6

FN9122.2

April 13, 2005

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 ISL6522A 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

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

voltages 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.

Overcurrent Protection

The overcurrent function protects the converter from a

shorted output by using the upper MOSFETs on-resistance,

converter’s efficiency and reduces cost by eliminating a

current sensing resistor.

The overcurrent function cycles the soft-start function in a

programs the overcurrent trip level. An internal 200

µA

FIGURE 3. SOFT-START INTERVAL

VOLTAGE

TIME

t

1

C

SS

I

SS

----------- V

OSC MIN

()

⋅

=

t

2

t

1

–

C

SS

I

SS

-----------

V

OUT

SteadyState

V

IN

------------------------------------------------

∆V

OSC

⋅⋅

==

Where:

∆V

RELEASED AT STEADY STATE

0A

0V

TIME (20ms/DIV)

5A

10A

15A

2V

4V

FIGURE 4. OVERCURRENT OPERATION

ISL6522A