6

FN9030.8

March 10, 2006

OCSET

the converter overcurrent (OC) trip point according to the

following equation:

An overcurrent trip cycles the soft-start function.

SS

Connect a capacitor from this pin to ground. This capacitor,

along with an internal 10

µA current source, sets the soft-start

interval of the converter.

COMP and FB

COMP and FB are the available external pins of the error

amplifier. The FB pin is the inverting input of the error

amplifier and the COMP pin is the error amplifier output.

These pins are used to compensate the voltage-control

feedback loop of the converter.

EN

This pin is the open-collector enable pin. Pull this pin below

1V to disable the converter. In shutdown, the soft-start pin is

discharged and the UGATE and LGATE pins are held low.

GND

Signal ground for the IC. All voltage levels are measured

with respect to this pin.

PHASE

Connect the PHASE pin to the upper MOSFET source. This

pin is used to monitor the voltage drop across the MOSFET

for overcurrent protection. This pin also provides the return

path for the upper gate drive.

UGATE

Connect UGATE to the upper MOSFET gate. This pin

provides the gate drive for the upper MOSFET. This pin is also

monitored by the adaptive shoot through protection circuitry to

determine when the upper MOSFET has turned off.

BOOT

This pin provides bias voltage to the upper MOSFET driver.

A bootstrap circuit may be used to create a BOOT voltage

suitable to drive a standard N-Channel MOSFET.

PGND

This is the power ground connection. Tie the lower MOSFET

source to this pin.

LGATE

Connect LGATE to the lower MOSFET gate. This pin provides

the gate drive for the lower MOSFET. This pin is also

monitored by the adaptive shoot through protection circuitry to

determine when the lower MOSFET has turned off.

PVCC

Provide a bias supply for the lower gate drive to this pin.

VCC

Provide a 12V bias supply for the chip to this pin.

Functional Description

Initialization

The ISL6522 automatically initializes upon receipt of power.

Special sequencing of the input supplies is not necessary.

The Power-On Reset (POR) function continually monitors

the input supply voltages and the enable (EN) pin. The POR

monitors the bias voltage at the VCC pin and the input

initiates soft-start operation after both input supply voltages

exceed their POR thresholds. For operation with a single

before POR initiates operation.

The POR function inhibits operation with the chip disabled

(EN pin low). With both input supplies above their POR

thresholds, transitioning the EN pin high initiates a soft-start

interval.

Soft-Start

The POR function initiates the soft-start sequence. An internal

10

the SS pin to 4V. Soft-start clamps the error amplifier output

(COMP pin) to the SS pin voltage. Figure 3 shows the soft-

reach the valley of the oscillator’s triangle wave. The

oscillator’s triangular waveform is compared to the ramping

error amplifier voltage. This generates PHASE pulses of

increasing width that charge the output capacitor(s). This

interval of increasing pulse width continues to t2, at which

point the output is in regulation and the clamp on the COMP

pin is released. This method provides a rapid and controlled

output voltage rise.

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ISL6522