LTC3520 Datasheet, PDF(12/24 Page) Linear Technology – Synchronous 1A Buck-Boost and 600mA Buck Converters

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LTC3520 Datasheet, PDF (12/24 Pages) Linear Technology – Synchronous 1A Buck-Boost and 600mA Buck Converters

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LTC3520

OPERATION

This leads to a reduced output current capability at large

step-down ratios. In contrast, the LTC3520 performs cur-

rent limiting prior to the addition of the slope compensation

ramp and therefore achieves a peak inductor current limit

that is independent of duty cycle.

Soft-Start

The buck converter incorporates a voltage mode soft-start

circuit which is adjustable via the value of an external

soft-start capacitor, CSS. The typical soft-start duration

is given by the following equation:

tSS(ms) = 0.15CSS(nF)

The buck converter remains in regulation during soft-start

and will therefore respond to output load transients which

occur during this time. In addition, the output voltage rise-

time has minimal dependency on the size of the output

capacitor or load current.

Error Ampliï¬er and Compensation

The LT3520 buck converter utilizes an internal trans-

conductance error ampliï¬er. Compensation of the feed-

back loop is performed internally to reduce the size of the

application circuit and simplify the design process. The

compensation network has been designed to allow use of

a wide range of output capacitors while simultaneously

ensuring a rapid response to load transients.

BUCK-BOOST CONVERTER OPERATION

PWM Mode Operation

When the PWM pin is held high, the LTC3520 buck-boost

converter operates in a constant-frequency PWM mode us-

ing voltage mode control. A proprietary switching algorithm

allows the converter to switch between buck, buck-boost,

and boost modes without discontinuity in inductor cur-

rent or loop characteristics. The switch topology for the

buck-boost converter is shown in Figure 1.

When the input voltage is signiï¬cantly greater than the

output voltage, the buck-boost converter operates in

buck mode. Switch D turns on continuously and switch C

remains off. Switches A and B are pulse width modulated

to produce the required duty cycle to support the output

regulation voltage. As the input voltage decreases, switch

A remains on for a larger portion of the switching cycle.

When the duty cycle reaches approximately 85%, the

switch pair AC begins turning on for a small fraction of the

switching period. As the input voltage decreases further,

the AC switch pair remains on for longer durations and

the duration of the BD phase decreases proportionally. As

the input voltage drops below the output voltage, the AC

phase will eventually increase to the point that there is no

longer any BD phase. At this point, switch A remains on

continuously while switch pair CD is pulse width modu-

lated to obtain the desired output voltage. In this case, the

converter is operating solely in boost mode.

This switching algorithm provides a seamless transition

between operating modes and eliminates discontinuities

in average inductor current, inductor current ripple, and

loop transfer function throughout all three operational

modes. These advantages result in increased efï¬ciency

and stability in comparison to the traditional four-switch

buck-boost converter.

LTC3520

PGND1 PGND1

3520 F01

Figure 1. Buck-Boost Switch Topology

3520f

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