Datasheet
SLUS431A – MAY 2000 – REVISED DECEMBER 2000
6
POST OFFICE BOX 655303 • DALLAS, TEXAS 75265
Terminal Functions
TERMINAL
I/O
DESCRIPTION
NAME NO.
I/O
DESCRIPTION
AGND 7 This pin is the reference point for grounding all analog functions and must be kept as clean as possible from all
switching noise. It should be connected to PGND in only one location as close to the IC as practical.
CS 9 I Pulse-by-pulse and shutdown overcurrent sense input pin
FB 4 I Control input for the signal from a secondary–side PWM controller
OUT 11 O Drive pin for the MOSFET power switch
OVS 1
I
This pin is used with an external resistor divider of VIN to terminate operation if a voltage is sensed above the
internal 4 V threshold. Activation sets the shutdown latch that requires recycling the voltage on VDD to restart.
PGND 10 This power ground for the PWM output stage conducts any current transients from the power switch gate drive. It
should be closely bypassed to VDD and connected to AGND in only one location as close to the IC as possible.
REF 6
O
This is a 5-V output usable with external loads of up to 10 mA. This voltage is also the source for all internal
analog threshold settings and should be bypassed with a minimum of 0.1-µF capacitance to AGND.
RT 5 I Sets the free-running startup oscillator frequency
SD 2 I Input to the shutdown circuit
SS 3
I
The pin implements the primary-side soft-start function. This is the connection point for an external capacitor that
determines the rate of increase in commanded pulse width for the power switch at startup. It also serves as the
ac ground return for the feedback pulse transformer to provide a tracking bias for the FB input.
START 13 O Used to develop a regulated 12 V at VDD and thereby minimize or eliminate continuous current drain
UVS 14
I
This pin is used with an external resistor divider of Vin to terminate operation if a voltage is sensed below the
internal 4-V threshold. Activation maintains the circuit in shutdown with the soft-start capacitor clamped low.
VDD 12 I Power input connection for all control circuitry
VS 8 I Volt-second clamp
detailed descriptions
current sense (CS)
This is the pulse-by-pulse and shutdown overcurrent sense input pin. This current-sense pin triggers a
pulse-by-pulse termination anytime a 1.0-V threshold is exceeded while a signal in excess of 1.375 V on this
pin initiates a complete shutdown. Each activation of pulse-by-pulse termination also sends a current pulse to
the SD pin where an external capacitor can be used to provide a delayed shutdown. Since the CS pin can be
noise sensitive, it is good practice to insert a small low-pass RC filter between this pin and the current sensor.
feedback (FB)
This is the control input for the signal from a secondary-side PWM controller whose pulse-width command has
been differentiated by the feedback pulse-edge transformer into positive
start
and negative
stop
pulses. These
signals are used to turn on and turn off the primary power switch and must have an amplitude of at least
V
SS
±2.0V (4 V peak-to-peak) for at least 25 ns per pulse and no more than 200 ns per pulse. The maximum
amplitude allowed on this pin is V
SS
± 7.0 V.
output drive (OUT)
This is the drive pin for the MOSFET power switch and sinks (1.5 A) and sources (0.75 A) fast, high-current
gate-drive pulses. During shutdown, this pin is self–biased to an active low state. A minimum of 4 W should
be added in series with the output to ensure that the on-chip driver safe operating area is not exceeded. (Data
from the IRF820/830/840 family of MOSFETs, commonly available in the TO–220 package, is used to derive
this value. The gate charge needed to provide full enhancement was used to establish an equivalent
capacitance of up to 4000 pF.)