
3
38 9 10
FAULT
CT
T
C
.
=
×
6
1 0 10
S D
C T
T
.
C
=
FAULT
IFLT
I
k
R
W
=
200
SLUS907D – JANUARY 2009 – REVISED AUGUST 2010
www.ti.com
Table 1. TERMINAL FUNCTIONS
FUNCTION
PIN NO.
DESCRIPTION
FLT
1
Fault low indicated the fault time has expired and the FET is switched off
EN
2
Device is enabled when this pin is pulled low
VIN
3
Power In and control supply voltage
GND
4
GND
ISET
5
A resistor to ground sets the fault current, the current limit is 125% of the fault current. TPS2421 only
CT
6
A capacitor to ground sets the fault time
VOUT
7
Output to the load
PG
8
Power Good low represents the output voltage is within 300 mV of the input voltage
PIN DESCRIPTION
CT: Connect a capacitor form CT to GND to set the fault time. The fault timer starts when the fault current
threshold is exceeded, charging the capacitor with 36 mA from GND towards an upper threshold of 1.4 V. If the
capacitor reaches the upper threshold, the internal pass MOSFET is turned off. The MOSFET will stay off until
EN is cycled if a latching version is used. If an auto-retry version is used, the capacitor will discharge at 5 mA to
0.2 V and then re-enable the pass MOSFET. When the device is disabled, CT is pulled to GND through a
100-k
resistor.
The timer period must be chosen long enough to allow the external load capacitance to charge. The fault timer
period is selected using Equation 1 where TFAULT is the minimum timer period in seconds and CCT is in Farads. (1)
This equation does not account for component tolerances. In autoretry versions, the second and subsequent
retry timer periods will be approximately 85% as long as the first retry period.
In autoretry versions, the fault timer discharges the capacitor for a nominal TSD in seconds with CCT in Farads per
(2)
The nominal ratio of on to off times represents about a 3% duty cycle when a hard fault is present on the output
of an autoretry version part.
FLT: Open-drain output that pulls low on any condition that causes the output to open. These conditions are
either an overload with a fault time-out, or a thermal shutdown. FLT becomes operational before UV, when VIN is
greater than 1 V.
GND: This is the most negative voltage in the circuit and is used as reference for all voltage measurements
unless otherwise specified.
ISET: A resistor from this pin to GND sets both the fault current (IFAULT) and current limit (IMAX) levels. The
current limit is internally set at 125% of the fault current. The fault timer function on CT starts charging CT if IVIN
exceeds the programmed fault current. If this current continues long enough for VCT to reach its upper trip
threshold, the output is turned off. If IVIN falls below the fault current threshold before CT reaches its upper
threshold, CT is discharged and normal operation continues.
The internal MOSFET actively limits current if IVIN reaches the current limit set point. The fault timer operation is
the same in this mode as described previously.
The fault current value is programmed as follows;
(3)
6
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