Datasheet
SLUS274A – JANUARY 1999 – REVISED APRIL 2003
4
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TERMINAL FUNCTIONS
TERMINAL
I/O
DESCRIPTION
NAME NO.
I/O
DESCRIPTION
CT 4 I A capacitor is connected to this pin in order to set the maximum fault time.
IMAX 2 I This pin programs the maximum allowable sourcing current.
OUT 7 O Output drive to the MOSFET pass element.
PL 8 I This feature ensures that the average MOSFET power dissipation is controlled.
SENSE 6 I Input voltage from the current sense resistor.
SD/FLT 1 O This pin provides fault output indication and shutdown control.
VDD 3 O Current driven with a resistor to a voltage at least 10V more positive than VSS.
VSS 5 O Ground reference for the device and the most negative voltage available.
DETAILED PIN DESCRIPTIONS
CT
A capacitor connected to this pin allows setting of the maximum fault time. The maximum fault time must be
more than the time to charge external load capacitance. The maximum fault time is defined as:
t
FAULT
+
ǒ
2 C
T
Ǔ
I
CH
where
I
CH
+ 36 mA ) I
PL
and I
PL
is the current into the power limit pin. Once the fault time is reached the output shuts down for a time
given by:
t
SD
+ 2 10
6
C
T
IMAX
This pin programs the maximum allowable sourcing current. Since V
DD
is a regulated voltage, a voltage divider
can be derived from V
DD
to generate the program level for the IMAX pin. The current level at which the output
appears as a current source is equal to the voltage on the IMAX pin over the current sense resistor. If desired,
a controlled current startup can be programmed with a capacitor on the IMAX pin, and a programmed start delay
can be achieved by driving the shutdown with an open collector/drain device into an R-C network.
PL
This pin’s feature ensures that the average MOSFET power dissipation is controlled. A resistor is connected
from this pin to the drain of the N-channel MOSFET pass element. When the voltage across the N-channel
MOSFET exceeds 5 V, current flows into the PL pin which adds to the fault timer charge current, reducing the
duty cycle from the 3% level. When I
PL
is much greater 36 µA, then the average MOSFET power dissipation
is given by:
P
FET(avg)
+ IMAX 1 10
*6
R
PL
SENSE
Input voltage from the current sense resistor. When there is greater than 50 mV across this pin with respect to
VSS, a fault is sensed, and C
T
starts to charge.
(1)
(2)
(3)
(4)