Datasheet

MAX4271/MAX4272/MAX4273
3V to 12V Current-Limiting Hot-Swap Controllers
with Autoretry, DualSpeed/BiLevel Fault Protection
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In each case, when a fault is encountered, the status
pin (STAT) goes low, and for the MAX4273, the device
discharges the output voltage through a 1kΩ resistor
from LLMON to GND. After a fault, the MAX4271 stays
latched off and the MAX4272 enters retry mode, while
the MAX4273 has selectable latched or retry mode.
Figure 4 shows the waveforms of a fault condition.
BiLevel Fault Protection
Slow Comparator
The slow comparator is disabled at startup while the
external MOSFET is turning on. This allows the part to
ignore the higher-than-normal inrush current charging
the board capacitors when a card is first plugged in.
If the slow comparator detects an overload condition
while in normal operation (after startup is completed), it
turns off the external MOSFET by discharging the gate
capacitance with a 200µA current. The slow compara-
tor threshold is set at 50mV and has a default delay of
20µs (CSPD floating), allowing it to ignore power-sup-
ply glitches and noise. The response time can be
lengthened with an external capacitor at CSPD (Figure
10).
If the overcurrent condition is not continuous, then the
duration above the threshold minus the duration below
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 6µA current
source. A fault is detected when CSPD is charged to
the trip point of 1.2V. Therefore, a pulsing current with a
duty cycle of 50% or greater (i.e., the current is above
the threshold level > 50% of the time) is considered a
fault condition even if it is never higher than the thresh-
old for longer than the slow comparator’s set response
time.
The discharge rate depends on the N-FET gate capaci-
tance and the external capacitance at GATE. In the
MAX4273, CEXT remains connected and capacitance
to this point has to be discharged by the same current.
This increases the discharge time. Once the fault con-
dition is detected, the STAT pin goes low and the
device goes into retry or latched mode.
Fast Comparator
The fast comparator behaves differently according to
the operating mode.
During startup, the fast comparator is part of a simple
current regulator. When the sensed current is above
the fast comparator threshold, the gate is discharged
with a 70µA 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
sawtooth shaped, and the load current will present a
20% ripple. The ripple can be reduced by adding a
capacitor from GATE to GND.
Figure 3. Startup Waveforms
t
START
t
ON
2.7V TO 6V
V
GATE
I
FAST, SET
V
GATE
STAT
ON
V
TH
V
OUT
C
BOARD
= LARGE
C
BOARD
= 0
V
OUT
I
LOAD
Figure 4. Response to a Fault Condition
t
OFF
V
GATE
STAT
V
OUT
I
LOAD
I
LIM
2.7V TO 6V