High Efficiency 4-Switch Buck-Boost Controller Provides Accurate Output Current Limit

introduction

When a converter's input voltage can be above or below the regulated output and isolation is not required, the four-switch buck-boost converter (Figure 1) is often a better alternative to transformer-based topologies. Buck-boost converters offer a wider input voltage range, higher efficiency, and do not require a large, bulky transformer. Buck-boost converters are much more efficient than equivalent SEPIC converters.

The LTC3789 is a buck-boost switching regulator controller that operates in current mode with a constant switching frequency. Current mode control simplifies loop compensation and achieves excellent load and line transient response while requiring very small output and input capacitors. In addition, an accurate inductor current limit allows the use of a small inductor size.

LTC3789 Features

The internal oscillator can be set with a resistor, applied voltage or synchronized to an external clock with a frequency range of 200kHz to 600kHz. The LTC3789's wide 4V to 38V (40V maximum) input and output range, as well as seamless and low noise transitions between operating regions make it ideal for automotive, telecom and battery-powered systems. Higher input voltages can be easily achieved by adding a high voltage gate driver (such as the LTC4444-5) for the input-side MOSFET.

Figure 1 shows a simplified schematic of a buck-boost converter. The LTC3789 controls four external N-channel MOSFETs (A, B, C, D). The power stage uses an inductor (L) and a current sense resistor (RSENSE) to provide current mode control and inductor current limiting. Figures 2 to 4 show the switching waveforms for buck, buck-boost and boost modes of operation.

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The LTC3789 is available in low profile 28-lead 4mm x 4mm QFN and narrow SSOP packages.

Generates 12V/5A output from 4V ~ 38V input

Figure 5 shows a buck-boost regulator that can take a 4V to 38V input and convert it to a fixed 12V output. The LTC3789 includes two internal series regulators that provide 5.5V to the gate driver and control circuitry. During startup, current is drawn from VIN (40V maximum rating), but once the voltage on the EXTVCC pin exceeds 4.8V, it immediately switches to EXTVCC (14V maximum rating) to reduce power losses.

The four-switch topology allows the output to be disconnected from the input during shutdown. The LTC3789 features adjustable soft-start, no reverse current during startup, and 1% output voltage accuracy.

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The MODE/PLLIN pin can be used to select between pulse-skipping mode and forced continuous mode operation, or to synchronize the IC to an external clock. Pulse-skipping mode allows for lowest ripple at light loads, while forced continuous mode operates at a constant frequency for noise-sensitive applications.

A power good output pin (PGOOD) indicates when the output is within 10% of its set point.

Accurate Output (or Input) Current Limit The optional output current feedback loop shown in Figure 6 supports battery charging and other constant current applications where accurate output current limiting is important. The output current limit function provides a constant current characteristic. The output current is limited to a constant level even when the output voltage is pulled low due to an overcurrent condition. Alternatively, the sense resistor can be simply moved to the input side to accurately control the maximum input current.

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in conclusion

The LTC3789 is a constant frequency, current mode, buck-boost switching regulator controller that accepts a wide input voltage range of 4V to 38V. The single inductor topology enables high power density and high efficiency in a compact footprint. Its optional current limit function is very useful for applications that require accurate current control, such as battery charging.