LTC3521 datasheet(11/20 Pages) LINER

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LTC3521

3521f

modes. These advantages result in increased efficiency

and stability in comparison to the traditional 4-switch

buck-boost converter.

Error Amplifier and Compensation

The buck-boost converter utilizes a voltage mode error

amplifierwithaninternalcompensationnetworkasshown

in Figure 2.

this case, the increased bandwidth created by decreasing

R2 is used to counteract the reduced converter bandwidth

caused by the large output capacitor.

Current Limit Operation

The buck-boost converter has two current limit circuits.

The primary current limit is an average current limit circuit

which injects an amount of current into the feedback node

which is proportional to the extent that the switch A cur-

rent exceeds the current limit value. Due to the high gain

of this loop, the injected current forces the error amplifier

outputtodecreaseuntiltheaveragecurrentthroughswitch

A decreases approximately to the current limit value. The

average current limit utilizes the error amplifier in an ac-

tive state and thereby provides a smooth recovery with

little overshoot once the current limit fault condition is

removed. Since the current limit is based on the average

current through switch A, the peak inductor current in

current limit will have a dependency on the duty cycle

(i.e., on the input and output voltages in the overcurrent

condition).

The speed of the average current limit circuit is limited by

the dynamics of the error amplifier. On a hard output short,

it would be possible for the inductor current to increase

substantially beyond current limit before the average cur-

rent limit circuit would react. For this reason, there is a

second current limit circuit which turns off switch A if the

current ever exceeds approximately 165% of the average

current limit value. This provides additional protection in

the case of an instantaneous hard output short.

Reverse Current Limit

The reverse current comparator on switch D monitors

the inductor current entering PVOUT. When this current

exceeds 375mA (typical), switch D will be turned off for

the remainder of the switching cycle.

operaTion

0.6V

GND

PVOUT

LTC3521

VOUT

FB1

3521 F02

–

Figure 2. Buck-Boost Error Amplifier and Compensation

Notice that resistor R2 of the external resistor divider

networkplaysanintegralroleindeterminingthefrequency

response of the compensation network. The ratio of R2 to

R1 must be set to program the desired output voltage but

this still allows the value of R2 to be adjusted to optimize

thetransientresponseoftheconverter.Increasingthevalue

of R2 generally leads to greater stability at the expense of

reduced transient response speed. Increasing the value of

R2canyieldsubstantialtransientresponseimprovementin

caseswherethephasemarginhasbeenreducedduetothe

use of a small value output capacitor or a large inductance

(particularly with large boost step-up ratios). Conversely,

decreasing the value of R2 increases the loop bandwidth

which can improve the speed of the converter’s transient

response. This can be useful in improving the transient

response if a large valued output capacitor is utilized. In

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