LTC4242
16
4242f
MOSFET shuts off (see Supply Transients). The compen-
sation network R
G
/C
G
assists the gate voltage recovery.
The ACL limits the current level to 2x the ECB threshold
by regulating the gate voltage.
For the internal switch, the ACL limits the supply current
to about 3x the circuit breaker current level of 550mA.
The ECB has a 20µs fi lter delay before latching off to prevent
unnecessary resets of the system due to minor transient
surges. An overcurrent fault on any of the main outputs
(12V or 3.3V) latches off both main outputs without af-
fecting the 3.3V auxiliary output. Similarly, an overcurrent
fault on the 3.3V auxiliary output latches off the auxiliary
output, without affecting the main outputs.
When there is a shorted load with signifi cant supply lead
inductance, the supply pin voltage could collapse before
the ACL brings down the gate of the external MOSFET. In
this case, the undervoltage lockout circuit, with 18µs fi lter
time, turns off the pass MOSFETs.
Undervoltage Fault
An undervoltage fault occurs when any of the input sup-
plies, 12V
IN
, 3V
IN
or AUXIN, falls below its undervoltage
threshold for more than 18µs. This turns off the switches
immediately. An undervoltage on the 3.3V auxiliary sup-
ply will not cause the main supplies to shut off and vice
versa. An undervoltage fault on any of the main supplies
shuts off both main supply switches. If V
CC
falls below
its UVLO threshold for more than 38µs, all switches are
turned off. The switches are allowed to turn on when
the supply voltages and V
CC
rise above their respective
undervoltage thresholds.
Power Good Fault
A power good fault occurs when any supply output drops
below its power good threshold for more than 20µs.
A power good fault on the main/AUX supplies causes
the PGOOD/AUXPGOOD to be pulled high. There are a
variety of conditions which must be satisfi ed for PGOOD/
AUXPGOOD to be asserted low:
1. The output voltage is above power good threshold
2. EN pin is low
3. The input voltage is above the undervoltage threshold
4. ON pin is high
5. Thermal shutdown not activated
Resetting Faults
To reset an overcurrent fault on the main outputs, bring ON
low or the faulting supply below its undervoltage lockout
(UVLO) threshold. To reset an overcurrent or thermal
shutdown fault on the auxiliary output, bring AUXON low
or the auxiliary suppy below its UVLO threshold. Bringing
V
CC
below its UVLO threshold resets all overcurrent and
thermal shutdown faults. The part cannot be reset when
fault overide, FON, is high.
Auto-Retry After a Fault
As shown in Figure 9, the LTC4242 can be confi gured to
automatically retry after a fault condition by connecting
both the FAULT and ON pins together with an RC network.
The auto-retry circuit will attempt to restart the LTC4242
after a circuit breaker trip, as shown in the timing diagram
of Figure 10.
t
RC V
RµA
OFF
AUTO AUTO OL
AUTO
≈
()
+
••.–
.•
1 235
2 065 9
For the component values shown, t
OFF
= 3.3ms. Since the
duration of a short is less than 40µs in the worst case, the
auto-retry duty cycle is 1.3%.
APPLICATIO S I FOR ATIO
WUU
U
Figure 8. Short-Circuit Fault on 3.3V
AUX
Output
AUXINn
5V/DIV
AUXOUTn
5V/DIV
I
LOAD
5A/DIV
5µs/DIV
4242 F08
AUXFAULTn
5V/DIV
