANTI-CROSS CONDUCTION R2 FB1 – + 4M 3 IPK COMP VREF1 IPK UVLO IZERO COMP ERROR AMP SLEEP COMP IZERO START-UP LOGIC + – MODE CONTROL CLK1 1MHz OSC R1 SLOPE COMP + – VREF COUT1 10μF 12 VREF CLAMP 5 VIN2 0.8V TO 5V TSD THERMAL SHUTDOWN L2 4.7μH 7 WAKE CSS SW2 VIN2 VIN2 CIN2 2.2μF VOUT2 VSEL VBEST WELL SWITCH VB VOUT2 VOUT2 1.5V TO 5.25V 4 ANTI-RING 8 SHDN2 SHUTDOWN SHUTDOWN GATE DRIVERS AND ANTI-CROSS CONDUCTION R4 FB2 – + 4M 3 IPK COMP VREF2 IPK UVLO START-UP 1MHz OSC + – MODE CONTROL CLK2 R3 ERROR AMP SLEEP COMP IZERO LOGIC IZERO COMP SLOPE COMP + – VREF COUT2 10μF 9 VREF CLAMP THERMAL SHUTDOWN TSD WAKE CSS GND EXPOSED PAD GND 3 13 6 3535 BD 3535f 8 LTC3535 OPERATION (Refer to Block Diagram) The LTC3535 is a dual channel 1MHz synchronous boost converter housed in a 12-lead 3mm × 3mm DFN package. Each channel is identical and fully independent. They can operate from the same source, or from different voltage sources. LOW VOLTAGE START-UP 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. 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. 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 9μA per channel. Operation can be best understood by referring to the Block Diagram. 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. 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. 3535f 9 LTC3535 OPERATION (Refer to Block Diagram) Oscillator Current Limit An internal oscillator (independent for each channel) sets the switching frequency to 1MHz. 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. 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. 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. 3535f 10 LTC3535 OPERATION (Refer to Block Diagram) 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 160°C, 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 15°C. 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. 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 9μA 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 (10μF 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. 3535f 11 LTC3535 APPLICATIONS INFORMATION VIN > VOUT OPERATION COMPONENT SELECTION 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). The LTC3535 can utilize small surface mount chip inductors due to their fast 1MHz switching frequency. Inductor values between 3.3μH and 6.8μH 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 10μH will increase component size while providing little improvement in output current capability. The minimum inductance value is given by: L> 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. 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. SHDN VOUT1 VIN1 GND GND SHDN ( VIN(MIN) • VOUT(MAX ) – VIN(MIN) ) Ripple • VOUT(MAX) where: Ripple = Allowable inductor current ripple (amps peakpeak) VIN(MIN) = Minimum input voltage PCB LAYOUT GUIDELINES VOUT2 Inductor Selection 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. VIN2 Figure 1. Recommended Component Placement 3535f 12 LTC3535 APPLICATIONS INFORMATION Table 1. Recommended Inductors VENDOR PART/STYLE Coilcraft (847) 639-6400 www.coilcraft.com LPO4815 LPS4012, LPS4018 MSS5131 MSS4020 MOS6020 ME3220 DS1605, DO1608 Coiltronics www.cooperet.com SD10, SD12, SD14, SD18, SD20, SD52, SD3114, SD3118 FDK (408) 432-8331 www.fdk.com MIP3226D4R7M, MIP3226D3R3M MIPF2520D4R7 MIPWT3226D3R0 Murata (714) 852-2001 www.murata.com LQH43C LQH32C (-53 series) 301015 Sumida (847) 956-0666 www.sumida.com CDRH5D18 CDRH2D14 CDRH3D16 CDRH3D11 CR43 CMD4D06-4R7MC CMD4D06-3R3MC Taiyo-Yuden www.t-yuden.com NP03SB NR3015T NR3012T TDK (847) 803-6100 www.component.tdk.com VLP VLF, VLCF Toko (408) 432-8282 www.tokoam.com D412C D518LC D52LC D62LCB Wurth (201) 785-8800 www.we-online.com WE-TPC type S, M 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 4.7μF to 10μF 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 4.7μF 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 2.2μF 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 WEBSITE AVX (803) 448-9411 www.avxcorp.com Murata (714) 852-2001 www.murata.com Taiyo-Yuden (408) 573-4150 www.t-yuden.com TDK (847) 803-6100 www.component.tdk.com Samsung (408) 544-5200 www.sem.samsung.com 3535f 13 LTC3535 TYPICAL APPLICATION Single Cell to 3.3V Converter with 20 Seconds of Holdup with 30mA Load VOUT 3.3V 30mA 4.7μH 499k SW VIN 0.8V TO 1.5V 2.2μF VIN1 VOUT1 SHDN1 VOUT2 4.25V 1M 1.78M VIN2 FB1 SHDN2 FB2 GND SW2 GND 1.5M VHOLDUP CHOLD* 0.47μF 10μF LTC3535 2.2μF + 392k 1M 4.7μH 3535 TA02 *POWERSTOR PA-5R0H474-R VIN 1V/DIV VHOLDUP 2V/DIV VOUT 2V/DIV 5s/DIV 3535 TA02b 3535f 14 LTC3535 PACKAGE DESCRIPTION DC Package 12-Lead Plastic DFN (3mm × 3mm) (Reference LTC DWG # 05-08-1725 Rev A) 0.70 p0.05 3.50 p0.05 2.10 p0.05 2.38 p0.05 1.65 p0.05 PACKAGE OUTLINE 0.25 p 0.05 0.45 BSC 2.25 REF RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED 3.00 p0.10 (4 SIDES) R = 0.115 TYP 7 0.40 p 0.10 12 2.38 p0.10 1.65 p 0.10 PIN 1 NOTCH R = 0.20 OR 0.25 s 45o CHAMFER PIN 1 TOP MARK (SEE NOTE 6) 6 0.200 REF 1 0.23 p 0.05 0.45 BSC 0.75 p0.05 2.25 REF (DD12) DFN 0106 REV A 0.00 – 0.05 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 3535f 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 DESCRIPTION COMMENTS LTC3400/LTC3400B 600mA ISW, 1.2MHz, Synchronous Step-Up DC/DC Converters 92% Efficiency VIN: 1V to 5V, VOUT(MAX) = 5V, IQ = 19μA/300μA, ISD < 1μA, ThinSOT Package LTC3421 3A ISW, 3MHz, Synchronous Step-Up DC/DC Converter with Output Disconnect 95% Efficiency VIN: 1V to 4.5V, VOUT(MAX) = 5.25V, IQ = 12μA, ISD < 1μA, QFN24 Package LTC3422 1.5A ISW, 3MHz Synchronous Step-Up DC/DC Converter with Output Disconnect 95% Efficiency VIN: 1V to 4.5V, VOUT(MAX) = 5.25V, IQ = 25μA, ISD < 1μA, 3mm × 3mm DFN Package LTC3426 2A ISW, 1.2MHz, Step-Up DC/DC Converter 92% Efficiency VIN: 1.6V to 4.3V, VOUT(MAX) = 5V, ISD < 1μA, SOT-23 Package LTC3427 500mA ISW, 1.2MHZ, Synchronous Step-Up DC/DC Converter with Output Disconnect 93% Efficiency VIN: 1.8V to 4.5V, VOUT(MAX) = 5V, 2mm × 2mm DFN Package LTC3428 500mA ISW, 1.25MHz/2.5MHz, Synchronous Step-Up DC/DC Converters with Output Disconnect 92% Efficiency VIN: 1.8V to 5V, VOUT(MAX) = 5.25V, ISD < 1μA, 2mm × 2mm DFN Package LTC3429 600mA ISW, 500kHz, Synchronous Step-Up DC/DC Converter with Output Disconnect and Soft-Start 96% Efficiency VIN: 1V to 4.4V, VOUT(MAX) = 5V, IQ = 20μA/300μA, ISD < 1μA, ThinSOT Package LTC3458/LTC3458L 1.4A ISW, 1.5MHz, Synchronous Step-Up DC/DC Converter/Output Disconnect/Burst Mode Operation 93% Efficiency VIN: 1.5V to 6V, VOUT(MAX) = 7.5V, IQ = 15μA, ISD < 1μA, DFN12 Package LTC3459 70mA ISW, 10V Micropower Synchronous Boost Converter/Output Disconnect/Burst Mode Operation VIN: 1.5V to 5.5V, VOUT(MAX) = 10V, IQ = 10μA, ISD < 1μA, ThinSOT Package LTC3499 750mA (ISW), 1.2MHz, Step-Up DC/DC Converter with Reverse Battery Protection and Output Disconnect 92% Efficiency VIN: 1.8V to 5.5V, VOUT(MAX) = 6V, IQ = 20μA, ISD < 1μA, 3mm × 3mm DFN-8 Package, MSOP-8 Package LTC3525-3 LTC3525-3.3 LTC3525-5 400mA Micropower Synchronous Step-Up DC/DC Converter with Output Disconnect 95% Efficiency VIN: 1V to 4.5V, VOUT(MAX) = 3.3V or 5V, IQ = 7μA, ISD < 1μA, SC-70 Package LTC3525L-3 400mA Micropower Synchronous Step-Up DC/DC Converter with Output Disconnect 93% Efficiency VIN: 0.88V to 4.5V, VOUT = 3V, IQ = 7μA, ISD < 1μA, SC-70 Package LTC3526/LTC3526B LTC3526-2 LTC3526B-2 500mA, 1MHz/2.2MHz, Synchronous Step-Up DC/DC Converters with Output Disconnect 94% Efficiency VIN: 0.85V to 5V, VOUT(MAX) = 5.25V, IQ = 9μA, ISD < 1μA, 2mm × 2mm DFN-6 Package LTC3526L LTC3526LB 550mA, 1MHz, Synchronous Step-Up DC/DC Converters with Output Disconnect 94% Efficiency VIN: 0.7V to 5V, VOUT(MAX) = 5.25V, IQ = 9μA, ISD < 1μA, 2mm × 2mm DFN-6 Package LTC3526/LTC3526B 500mA (ISW), 1MHz Synchronous Step-Up DC/DC Converter with Output Disconnect 94% Efficiency VIN: 0.8V to 5V, VOUT(MAX) = 5.25V, IQ = 9μA, ISD < 1μA, 2mm × 2mm DFN-6 Package LTC3527/LTC3527-1 Dual 800mA and 400mA (ISW), 2.2MHz, Synchronous Step-Up DC/DC Converter with Output Disconnect 94% Efficiency VIN: 0.7V to 5V, VOUT(MAX) = 5.25V, IQ = 12μA, ISD < 1μA, 3mm × 3mm QFN-16 Package LTC3528 LTC3528-2 1A (ISW), 1MHz Synchronous Step-Up DC/DC with Output DisconnectConverter 94% Efficiency VIN: 0.7V to 5V, VOUT(MAX) = 5.25V, IQ = 12μA, ISD < 1μA, 2mm × 3mm DFN-8 Package LTC3537 600mA , 2.2MHz, Synchronous Step-Up DC/DC Converter with Output Disconnect and 100mA LDO 94% Efficiency VIN: 0.7V to 5V, VOUT(MAX) = 5.25V, IQ = 30μA, ISD < 1μA, 3mm × 3mm QFN-16 Package LTC3539 LTC3539-2 2A (ISW), 1/2MHz, Synchronous Step-Up DC/DC Converter with Output Disconnect 94% Efficiency VIN: 0.7V to 5V, VOUT(MAX) = 5.25V, IQ = 10μA, ISD < 1μA, 2mm × 3mm DFN-8 Package ThinSOT is a trademark of Linear Technology Corporation. 3535f 16 Linear Technology Corporation LT 0109 • PRINTED IN USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 2009