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LTC3532

3532fc

be minimized. Note that if the load current applied during

forced Burst Mode operation (BURST pin is grounded)

exceeds the current that can be supplied, the output voltage

will start to droop and the IC will automatically come out

of Burst Mode operation and enter ﬁxed frequency mode,

raising VOUT. Once regulation is achieved, the IC will then

enter Burst Mode operation once again, and the cycle will

repeat, resulting in about 4% output ripple. Note that Burst

Mode operation is inhibited during soft-start.

Burst Mode Operation to Fixed Frequency Transient

Response

In Burst Mode operation, the compensation network is

not used and VC is disconnected from the error ampliﬁer.

During long periods of Burst Mode operation, leakage

currents in the external components or on the PC board

could cause the compensation capacitor to charge (or

discharge), which could result in a large output transient

when returning to ﬁxed frequency mode of operation, even

at the same load current. To prevent this, the LTC3532

incorporates an active clamp circuit that holds the voltage

on VC at an optimal voltage during Burst Mode operation.

This minimizes any output transient when returning to

ﬁxed frequency mode operation. For optimum transient

response, Type 3 compensation is also recommended

to broad band the control loop and roll off past the two

pole response of the output LC ﬁlter. (See Closing the

Feedback Loop.)

Soft-Start

The soft-start function is combined with shutdown. When

the SHDN/SS pin is brought above 1V typical, the IC is

enabled but the EA duty cycle is clamped from VC. A de-

tailed diagram of this function is shown in Figure 5. The

components RSS and CSS provide a slow ramping voltage

on SHDN/SS to provide a soft-start function. To ensure

that VC is not being clamped, SHDN/SS must be raised

above 2.4V. To enable Burst Mode operation, SHDN/SS

must be raised to within 0.5V of VIN.

–

VIN

ERROR AMP

1.22V

15μA

CP1

VOUT

SHDN/SS

CSS

ENABLE SIGNAL

RSS

SOFT-START

CLAMP

TO PWM

COMPARATORS

CHIP

ENABLE

3532 F05

–

VCI

Figure 5. Soft-Start Circuitry

OPERATIO

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