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LTC3113

3113f

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 efﬁciency

and stability in comparison to the traditional four-switch

buck-boost converters.

Burst Mode Operation

With the BURST pin held high, the buck-boost converter

operates utilizing a variable frequency switching algorithm

designed to improve efﬁciency at light load and reduce

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

the inductor is charged with ﬁxed peak amplitude current

pulses and as a result only a fraction of the maximum

output current can be delivered when in this mode.

These current pulses are repeated as often as necessary

to maintain the output regulation voltage. The maximum

output current, IMAX, which can be supplied in Burst Mode

operation is dependent upon the input and output voltage

as given by the following formula:

MAX ≅

•

IN + VOUT

•

η A

()

where IPK is the Burst Mode peak current limit in amps

and is the

η efﬁciency.

If the buck-boost load exceeds the maximum Burst Mode

current capability, the output rail will lose regulation. In

Burst Mode operation, the error ampliﬁer is conﬁgured

in a low power mode of operation and used to hold the

compensation pin, VC, to reduce transients that may oc-

cur during transitions from Burst Mode to PWM mode

operation.

OPERATION

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