LTC4232
10
4232fc
For more information www.linear.com/LTC4232
applicaTions inForMaTion
An overcurrent fault occurs when the current limit circuitry
has been engaged for longer than the timeout delay set
by the TIMER. Current limiting begins when the MOSFET
current reaches 1.5A to 5.6A (depending on the foldback).
The GATE pin is then brought down with a 140mA GATE-
to-OUT current. The voltage on the GATE is regulated in
order to limit the current to less than 5.6A. At this point,
a circuit breaker time delay starts by charging the external
timing capacitor with a 100µA pull-up current from the
TIMER pin. If the TIMER pin reaches its 1.235V threshold,
the internal switch turns off (with a 250µA current from
GATE to ground). Included in the Typical Performance
Characteristics curves is a graph of the Safe Operating
Area for the MOSFET. From this graph one can determine
the MOSFET’s maximum time in current limit for a given
output power.
Tying the TIMER pin to INTV
CC
will force the part to use
the internally generated (circuit breaker) delay of 2ms.
In either case the F LT pin is pulled low to indicate an
overcurrent fault has turned off the pass MOSFET. For a
given circuit breaker time
delay, the equation for setting
the timing capacitor’s value is as follows:
C
T
= t
CB
• 0.083[µF/ms]
After the switch is turned off, the TIMER pin begins dis-
charging the
timing capacitor with a 2µA pull-down cur-
rent.
When the TIMER pin reaches its 0.21V threshold, an
internal
100ms timer is started. After the 100ms delay, the
switch is allowed to turn on again if the overcurrent fault
latch has been cleared. Bringing the UV pin below 0.6V
for a minimum of 1µs and then high will clear the fault
latch. If the TIMER pin is tied to INTV
CC
then the switch is
allowed to turn on again (after an internal 100ms delay),
if the overcurrent fault latch is cleared.
Tying the F LT pin to the UV pin allows the part to self-clear
the fault and turn the MOSFET on as soon as TIMER pin
has ramped below 0.21V. In this auto-retry mode the
LTC4232 repeatedly tries to turn on after an overcurrent
at a period determined by the capacitor on the TIMER pin.
The auto-retry mode also functions when the TIMER pin
is tied to INTV
CC
.
The waveform in Figure 4 shows how the output latches
off
following a short-circuit. The current in the MOSFET
is 1.4A as the timer ramps up.
10µF, especially if the wiring inductance from the supply
to the V
DD
pin is greater than 3µH. The possibility of oscil-
lation will increase as the load current (during power-up)
increases.
There are two ways to prevent this type of
oscillation. The simplest way is to avoid load capacitances
below 10µF. For wiring inductance larger than 20µH, the
minimum load capacitance may extend to 100µF. A second
choice is to connect an external gate capacitor C
P
>1.5nF
as shown in Figure 3.
Figure 3. Compensation for Small C
LOAD
4232 F03
LTC4232
OPTIONAL
RC TO LOWER
INRUSH CURRENT
GATE
C
P
2.2nF
Turn-Off Sequence
The switch can be turned off by a variety of conditions. A
normal turn-off is initiated by the UV pin going below its
1.235V threshold. Additionally, several fault conditions
will turn off the switch. These include an input overvolt
-
age (OV pin
), overcurrent circuit breaker (SENSE pin) or
overtemperature.
Normally the switch is turned off with
a 250µA current pulling down the GATE pin to ground.
With the switch turned off, the OUT voltage drops which
pulls the FB pin below its threshold. PG then pulls low to
indicate output power is no longer good.
If V
DD
drops below 2.65V for greater than 5µs or INTV
CC
drops below 2.5V for greater than 1µs, a fast shutdown
of the switch is initiated. The GATE is pulled down with a
140mA current to the OUT pin.
Overcurrent Fault
The LTC4232 features an adjustable current limit with
foldback that protects against short-circuits and excessive
load current. To prevent excessive power dissipation in the
switch during active current limit, the available current is
reduced as a function of the output voltage sensed by the
FB pin. A graph in the Typical Performance Characteristics
curves shows the Current Limit
Threshold Foldback.
