LINEAR High Efficiency 4-Switch Buck-Boost Controller for Precise Output Current Limit - Design Note 499 Tage Bjorklund Owner's Manual User Guide

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High Effi ciency 4-Switch Buck-Boost Controller Provides Accurate Output Current Limit – Design Note 499 Tage Bjorklund Introduction The LTC3789 includes two internal series regulators that

A 4-switch buck-boost converter (Figure 1) is often a provide 5.5V for gate drivers and control circuitry. During better alternative to a transformer-based topology when start-up the current is drawn from VIN (40V maximum a converter’s input voltage can be above or below the rating), but switches to EXTVCC (14V maximum rating) regulated output voltage and isolation is not required. as soon as the voltage on EXTVCC exceeds 4.8V, to A buck-boost converter provides a wider input voltage decrease power loss.

range, better effi ciency and has no need of a bulky

This 4-switch topology allows the output to be dis-

transformer. A buck-boost converter is much more

connected from the input during shutdown. LTC3789

effi cient than a comparable SEPIC converter.

features adjustable soft-start, no reverse current during The LTC®3789 is a buck-boost switching regulator start-up and 1% output voltage accuracy.

controller that operates in current mode at a constant

SW2A SW1

switching frequency. Current mode control simplifi es

VIN VOUT

loop compensation and yields excellent load and line

CIN COUT

transient response with only small output and input

SENSE+

capacitance. Furthermore, an accurate inductor current

RSENSE

limit allows use of a small size inductor.

SENSE–

LTC3789 Features

The internal oscillator can be programmed by a resis- R1

tor, applied voltage, or phase-locked to an external

LTC3789 SENSE+

SENSE– R2

clock within a 200k Hz to 600k Hz frequency range. The

LTC3789’s wide 4V to 38V (40V maximum) input and DN499 F01

output range and seamless, low noise transitions be-

Figure 1. 4-Switch Buck-Boost Converter Using the

tween operating regions is ideal for automotive, telecom

LTC3789

and battery-powered systems. Higher input voltage is easily enabled by adding a high voltage gate driver for the input side MOSFETs, for example the LTC4444-5. Figure 1 shows a simplifi ed schematic of a buck-boost converter. The LTC3789 controls four external N-channel

5V/DIV

MOSFETs (A, B, C, D). The power stage uses one inductor 1A/DIV (L), and a current sense resistor (RSENSE) for current

5V/DIV

mode control and inductor current limit. Figures 2 to 4 show the switch waveforms for buck, buck-boost, and boost modes of operation. The LTC3789 is available in low profi le 28-pin 4mm ×

DN499 F02

1μs/DIV

5mm QFN and narrow SSOP packages.

Figure 2. Buck Mode [VIN = 20V, VOUT = 12V]

12V, 5A Output from a 4V to 38V Input

L, LT, LTC, LTM, Linear Technology, the Linear logo are registered trademarks

Figure 5 shows a buck-boost regulator that takes a 4V

of Linear Technology Corporation. All other trademarks are the property of their

to 38V input and converts it to a fi xed 12V. respective owners.