7

FN6348.0

August 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

measures the VIN and VO pin voltages. The positive slope

proportional to the slew rates of the positive and negative

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

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

rise. The output voltage enters regulation in approximately

has elapsed.

The PGOOD pin indicates when the converter is capable of

supplying regulated voltage. The PGOOD pin is an

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

Ω, or 32Ω for an overcurrent fault,

or 63

Ω for an overvoltage fault.

V

RPOS

g

m

() V

IN

V

OUT

–

()

•

=

(EQ. 1)

V

RNEG

g

⋅

=

(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

ISL6268