8

FN6703.1

September 11, 2008

(Figure 4), results from Equation 2 would differ from the

calculated frequency. The typical performance curves may

be used as a tool along with the previous equations as a

more accurate tool to estimate the operating frequency more

accurately.

The maximum duty cycle, D, and deadtime, DT, can be

calculated from:

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.

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.

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.

Thermal Protection

An internal temperature sensor protects the device should

the junction temperature exceed +145°C. There is

approximately +15°C of hysteresis.

Ground Plane Requirements

Careful layout is essential for satisfactory operation of the

be bypassed directly to GND with good high frequency

capacitance.

DT

⁄

=

(EQ. 5)

DT

1

D

–

() T

OSC

⋅

=

s(EQ. 6)

ISL6745A