SP6121 datasheet(6/16 Pages) SIPEX

6 / 16 page

Date: 5/25/04

SP6121 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 SP6121

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

SP6121 holds the high side and low side

MOSFETs off for a finite period of time. Pro-

vided that the SP6121 is enabled, this time is set

by the internal charge of the SS capacitor. In the

event of an over-current 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 condition, the SP6121 stays off for

the time it takes to discharge the COMP pin

down below the 0.3V shutdown threshold. As

soon as the COMP pin reaches 0.3V, the fault

latch is reset and the SP6121 is allowed to

attempt restart just like during a normal soft start

cycle. The COMP pin has to charge back to 1V

before any output switching can take place. At

this point, the regulator attempts to restart nor-

mally by delivering short gate pulses to the

output switches. If the over-current condition

persists, the regulator will be kept off for the

total time that it takes to charge the internal soft-

start capacitor to within 1V from the input

supply voltage VCC plus the time required by the

COMP voltage to cross the 1V threshold. This

total time is typically several milli-seconds and

minimizes thermal stress to the regulator com-

ponents as the over-current condition persists.

The waveforms that describe the hiccup mode

operation are shown in Figure 2.

160 mV

3 V

0 V

1.0 V

V(VCC)

V(ISET)-V(ISENSE)

PDRV

Voltage

TIME

0 V

0.3 V

COMP

Voltage

Figure 2. SP6121 Hiccup mode waveforms

dVss/dt = 0.4V/ms

1.25V

I(L)

TIME

0.3 V

0.7 V

1 V

0 V

VOUT = V(REF) * (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)

Figure 1. SP6121 Soft Start Waveforms

THEORY OF OPERATION: continued

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