LTC3535 Dual Channel 550mA 1MHz Synchronous Step-Up DC/DC Converter DESCRIPTION FEATURES Two Independent Step-Up Converters n Each Channel Delivers 3.3V at 100mA from a Single Alkaline/NiMH Cell or 3.3V at 200mA from Two Cells nV Start-Up Voltage: 680mV IN n 1.5V to 5.25V V OUT Range n Up to 94% Efficiency n Output Disconnect n 1MHz Fixed Frequency Operation nV > V IN OUT Operation n Integrated Soft-Start n Current Mode Control with Internal Compensation n Burst Mode® Operation with 9µA I Each Channel Q n Internal Synchronous Rectifier n Logic Controlled Shutdown (I < 1µA) Q n Anti-Ring Control n 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. n 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 L, LT, LTC, LTM, Linear Technology, Burst Mode and the Linear logo are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. Medical Instruments Noise Canceling Headphones Energy Harvesting Bluetooth Headsets TYPICAL APPLICATION Efficiency vs Load Current 4.7µH OFF ON VIN 0.8V TO 1.5V SW1 SHDN1 OFF ON VOUT2 3.3V 50mA 10µF LTC3535 2.2µF 10µF VOUT2 VIN2 FB1 SHDN2 FB2 1.78M GND SW2 GND 100 VOUT1 1.8V 100mA VOUT1 90 80 511k 1M 1M EFFICIENCY (%) VIN1 70 VOUT = 1.8V VOUT = 3.3V 60 50 40 30 20 4.7µH 3535 TA01 10 VIN = 1.2V 0 0.01 0.1 10 100 1 LOAD CURRENT (mA) 1000 3535 TA01b 3535fa 1 LTC3535 ABSOLUTE MAXIMUM RATINGS PIN CONFIGURATION (Note 1) TOP VIEW VIN1, VIN2 Voltage.......................................... –0.3V to 6V SW1, SW2 Voltage DC............................................................. –0.3V to 6V Pulsed <100ns.......................................... –0.3V to 7V SHDN1, SHDN2, FB1, FB2 Voltage................ –0.3V to 6V VOUT1, VOUT2................................................. –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 × 3mm) PLASTIC DFN θJA = 43°C/W, θJC(PAD) = 3°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 MIN TYP MAX 0.68 0.8 UNITS V l 0.5 5 V l 1.5 5.25 V l 1.165 VFB = 1.30V 1.195 1.225 V 1 50 nA Quiescent Current—Shutdown VSHDN= 0V, Not Including Switch Leakage, VOUT = 0V 0.01 1 µA Quiescent Current—Active Measured on VOUT, Non-Switching 250 500 µA Quiescent Current—Burst Measured on VOUT, FB > 1.230V N-Channel MOSFET Switch Leakage Current VSW = 5V 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 P-Channel MOSFET Switch On Resistance VOUT = 3.3V N-Channel MOSFET Current Limit Current Limit Delay to Output (Note 3) Maximum Duty Cycle VFB = 1.15V Ω 0.6 Ω l 550 750 mA 60 ns l 87 90 % 3535fa 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 0 l Switching Frequency UNITS l 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 43°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 EFFICIENCY 90 60 10 50 POWER LOSS 40 1 30 20 VIN = 1.0V VIN = 1.2V VIN = 1.5V 10 0.1 1 10 100 LOAD CURRENT (mA) 0.1 0.01 1000 3535 G01 EFFICIENCY 90 70 60 10 50 POWER LOSS 40 30 1 VIN = 1.2V VIN = 1.8V VIN = 2.4V VIN = 3.0V 20 10 0 0.01 100 100 0.1 1 10 100 LOAD CURRENT (mA) POWER LOSS (mW) 70 No-Load Input Current vs VIN 1000 80 100 POWER LOSS (mW) EFFICIENCY (%) 80 0 0.01 100 EFFICIENCY (%) 90 1000 0.1 0.01 1000 VOUT = 5V 80 VOUT = 3.3V 70 IIN (µA) 100 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 3535fa 3 LTC3535 TYPICAL PERFORMANCE CHARACTERISTICS (Each Channel) TA = 25°C, unless otherwise noted. Efficiency vs Load Current and VIN for VOUT = 5V 100 10 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 1000 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 VOUT = 1.8V L = 4.7µH LOAD CURRENT (mA) 80 60 50 40 30 40 LEAVE BURST 20 ENTER BURST 15 10 20 5 10 2.0 2.5 3.0 VIN (V) 3.5 0 4.5 4.0 1 1.25 VIN (V) LEAVE BURST 25 ENTER BURST 15 10 1.5 2.0 VIN (V) 2.5 10 3.0 3535 G08c 1.25 1 1.5 1.75 2 3535 G08b Oscillator Frequency Change vs VOUT 30 ENTER BURST 20 0 1.0 NORMALIZED TO VOUT = 3.3V 2 LEAVE BURST 10 1.0 15 VIN (V) 1 0 –1 –2 5 0 ENTER BURST 20 3 VOUT = 5V L = 4.7µH 40 20 25 0 1.5 50 30 LEAVE BURST 30 Burst Mode Threshold Current vs VIN LOAD CURRENT (mA) LOAD CURRENT (mA) 35 3526 G06 3535 G08a Burst Mode Threshold Current vs VIN VOUT = 3.3V L = 4.7µH 1.15 5 3535 G07 40 VOUT = 2.5V L = 4.7µH 35 FREQUENCY CHANGE (%) DELAY (µs) 70 1.05 Burst Mode Threshold Current vs VIN LOAD CURRENT (mA) 30 0.95 VIN (V) Burst Mode Threshold Current vs VIN 25 1.5 0.85 3535 G05 Start-Up Delay Time vs VIN 1.0 0.75 VIN (V) 90 0 10 0.65 4.5 4.0 3535 G03 100 VOUT = 3.3V VOUT = 2.5V 300 IOUT (mA) 60 POWER LOSS (mW) 70 10000 VOUT = 3.3V 350 80 EFFICIENCY (%) 400 1000 EFFICIENCY 90 LOAD (Ω) 100 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 3535fa 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 NMOS 0.40 4 2 0 –2 –4 –6 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) 1.0 0.9 70 0.7 –50 90 0.80 10.0 0.25 0.75 9.5 0 0.70 1mA LOAD –0.25 0.65 NO LOAD 0.60 8.0 –0.75 0.55 7.5 80 100 0.50 –50 25 0 25 50 TEMPERATURE (°C) Fixed Frequency Switching Waveform and VOUT Ripple VOUT 10mV/DIV AC-COUPLED VIN = 1.2V 500ns/DIV VOUT = 3.3V AT 100mA COUT = 10µF 75 100 3535 G16 MEASURED ON VOUT 7.0 1.5 2.0 2.5 3.0 3.5 VOUT (V) SW PIN 2V/DIV VOUT 20mV/DIV AC-COUPLED INDUCTOR CURRENT 0.2A/DIV 4.0 4.5 5.0 3535 G15 VOUT and IIN During Soft-Start Burst Mode Waveforms SW PIN 2V/DIV Burst Mode Quiescent Current vs VOUT 3526 G14 3535 G13 90 8.5 –0.50 –1.00 20 40 60 –60 –40 –20 0 TEMPERATURE (°C) 70 9.0 IQ (µA) NORMALIZED TO 25°C VIN (V) 0.50 –10 10 30 50 TEMPERATURE (°C) 3535 G12 Start-Up Voltage vs Temperature VFB vs Temperature –30 3535 G11 3535 G10 CHANGE IN VFB (%) 1.1 0.8 –8 0.35 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 VOUT 1V/DIV 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 3535fa 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 3535fa 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. 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, Exposed Pad Pin 13): Signal and Power Ground. Provide a short direct PCB path between GND and the (–) side of the input and output capacitors. 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. 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. 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 1µF ceramic decoupling capacitor from this pin to ground. SHDN2 (Pin 8): Logic Controlled Shutdown Input for Channel 2. There is an internal 4MΩ pull-down on this pin. SHDN = High: Normal operation. SHDN = Low: Shutdown, quiescent current < 1µA. 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)] VIN1 (Pin 10): Battery Input Voltage for Channel 1. Connect a minimum of 1µF ceramic decoupling capacitor from this pin to ground. SHDN1 (Pin 11): Logic Controlled Shutdown Input for Channel 1. There is an internal 4MΩ pull-down on this pin. SHDN = High: Normal operation. SHDN = Low: Shutdown, quiescent current < 1µA. 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)]. 3535fa 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 FB1 – + 4M Σ IPK UVLO IZERO IZERO COMP CLK1 1MHz OSC R1 ERROR AMP SLEEP COMP START-UP LOGIC COUT1 10µF SLOPE COMP + – IPK COMP VREF1 VREF R2 12 + – MODE CONTROL VREF CLAMP 5 VIN2 0.8V TO 5V CIN2 2.2µF TSD THERMAL SHUTDOWN L2 4.7µH 7 WAKE CSS SW2 VIN2 VIN2 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 – + 4M Σ IPK COMP VREF2 IPK UVLO IZERO 1MHz OSC CLK2 IZERO COMP COUT2 10µF R3 ERROR AMP SLEEP COMP START-UP LOGIC R4 9 SLOPE COMP + – VREF FB2 + – MODE CONTROL VREF CLAMP THERMAL SHUTDOWN TSD WAKE CSS GND EXPOSED PAD GND 3 13 6 3535 BD 3535fa 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. 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. 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 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. Soft-Start 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. An internal oscillator (independent for each channel) sets the switching frequency to 1MHz. 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 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. Oscillator Shutdown Shutdown is accomplished by pulling the SHDN pin below 0.3V and enabled by pulling the SHDN pin above 0.8V. Although SHDN can be driven above VIN or VOUT (up to the absolute maximum rating) without damage, the LTC3535 has a proprietary test mode that may be engaged if SHDN is held in the range of 0.5V to 1V higher than the greater of VIN or VOUT. If the test mode is engaged, normal PWM switching action is interrupted, which can cause undesirable operation in some applications. Therefore, in applications where SHDN may be driven above VIN, a resistor divider or other means must be employed to keep the SHDN voltage below (VIN + 0.4V) to prevent the possibility of 3535fa 9 LTC3535 OPERATION (Refer to Block Diagram) the test mode being engaged. Refer to Figure 1 for two possible implementations. LTC3535 Zero Current Comparator LTC3535 VIN 4M ±30% 4M ±30% SHDN R SHDN 3535 F01 VCNTRL 1M R > (VCNTRL/VIN + 0.4) – 1)MΩ independent of input or output voltage, unless VOUT falls below 0.7V, in which case the current limit is cut in half. ZETEX ZC2811E 1M VCNTRL Figure 1. Recommended Shutdown Circuits When Driving SHDN Above VIN 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). Error Amplifier Anti-Ringing Control 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. 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. 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. 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, Output Disconnect The LTC3535 is designed to allow 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. It also allows for inrush current limiting at turn-on, 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. 3535fa 10 LTC3535 OPERATION (Refer to Block Diagram) 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. APPLICATIONS INFORMATION VIN > VOUT OPERATION PCB LAYOUT GUIDELINES 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. The high speed operation of the LTC3535 demands careful attention to board layout. A careless layout will result in reduced performance. Figure 2 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. 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). SHDN VOUT1 VIN1 GND GND 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. VOUT2 SHDN VIN2 Figure 2. Recommended Component Placement 3535fa 11 LTC3535 APPLICATIONS INFORMATION COMPONENT SELECTION Table 1. Recommended Inductors Inductor Selection 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> ( VIN(MIN) • VOUT(MAX ) – VIN(MIN) Ripple • VOUT(MAX) ) 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 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. 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 3535fa 12 LTC3535 APPLICATIONS INFORMATION 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. 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. 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. Table 2. Capacitor Vendor Information 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. 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 TYPICAL APPLICATION Single Cell to 3.3V Converter with 20 Seconds of Holdup with 30mA Load VOUT 3.3V 30mA 4.7µH 499k VIN 0.8V TO 1.5V SW VIN1 SHDN1 2.2µF VOUT1 VOUT2 10µF LTC3535 2.2µF VIN2 FB1 SHDN2 FB2 GND SW2 GND 1.5M + VHOLDUP CHOLD* 0.47F 4.25V 1M VIN 1V/DIV 1.78M VHOLDUP 2V/DIV 392k 1M VOUT 2V/DIV 5s/DIV 4.7µH 3535 TA02b 3535 TA02 *POWERSTOR PA-5R0H474-R 3535fa 13 LTC3535 PACKAGE DESCRIPTION DC Package 12-Lead Plastic DFN (3mm × 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 3.00 ± 0.10 (4 SIDES) R = 0.115 TYP 7 0.40 ± 0.10 12 2.38 ± 0.10 1.65 ± 0.10 PIN 1 NOTCH R = 0.20 OR 0.25 × 45° CHAMFER PIN 1 TOP MARK (SEE NOTE 6) 0.200 REF 0.75 ± 0.05 6 1 0.23 ± 0.05 0.45 BSC 2.25 REF 0.00 – 0.05 (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 3535fa 14 LTC3535 REVISION HISTORY REV DATE DESCRIPTION PAGE NUMBER A 9/10 Updated Applications section 1 Updated Pin Configuration 2 Updated Note 6 3 Updated Pins 3, 6 and 13 text 7 Updated Shutdown section 9, 10 Corrected CHOLD capacitor value in Typical Application 13 Added new Typical Application and Updated Related Parts table 16 3535fa 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 TYPICAL APPLICATION 3.3V Converter Operates from a Single Cell or from Harvested Thermal Energy, as Low as 1°C ∆T TEG FERROTEC 9500/127/100B COILCRAFT LPR6235-752SML T1 1:100 47µF • COILCRAFT XFL4020-472 4.7µH SINGLE AAA CELL 1nF C1 • VSTORE + VOUT2_EN 330pF C2 LTC3108 VOUT 2.35V PEAK SW D1A + C1 1.5mF SHDN1 VIN2 VOUT1 3.3V VOUT2 10µF LTC3535 FB2 1M 4.7µH 1µF 1µF 3535 TA03 COILCRAFT XFL4020-472 D1B GND 1.78M 30.1k SHDN2 VLDO VOUT FB1 2.2µF GND SW2 GND VS2 VAUX 2.2µF VOUT2 VS1 SW1 VIN1 PGD BAT54C RELATED PARTS PART NUMBER DESCRIPTION COMMENTS 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 Disconnect Converter 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 3535fa 16 Linear Technology Corporation LT 0910 REV A • PRINTED IN USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com LINEAR TECHNOLOGY CORPORATION 2009