Datasheet

IMAX
IMAX
201k
R
I
W
=
IFLT
FAULT
200 k
R
I
W
=
6
SD CT
T 1.0 10 C= ´ ´
FAULT
CT
3
T
C
38.9 10
=
´
TPS2420
www.ti.com SLUS903E –JANUARY 2009–REVISED MAY 2013
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 μA 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 μA to
0.2 V and then re-enable the pass MOSFET. When the device is disabled, CT is pulled to GND through a 100-
kΩ resistor.
The timer period must be chosen long enough to allow the external load capacitance to charge. The fault timer
period is selected using the following formula where T
FAULT
is the minimum timer period in seconds and C
CT
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 T
SD
in seconds with C
CT
in Farads per
the following equation.
(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 device.
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 V
IN
is
greater than 1 volt.
GND: This is the most negative voltage in the circuit and is used as reference for all voltage measurements
unless otherwise specified.
IFLT: A resistor connected from this pin to ground sets the fault current threshold (I
FAULT
). Currents between the
fault current threshold and the current limit are permitted to flow unimpeded for the period set by the fault timer
programmed on C
T
. This permits loads to draw momentary surges while maintaining the protection provided by a
lower average-current limit. IFLT may not be set below 1 A to maintain the Fault Current Limit threshold accuracy
listed in Electrical Characteristics. Some parts may not current limit or fault as expected.
The fault timer implemented by C
T
starts charging C
T
when current through V
IN
exceeds I
FAULT
. If the current
doesn’t drop below the I
FAULT
level before V
CT
reaches its upper threshold, the output will be shut off. The fault
current resistor is set by the following formula where I
FAULT
is in Amperes and R
RFLT
is in ohms.
(3)
IMAX: A resistor connected from this pin to ground sets I
MAX
. The TPS2420 will limit current to I
MAX
. If the current
does not drop below the I
FAULT
level before the timer times out then the output will be shut off. R
MAX
is set by the
formula:
(4)
I
MAX
must be set sufficiently larger than I
FAULT
to ensure that l
MAX
could never be less than I
FAULT
, even after
taking tolerances into account.
EN: When this pin is pulled low, the device is enabled. The input threshold is hysteretic, allowing the user to
program a startup delay with an external RC circuit. EN is pulled to V
IN
by a 10-MΩ resistor, pulled to GND by
16.8 MΩ and is clamped to ground by a 7-V Zener diode. Because high impedance pullup/down resistors are
used to reduce current draw, any external FET controlling this pin should be low leakage.
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