SP6123 datasheet(6/18 Pages) EXAR

6 / 18 page

Date: 9/13/04

SP6123 Low Voltage, Synchronous Step Down PWM Controller

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 SP6123

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

SP6123 holds the high side and low side

MOSFETs off for a finite period of time. Pro-

vided that the SP6123 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 capaci-

tor, the COMP pin and holds the output drivers

off. During this condition, the SP6123 stays off

for the time it takes to discharge the COMP pin

down to the 0.27V shutdown threshold. At this

point, the fault latch is reset, but before the

SP6123 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. How-

ever, if upon restart, the overcurrent condition is

still present, the SP6123 will detect the fault and

remain in a fault state until COMP reaches about

VCC-1V thereby increasing the MOSFET off-

time. This protection scheme minimizes ther-

mal stress to the regulator components as the

overcurrent condition persists.

The simplified waveforms that describe the hic-

cup mode operation are shown below.

200 mV

3 V

0.3 V

VCOMP

1.0 V

VSWN

VBST

VCC-VSWN

Voltage

TIME

0 V

0.8 V

I(L)

TIME

0.3 V

0.7 V

1 V

0 V

VOUT = VREF * (1+R1/R2)

COMP

Internal SS

0 V

SWN

Voltage

FAULT

Current

Inductor

0 A

Voltage

Reference

Error Amp

0 V

V(VCC)

0 V

V(VCC)

OPERATION

SP6123 Datasheet(PDF) 6 Page - Exar Corporation

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