LTC4242 Linear Technology, LTC4242 Datasheet - Page 15

• Manufacturer Part Number: LTC4242
• Description: Dual Slot Hot Swap Controller
• Manufacturer: Linear Technology

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Thermal Shutdown
Each of the two internal switches for the 3.3V auxiliary
supplies is protected by an independent thermal shutdown
circuit. If the temperature of an internal switch reaches
150°C, the switch shuts down immediately and AUXFAULT
is latched low. All other power switches are not affected.
The switch is allowed to turn on again by recycling the
AUXON pin low then high with the temperature falling
below 120°C.
Overcurrent Fault
The LTC4242 features dual level glitch tolerant protection
against overcurrent faults for all the supplies. The sense
resistor (both internal and external) voltage drop is moni-
tored by an electronic circuit breaker (ECB) comparator
and an active current limit (ACL) amplifi er. In the event that
a supply’s current exceeds the ECB threshold, an internal
timer is started. If the supply is still overcurrent after 20µs,
the ECB trips and the MOSFET turns off immediately, as
shown in Figure 6.
During start-up, a supply output could be shorted to ground
in the worst case. The inrush current would be limited to
the ACL threshold, which is 2x the ECB threshold, and the
part will latch off after 20µs.
APPLICATIO S I FOR ATIO
AUXOUTn
PGOODn
12V
3V
5V/DIV
5V/DIV
5V/DIV
5V/DIV
5V/DIV
OUTn
OUTn
ENn
Figure 5. Normal Power-Down Sequence
U
100ms/DIV
U
W
4242 F05
U
During an output short circuit, the surge current must be
brought to a controlled level within the shortest amount of
time to protect the system. The LTC4242’s active current
limit enters a high current protection mode that immediately
turns off the output MOSFET by pulling its gate-to-source
voltage to zero. Current in the output MOSFET drops from
tens of amps to zero in a few hundred nanoseconds. The
input voltage drops during the high current and then
spikes upwards due to lead parasitic inductances as the
3V
FAULTn
3V
5V/DIV
5V/DIV
5V/DIV
3A/DIV
GATEn
I
OUTn
LOAD
3V
10A/DIV
FAULTn
3V
5V/DIV
5V/DIV
5V/DIV
GATEn
I
LOAD
OUTn
Figure 7. Short-Circuit Fault on 3.3V Output
Figure 6. Overcurrent Fault on 3.3V Output
5µs/DIV
5µs/DIV
LTC4242
4242 F06
4242 F07
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