ISL6268

FN6348 Rev 0.00

Page 7 of 14

Aug 22, 2006

Theory of Operation

Modulator

The ISL6268 is a hybrid of fixed frequency PWM control, and

can simultaneously affect the PWM switching frequency and

PWM duty cycle in response to input voltage and output load

transients. The term “Ripple” in the name “Robust Ripple

Regulator” refers to the converter output inductor ripple

representation of the output inductor ripple current. The duty-

written as:

is set by a resistor connected across the FSET and GND pins.

thresholds . The PWM switching frequency is proportional to

PWM switching frequency is inversely proportional to the

Power-On Reset

The ISL6268 is disabled until the voltage at the VCC pin has

threshold voltage. The controller will become once again

disabled when the voltage at the VCC pin decreases below the

EN, Soft-Start, and PGOOD

The ISL6268 uses a digital soft-start circuit to ramp the output

voltage of the converter to the programmed regulation setpoint

at a predictable slew rate. The slew rate of the soft-start

sequence has been selected to limit the inrush current through

the output capacitors as they charge to the desired regulation

voltage. When the EN pin is pulled above the rising EN

begins and the output voltage begins to rise. The output voltage

enters regulation in approximately 1.5ms and the PGOOD pin

The PGOOD pin indicates when the converter is capable of

supplying regulated voltage. The PGOOD pin is an undefined

ISL6268 features a unique fault-identification capability that

can drastically reduce trouble-shooting time and effort. The

pull-down resistance of the PGOOD pin corresponds to the

fault status of the controller. During soft-start or if an

undervoltage fault occurs, the PGOOD pull-down resistance is

95

, or 32 for an overcurrent fault, or 63 for an overvoltage

fault.

 V

IN

–





=

(EQ. 1)



=

(EQ. 2)

Ripple Capacitor Voltage CR

Error Amplifier Voltage VCOMP

Window Voltage VW

PWM

FIGURE 2. MODULATOR WAVEFORMS DURING LOAD

TRANSIENT

EN

VCC and PVCC

VOUT

PGOOD

1.5ms

2.75ms

FIGURE 3. SOFT-START SEQUENCE