Datasheet

LTC4224-1/LTC4224-2
9
422412fa
APPLICATIONS INFORMATION
time, a 5ms start-up blanking delay begins during which
the circuit breaker is not allowed to latchoff the MOSFET.
If the ECB is tripped at the end of 5ms (time point t
3
), the
MOSFET is latched off by pulling GATE down with 1.5mA
and FAULT is latched low. The waveform in Figure 6 shows
an unsuccessful start-up due to a short circuit at the output.
To ensure start-up, the load capacitor must be charged
up suffi ciently to exit current limit before the end of the
5ms blanking delay. For large load capacitors, it may be
necessary to connect an external capacitor from GATE to
GND as described in the Turn-On Sequence section.
After start-up, any transient overcurrent faults lasting
less than 100µs are ignored. Any overcurrent condition
lasting more than 100µs will initiate the 5ms ECB blank-
ing delay (time point t
4
). After the ECB blanking delay,
the ECB is armed for the following 5ms (time point t
5
to
t
6
). Any 100µs overcurrent pulse during this time latches
off the MOSFET. In summary, for 5ms to 10ms after a
100µs or greater overcurrent fault is detected, a second
100µs fault condition causes the MOSFET to latchoff. If
no overcurrent condition is detected during this time, the
part re-enters the IDLE state and another blanking delay
following an overcurrent condition is again required before
the MOSFET latches off. Figure 7 shows how the output
latches off following an overcurrent fault.
During a severe short-circuit (see Figure 8), the output load
current can briefl y surge to tens of amperes. The LTC4224
rapidly brings the current under control by discharging
the MOSFET’s gate with 125mA towards ground. After a
short delay, the ACL amplifi er regulates the gate voltage
until the ECB trips at the end of 5ms.
Undervoltage Fault
An undervoltage fault occurs if either V
CC1
or V
CC2
falls
below 0.8V for longer than 8s. This turns off the affected
supply’s switch by discharging GATE with 1.5mA and
clears its fault latch. An undervoltage fault on one supply
does not affect the operation of the other supply. If V
CC
,
the higher of V
CC1
and V
CC2
, falls below 2.4V for more
than 12s, all supply switches are turned off and all fault
latches are cleared.
Figure 6. Start-Up with Short at Output
5ms/DIV
ON1
2V/DIV
FAULT
5V/DIV
I
OUT1
1A/DIV
GATE1
5V/DIV
422412 F06
Figure 8. Severe Short-Circuit on Output
0.5µs/DIV
I
OUT2
8A/DIV
V
OUT2
5V/DIV
GATE2
5V/DIV
422412 F08
Figure 7. Overcurrent Fault on Output
2ms/DIV
I
OUT1
1A/DIV
V
OUT1
5V/DIV
GATE1
5V/DIV
422412 F07