Single-IC Converter Operates Buck and Boost to Provide an Output that is Within the Input Voltage Range

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 parts-count, single-converter solution that is easy to implement and does not have any of the drawbacks associated with conventional circuits.

3.3V, 3A Converter Operates from 2.7V–10V Source

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 short-circuit protection, offering a choice of burp-mode or latch-off operation for severe overload faults. Other features include soft-start, overvoltage protection (OVP) and a 2.7V–10V output range.

Figure 1. Schematic of buck-boost converter using LTC3785 to provide 3.3V at 3A out from a 2.7V–10V source.

The circuit produces seamless operation throughout the input voltage range, operating as a synchronous buck converter, synchronous boost 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 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 output-side switch waveforms along with inductor current for buck (10VIN), boost (2.7VIN), and buck-boost (3.8VIN) modes of operation.

Figure 2. Input-side and output-side switch waveforms along with inductor current for buck mode (10VIN).

Figure 3. Input-side and output-side switch waveforms along with inductor current for boost mode (2.7VIN).

Figure 4. Input-side and output-side switch waveforms along with inductor current for buck-boost mode (3.8VIN).

95% Efficiency

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 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 low-loss 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.

Figure 5. Efficiency in normal mode and Burst Mode operation.

Conclusion

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.

作者

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David Burgoon