MAX4370
Current-Regulating Hot-Swap Controller with
DualSpeed/BiLevel Fault Protection
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Slow Comparator
The slow comparator is disabled at start-up while the
external MOSFET is turning on. This allows the part to
ignore the higher-than-normal inrush current charging
the board capacitors (C
BOARD
) when a card is first
plugged in.
If the slow comparator detects an overload current
while in normal operation (after start-up is completed),
it turns off the external MOSFET by discharging the
gate capacitance with a 200µA current. The slow com-
parator threshold is set at 50mV and has a default
delay of 20µs (CSPD floating), allowing it to ignore
power-supply glitches and noise. The response time
can be lengthened with an external capacitor at CSPD
(Figure 8).
If the overcurrent condition is not continuous, the dura-
tion above the threshold minus the duration below it
must be greater than 20µs (or the external programmed
value) for the device to trip. When the current is above
the threshold, CSPD is charged with a 6 µA current
source; when the current is below the threshold, CSPD
is discharged with a 6 µA current source. A fault is
detected when CSPD is charged to the trip point of
1.2V. A pulsing current with a duty cycle greater than
50% (i.e., > 50% of the time the current is above the
threshold level) will be considered a fault condition
even if it is never higher than the threshold for more
than the slow comparator’s set response time.
Once the fault condition is detected, the STAT pin goes
low and the device goes into latched mode. The GATE
voltage discharge rate depends on the gate capaci-
tance and the external capacitance at GATE.
Fast Comparator
The fast comparator behaves differently according to
the operating mode. During start-up, the fast compara-
tor is part of a simple current regulator. When the
sensed current is above the threshold (V
FC,TH
=
200mV), the gate is discharged with a 80µA current
source. When the sensed current drops below the
threshold, the charge pump turns on again. The sensed
current will rise and fall near the threshold due to the
fast comparator and charge-pump propagation delay.
The gate voltage will be roughly saw-tooth shaped, and
the load current will present a 20% ripple. The ripple
can be reduced by adding a capacitor from GATE to
GND. Once C
BOARD
is completely charged, the load
current drops to its normal operating levels. If the
sensed current is still high after the start-up timer
expires, the MOSFET gate is discharged completely.
In normal operation (after start-up), the fast comparator
is used as an emergency off switch. If the load current
reaches the fast comparator threshold, the device
immediately forces the MOSFET off completely by dis-
charging the GATE with a 200 µA current. This can
occur in the event of a serious current overload or a
dead short. Given a 1000pF gate capacitance and 12V
gate voltage, the MOSFET will be off in less than 60 µs.
Any additional capacitance connected between GATE
and GND to slow down the turn-on time also increases
the turn-off time.
Latched Mode and Reset
The MOSFET driver of the MAX4370 stays latched off
after a fault condition until it is reset by a negative-
going pulse on the ON pin. Pulse ON low for 20 µs
(min), then high to restart after a fault. During start-up, a
negative-going edge on ON will force the device to turn
off the MOSFET and place the device in latched mode.
Keep ON low for 20µs (min) to restart.
