L DESIGN IDEAS Single-IC Converter Operates Buck and Boost to Provide an Output that is Within the Input Voltage Range by David Burgoon Introduction Generating an output voltage that is always above or below the input voltage range can easily be handled by conventional boost or buck regulators, respectively. However, when the output voltage is within the input voltage range, as in many Li-Ion battery powered applications requiring a 3V or 3.3V output, conventional designs fall short, suffering variously from low efficiency, complex magnetics, polarity inversion and circuit complexity. The LTC3785 buck-boost controller facilitates a simple, efficient, low partscount, single-converter solution that is easy to implement and does not have any of the drawbacks associated with conventional circuits. tion (OVP) and a 2.7V–10V output range. The circuit produces seamless operation throughout the input voltage range, operating as a synchronous buck converter, synchronous boost 2.2nF VOUT 215k 215k 3 127k 470pF 4 42.2k 5 6 49.9k 7 8 9 15 14 Figure 1 shows a synchronous, 4-switch, buck-boost design that provides a 3.3V, 3A output from a 2.7V–10V input—perfect for a Li-Ion and/or loosely regulated wall adapter input. The controller provides shortcircuit protection, offering a choice of burp-mode or latch-off operation for severe overload faults. Other features include soft-start, overvoltage protec- 13 RUN/SS VIN FB VCC LT3785EMS VC ISVIN VSENSE VBST1 ILSET TG1 CCM SW1 RT ISSW1 MODE BG1 NC VDRV BG2 ISVOUT ISSW2 SW2 VBST2 GND Figure 2. Input-side and output-side switch waveforms along with inductor current for buck mode (10VIN) 4.7µF VIN 2.7V TO 10V 22 CMDSH-3 21 20 0.22µF Q1A FDS6894A 19 18 L1 2.2µH TDK RLF7030T 17 16 Q1B VOUT 3.3V 3A 10 CMDSH-3 11 47µF 6.3V 12 0.22µF Q2A FDS6894A Figure 1. Schematic of buck-boost converter using LTC3785 to provide 3.3V at 3A out from a 2.7V–10V source VSW1 5V/DIV IL1 2A/DIV VSW2 5V/DIV VSW2 5V/DIV 1µs/DIV 23 22µF 16V Q2B IL1 2A/DIV VSW2 5V/DIV TG2 24 25 VSW1 5V/DIV IL1 2A/DIV 34 continued on page 36 1 31.6k 2 3.3V, 3A Converter Operates from 2.7V–10V Source VSW1 5V/DIV 127k converter, or a combination of the two through the transition region. At input voltages well above the output, the converter operates in buck mode. Switches Q1A and Q1B commutate the input voltage, and Q2A stays 1µs/DIV Figure 3. Input-side and output-side switch waveforms along with inductor current for boost mode (2.7VIN) 1µs/DIV Figure 4. Input-side and output-side switch waveforms along with inductor current for buck-boost mode (3.8VIN) Linear Technology Magazine • October 2007 L DESIGN IDEAS inrush currents while charging the output caps during startup, as well as minimizing voltage overshoot when starting into light loads. For those applications requiring a power good output on the third channel, the LTC3545-1 version of the part substitutes a PGOOD3 output in place of the MODE/SYNC pin. The option of an external clock is not available on this version, and the part enters Burst Mode operation at light load currents. Minimal Channel Crosstalk High Efficiency A potential problem with multiple with Low Ripple output regulators is the interaction between channels when one of the channels undergoes a load transient. Figure 4 shows the response on channels 2 and 3 to a 0mA to 500mA load step on channel 1. Channels 2 and 3 are each loaded at 400mA. In each case, the crosstalk is on the order of 1mV to 2mV. 100 1 90 EFFICIENCY SW 2V/DIV VOUT 20mV /DIV IL 100mA /DIV 70 60 50 POWER LOSS 30 10 0 0.0001 Figure 5. At low load currents, Burst Mode operation improves efficiency without degrading output voltage ripple. 0.01 40 20 1µs/DIV 0.1 VIN = 2.5V VIN = 3.6V VIN = 4.2V POWER LOSS (W) EFFICIENCY (%) 80 TA = 25°C VOUT = 2V fOSC = 2.25MHz SINGLE CHANNEL Burst Mode OPERATION 0.001 0.01 0.1 LOAD CURRENT (A) 0.001 1 0.0001 Figure 6. Burst Mode operation maintains high efficiency at low load currents. LTC3785, continued from page 34 36 100 10 90 EFFICIENCY 80 POWER LOSS 60 50 40 30 20 10 0 0.001 0.1 VIN = 2.7V VIN = 3V VIN = 4.2V VIN = 10V VIN = 2.7V VIN = 3V VIN = 4.2V VIN = 10V 0.01 0.1 1 LOAD CURRENT (A) POWER LOSS (W) 1 70 BURST MODE OPERATION EFFICIENCY (%) on, connecting L1 to the output. As the input voltage is reduced and approaches the output, the converter approaches maximum duty cycle on the input (buck) side of the bridge, and the output (boost) side of the bridge starts to switch, thus entering the buck-boost or 4-switch region of operation. As the input is reduced further, the converter enters the boost region at the minimum boost duty cycle. Switch Q1A stays on, connecting the inductor to the input, while switches Q2A and Q2B commutate the output side of the inductor between the output capacitor and ground. In boost mode, this converter has the ability to limit input current and to shut down and disconnect the source from the output—two very desirable features that a conventional boost converter cannot provide. Figures 2, 3, and 4 show input-side and outputside switch waveforms along with inductor current for buck (10VIN), boost (2.7VIN), and buck-boost (3.8VIN) modes of operation. 0.01 10 0.001 Figure 5. Efficiency in normal mode and Burst Mode operation At low load currents, the LTC3545 operates in either pulse-skipping mode or Burst Mode operation depending on the state of the MODE/SYNC pin. Though pulse-skipping mode exhibits lower output ripple, the ripple in Burst Mode operation is still quite low while maintaining the added advantage of better efficiency at the lightest loads. The Burst Mode operation and Burst Mode efficiency are shown in Figures 5 and 6. Conclusion The LTC3545 is a unique part with tremendous flexibility. It greatly simplifies system and board design where multiple voltage supply rails are needed without sacrificing the features and performance found in individual regulators. The LTC3545 is ideally suited for battery powered applications where multiple or isolated voltage rails are required and board space is at a premium. L limit. Even higher efficiencies are possible by using a larger inductor and better MOSFETs as they become available. Efficiency at 10V in would benefit from an inductor with a lowloss ferrite core, especially at light loads. This circuit easily fits in 0.6in2 with components on both sides of the board. The curves show how Burst Mode operation improves efficiency at extremely light loads, dramatically enhancing battery life in applications such as memory that must maintain housekeeping functions even when the system is turned off. 95% Efficiency Conclusion Figure 5 shows efficiency in both normal (not forced continuous conduction) and Burst Mode operation. Very high efficiency of 95% is achieved at typical loads. This level of performance results in part from sophisticated controller features including high side drivers for N-channel MOSFETs and RDS(ON) current sensing for current The LTC3785 buck-boost controller overcomes the deficiencies of traditional designs with a smooth-transition, 4-switch, single-IC solution. It is elegant in its simplicity, high in efficiency and requires only a small number of inexpensive external components. The LTC3785 is available in a small 4mm × 4mm QFN package as well as a 28-lead SSOP. L Linear Technology Magazine • October 2007