6

Rev. 08/19/05

SP6128A Low Voltage, Synchronous Step Down PWM Controller

© Copyright 2005 Sipex Corporation

As the figure shows, the SS voltage controls a

variety of signals. First, provided all the exter-

nal fault conditions are removed, an internal

5

µA pull-up at the COMP pin brings the

SP6128A out of shutdown mode. The internal

timing circuit is then activated and controls the

ramp-up of the error amp reference voltage. The

COMP pin is pulled to 0.7V by the internal

clamp and then gradually charges preventing

the error amplifier from forcing the loop to

maximum duty cycle. As the COMP voltage

crosses about 1V (valley voltage of the PWM

ramp), the driver begins to switch the high side

MOSFET with narrow pulses in an effort to

keep the converter output regulated . The

SP6128A operates at low duty cycle as the

COMP voltage increases above 1V. As the error

amp reference ramps upward, the driver pulses

widen until a steady state value is reached and

the output voltage is regulated to the final value

ending the soft start charge cycle.

Hiccup Mode

When the converter enters a fault mode, the

SP6128A holds the high side and low side

MOSFETs off for a finite period of time. Provided

that the SP6128A is enabled, this time is set by the

internal charge of the soft-start capacitor. In the

event of an overcurrent condition, the current

sense comparator sets the fault latch, which in turn

discharge the internal SS capacitor, the COMP pin

and holds the output drivers off. During this con-

dition, the SP6128A stays off for the time it takes

to discharge the COMP pin down to the 0.29V

shutdown threshold. At this point, the fault latch

is reset, but before the SP6128A is allowed to

attempt restart, the COMP pin has to charge back

to 1V before any output switching can be initiated.

Then, the regulator attempts to restart normally by

delivering short gate pulses and if the overcurrent

condition is still present, the cycle will repeat itself.

However, if upon restart, the overcurrent condi-

tion is still present, the SP6128A will detect the

fault and remain in a fault state until the internal

increasing the MOSFET off-time. This protection

scheme minimizes thermal stress to the regulator

components as the overcurrent condition persists.

OPERATION: continued

0.8 V

I(L)

TIME

0.3 V

0.7 V

1 V

0 V

COMP

Internal SS

0 V

SWN

Voltage

Voltage

FAULT

Current

Inductor

0 A

Voltage

Reference

Error Amp

0 V

0 V