LINEAR Industry's First 4-Switch Buck-Boost Controller Achieves Highest Efficiency Using a Single Inductor – Design Note 369

advertisement

Industry’s First 4-Switch Buck-Boost Controller Achieves Highest Efficiency Using a Single Inductor – Design Note 369 Wilson Zhou and Theo Phillips Introduction High Efficiency 4-Switch Buck-Boost Converter One of the most common DC/DC converter problems is Figure 1 shows a simplified LTC3780 4-switch buck- generating a regulated voltage that falls somewhere in the boost converter. When VIN exceeds VOUT, the LTC3780 middle of a wide range of input voltages. When the input operates in buck mode. With switch D on and switch C off, voltage can be above, below or equal to the output voltage, switches A and B turn on and turn off alternately, as they the converter must perform step-down and step-up func- would in a typical synchronous buck regulator. Con- tions. Unlike solutions requiring bulky transformers, the versely, when VIN is lower than VOUT, the LTC3780 oper- LTC®3780 meets these requirements in the most compact ates in boost mode. With switch A on and synchronous and efficient manner, using just one off-the-shelf inductor switch B off, switch C and synchronous switch D turn on and a single current sense resistor. and turn off alternately, behaving as a typical synchro-

nous boost regulator.

The LTC3780 uses a constant frequency current mode architecture which allows seamless transitions between When VIN is close to VOUT, the controller is in buck-boost buck, boost and buck/boost modes with a wide 4V to 30V mode. Switches A and D are on for most of each period. (36V maximum) input and output range. Burst Mode® Brief connections between VIN and ground, and VOUT and

operation and skip cycle mode provide high efficiency ground, are made through the inductor and switches B-D operation at light loads, while forced continuous mode and A-C to regulate the output voltage. In buck-boost and discontinuous mode reduce output voltage ripple by mode, inductor peak-to-peak current is much lower than operating at a constant frequency. A soft-start feature that of SEPIC converters and traditional buck/boost con- reduces output overshoot and inrush currents during verters. Figure 2 shows the inductor current and switch start-up. Overvoltage protection, current foldback and node waveforms.

on-time limitation provide protection for fault conditions,

Low inductor ripple current and the use of synchronous

including short circuit, overvoltage and inductor current

rectifiers allow the LTC3780 to achieve very high

runaway. The LTC3780 is available in low profile 24-pin

, LTC, LT and Burst Mode are registered trademarks of Linear Technology

TSSOP and 32-lead 5mm 5mm QFN packages.

Corporation. Easy Drive is a trademark of Linear Technology Corporation. All other trademarks are the property of their respective owners.

SW2A SW1

VIN VOUT

CIN COUT

10V/DIV

RSENSE

10V/DIV

VOUT 100m V/DIV AC COUPLED

LTC3780 SNS+

R2 2A/DIV

DN369 F01 DN369 F02

5µs/DIV

Figure 1. 4-Switch Buck-Boost Converter Figure 2. Switch Nodes and Inductor Current Waveforms

(VIN = VOUT = 12V)