ISL6745A

4. Functional Description

FN6703 Rev.2.00

Page 10 of 13

Aug 14, 2017

4.3

Soft-Start Operation

The ISL6745A features a soft-start using an external

capacitor in conjunction with an internal current source.

Soft-start reduces stresses and surge currents during

start-up.

soft-start voltage, SS, in the SS comparator which drives

the PWM latch. While the SS voltage is less than 3.5V,

duty cycle is limited. The output pulse width increases

as the soft-start capacitor voltage increases up to 3.5V.

This has the effect of increasing the duty cycle from zero

to the maximum pulse width during the soft-start period.

When the soft-start voltage exceeds 3.5V, soft-start is

completed. Soft-start occurs during start-up and after

recovery from an overcurrent shutdown. The soft-start

voltage is clamped to 4V.

Please note the capacitance of the soft-start capacitor,

in series with the capacitor, 100Ω/µF (100Ω at least; 1k

at most).

4.4

Gate Drive

The ISL6745A is capable of sourcing and sinking 1A

peak current, and may also be used in conjunction with a

MOSFET driver such as the ISL6700 for level shifting.

To limit the peak current through the IC, an external

resistor may be placed between the totem-pole output of

the IC (OUTA or OUTB pin) and the gate of the

MOSFET. This small series resistor also damps any

oscillations caused by the resonant tank of the parasitic

inductances in the traces of the board and the FET’s

input capacitance.

4.5

Overcurrent Operation

Overcurrent delayed shutdown is enabled once the soft-

start cycle is complete. If an overcurrent condition is

detected, the soft-start charging current source is

disabled and the soft-start capacitor is allowed to

discharge through a 15µA source. At the same time a

50µs retriggerable one-shot timer is activated. It remains

active for 50µs after the overcurrent condition ceases. If

the soft-start capacitor discharges to 3.9V, the output is

disabled. This state continues until the soft-start voltage

reaches 270mV, at which time a new soft-start cycle is

initiated. If the overcurrent condition stops at least 50µs

prior to the soft-start voltage reaching 3.9V, the soft-start

charging currents revert to normal operation and the

soft-start voltage is allowed to recover.

4.6

Thermal Protection

An internal temperature sensor protects the device if the

junction temperature exceeds +145°C. There is

approximately +15°C of hysteresis.

4.7

Ground Plane Requirements

Careful layout is essential for satisfactory operation of

the device. A good ground plane must be employed.

high-frequency capacitance.