ISL6269_06 Datasheet, PDF(9/17 Page) Intersil Corporation – High-Performance Notebook PWM Controller with Bias Regulator and Audio-Frequency Clamp

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ISL6269_06 Datasheet, PDF (9/17 Pages) Intersil Corporation – High-Performance Notebook PWM Controller with Bias Regulator and Audio-Frequency Clamp

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ISL6269

Theory of Operation

Modulator

The ISL6269 is a hybrid of fixed frequency PWM control, and

variable frequency hysteretic control. The term âRippleâ in

the name âRobust-Ripple-Regulatorâ refers to the converter

output inductor ripple current, not the converter output ripple

voltage. The output voltage is regulated to 600mV at the FB

pin with respect to the GND pin. The FB pin is the inverting

input of the error amplifier. The frequency response of the

feedback control loop is tuned with a type-two compensation

network connected across the FB pin and COMP pin.

The R3 modulator synthesizes an AC signal VR, which is an

ideal representation of the output inductor ripple current. The

duty-cycle of VR is derived from the voltage measured at the

VIN pin and VO pin with respect to the GND pin.

Transconductance amplifiers convert the VIN and VO

voltages into currents that charge and discharge the ripple

capacitor CR. The positive slope of VR can be written as:

VRPOS = (gm) â¢ (VIN â VO)

(EQ. 1)

The negative slope of VR can be written as:

VRNEG = gm â¢ VO

(EQ. 2)

A voltage VW is referenced with respect to the error amplifier

output voltage VCOMP, creating a window-voltage envelope

into which voltage VR is compared. The VR, VCOMP, and VW

signals feed into a hysteretic window comparator in which

VCOMP is the lower threshold voltage and VW is the higher

threshold voltage. PWM pulses are generated as VR

traverses the VW and VCOMP thresholds. The charging and

discharging rates of capacitor CR determine the PWM

switching frequency for a given amplitude of VW with respect

to VCOMP. The R3 regulator simultaneously affects switching

frequency and duty cycle because it modulates both edges

of the PWM pulses.

RIPPLE CAPACITOR VOLTAGE CR

WINDOW VOLTAGE VW

ERROR AMPLIFIER VOLTAGE VCOMP

PWM

Âµs

FIGURE 3. MODULATOR WAVEFORMS DURING LOAD TRANSIENT

FN9177.1

August 7, 2006

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