TK83854
PIN DESCRIPTION (CONT.)
VOLTAGE AMPLIFIER INVERTING OUTPUT (V
SENSE
)
This is normally connected to a feedback network and to
the boost converter output through a divider network.
OSCILLATOR CHARGING CURRENT AND MULTIPLIER
LIMIT SET
(R
SET
)
A resistor from R
SET
to ground will program oscillator
charging current and maximum multiplier output. Multiplier
output current will not exceed 3.75 V divided by the resistor
from R
SET
to ground.
SOFT-START (SS)
SS will remain at GND as long as the IC is disabled or V
CC
is too low. SS will pull up to over 8 V by an internal 14
µA
current source when both V
CC
becomes valid and the IC is
enabled. SS will act as the reference input to the voltage
amplifier if SS is below V
ref
. With a large capacitor from SS
to GND, the reference to the voltage regulating amplifier
will rise slowly, and increase the PWM duty cycle slowly.
In the event of a disable command or a supply dropout, SS
will quickly discharge to ground and disable the PWM.
OSCILLATOR TIMING CAPACITOR (C
T
)
A capacitor from C
T
to GND will set the PWM oscillator
frequency according to this relationship:
interaction between the gate impedance and the GTDRV
output driver that might cause the GTDRV output to
overshoot excessively. Some overshoot of the GTDRV
output is always expected when driving a capacitive load.
f
OSC
= 1.25 / (R
SET
x C
T
)
POSITIVE SUPPLY VOLTAGE (V
CC
)
Connect V
CC
to a stable source of at least 20 mA above 17
V for normal operation. Also bypass V
CC
directly to GND to
absorb supply current spikes required to charge external
MOSFET gate capacitances. To prevent inadequate
GTDRV signals, these devices will be inhibited unless V
CC
exceeds the upper undervoltage lockout threshold and
remains above the lower threshold.
GATE DRIVER (GTDRV)
The output of the PWM is a totem pole MOSFET gate
driver on GTDRV. This output is internally clamped to
15 V so the IC can be operated with V
CC
as high as 35 V.
Use a series gate resistor of at least 5 ohms to prevent
Page 8
January 1999 TOKO, Inc.
TOKO [ TOKO, INC ]
