LTC3520 datasheet(13/24 Pages) LINER

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LTC3520

3520f

Error Ampliﬁer

The error ampliﬁer operates in voltage mode. Appropriate

loop compensation components must be utilized around

the ampliﬁer (between the FB1 and VC1 pins) in order

to ensure stable operation. For improved bandwidth, an

additional RC feedforward network can be placed across

the upper feedback divider resistor.

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

output to decrease until the average current through switch

A decreases approximately to the current limit value. The

average current limit utilizes the error ampliﬁer in an active

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 150% 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 the VOUT1 pin. If this current

exceeds 560mA (typical) switch D is turned off for the

remainder of the switching cycle.

Burst Mode Operation

With the PWM1 pin held low, the buck-boost converter

operates utilizing a variable frequency switching algorithm

designed to improve efﬁciency at light loads and reduce

the standby current at zero load. In Burst Mode operation,

the inductor is charged with ﬁxed peak amplitude current

pulses. These current pulses are repeated as often as

necessary to maintain the output regulation voltage. The

typical output current which can be supplied in Burst Mode

operation is dependent upon the input and output voltage

as given by the following formula:

OUT MAX BURST

OUT

(),

.•

013

In Burst Mode operation, the error ampliﬁer is not used but

is instead placed in a low current standby mode to reduce

supply current and improve light load efﬁciency.

Soft-Start

The buck-boost 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 converter remains in regulation during soft-start and

will therefore respond to output load transients that occur

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

has minimal dependency on the size of the output capaci-

tor or load. During soft-start, the buck-boost converter is

forced into PWM operation regardless of the state of the

PWM1 pin.

Transition From Burst to PWM Operation

In Burst Mode operation, the compensation network is

not used and the VC1 pin is disconnected from the error

ampliﬁer. During long periods of Burst Mode operation,

leakage currents in the external components or on the

PCB could cause the compensation capacitor to charge

or discharge resulting in a large output transient when

returning to the ﬁxed frequency mode of operation. To

prevent this from happening, the LTC3520 employs an

active clamp circuit that holds the voltage on the VC1 pin

to the optimal level during Burst Mode operation. This

minimizes any output transient when returning to ﬁxed

frequency operation.

OPERATION

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