LTC3522_15 Datasheet, PDF(10/20 Page) Linear Technology – Synchronous 400mA Buck-Boost and 200mA Buck Converters

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LTC3522_15 Datasheet, PDF (10/20 Pages) Linear Technology – Synchronous 400mA Buck-Boost and 200mA Buck Converters

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LTC3522

OPERATION

Soft-Start

The buck converter has an internal voltage mode soft-start

circuit with a nominal duration of 600Î¼s. The 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 LT3522 buck converter utilizes an internal transcon-

ductance error ampliï¬er. Compensation of the feedback

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 rapid response to load transients.

PGOOD2 Comparator

The PGOOD2 pin is an open-drain output which indicates

the status of the buck converter. If the buck output volt-

age falls 7.7% below the regulation voltage, the PGOOD2

open-drain output will pull low. The output voltage must

rise 2.5% above the falling threshold before the pull-down

will turn off. In addition, there is a 60Î¼s typical deglitch-

ing delay in the ï¬ag in order to prevent false trips due

to voltage transients on load steps. The PGOOD2 output

will also pull low during overtemperature shutdown and

undervoltage lockout to indicate these fault conditions.

The PGOOD2 output is only active if the buck converter

is enabled.

BUCK-BOOST CONVERTER OPERATION

PWM Mode Operation

When the PWM pin is held high, the LTC3522 buck-boost

converter operates in a constant frequency PWM mode with

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.

PVIN1 A SW1A

SW1B D VOUT1

LTC3522

PGND1 PGND2

3522 F01

Figure 1. Buck-Boost Switch Topology

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 modu-

lated 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. At this point,

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 4-switch

buck-boost converter.

Error Ampliï¬er and Compensation

The buck-boost converter utilizes a voltage mode error

ampliï¬er with an internal compensation network as shown

in Figure 2.

Notice that resistor R2 of the external resistor divider

network plays an integral role in determining the frequency

3522fa

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