7
variation in dV with R
TD
(Figure 5), and in charge current
(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 ISL6745 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.
The oscillator capacitor signal, C
T
, is compared to the 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 ISL6745 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
device. A good ground plane must be employed. V
DD
should
be bypassed directly to GND with good high frequency
capacitance.
DT
C
T
OSC
⁄= (EQ. 5)
DT 1 D–()T
OSC
⋅= s(EQ. 6)
ISL6745
