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3V to 12V Current-Limiting Hot-Swap Controllers
with Autoretry, DualSpeed/BiLevel Fault Protection
If the sensed current is still high after the startup timer
expires, the MOSFET gate is discharged completely as
described below.
In normal operation (after startup), the fast comparator is
used as an emergency off switch. If the load current
reaches the fast comparator threshold, the device quickly
forces the MOSFET off completely. This could happen in
the event of a serious current overload or a dead short.
The fast comparator has a 350ns response time and dis-
charges GATE with a 1mA current. Given a 1000pF gate
capacitance and 12V gate voltage, the MOSFET will be
off in about 12μs. Any additional capacitance connected
between GATE and GND to slow down the startup time
also increases the turn-off time.
In the MAX4273, CEXT goes high impedance during
the fast discharge. This reduces the effective capaci-
tance on GATE if a capacitor is used between GATE
and CEXT, and allows the MOSFET to quickly turn off.
In turn, this allows adjustment of the MOSFET charging
time without affecting the fast discharge rate, although
it does affect the slow discharge rate.
The MAX4271/MAX4272 fast comparator threshold is set
to four times the slow comparator threshold (i.e., 200mV).
The MAX4273 fast comparator threshold is set to 200mV
by connecting RTH to GND, is disabled by connecting
RTH to IN, or is adjustable by an external resistor con-
nected to IN (see Fast Comparator Threshold (RTH) ).
Latched/Autoretry
The MAX4271 MOSFET driver stays latched off after a
fault condition until it is reset by a negative-going pulse
on the ON pin. The MAX4272 is periodically turned on
after a fault condition with a timeout duration set by an
external timing capacitor on CTIM. The MAX4273 has a
selectable latched mode or retry mode. Connect CTIM
to IN to set the device in latched mode, or use an exter-
nal capacitor at CTIM to set the retry timeout.
Pulse ON low for 20μs (min) to restart after a fault
(MAX4271/MAX4273 in latched mode). Negative pulses
are ignored during autoretry (MAX4273 in autoretry
mode, or MAX4272).
The capacitor on CTIM affects the MAX4272’s retry
timeout period (time the part is shut down after an over-
current event) and the startup time (see the Electrical
Characteristics ). The retry timeout period is fixed at 32
times the startup time in order to minimize power dissi-
pation in the external MOSFET in case of a short-circuit
condition (see MOSFET Thermal Considerations ). This
is not an issue for parts latched off during a fault condi-
tion since they stay off until commanded on. The
MAX4273 configured in retry mode has a separate
startup timer capacitor (CTON) and retry timeout
capacitor (CTIM). This allows the user to change the
ratio between startup time and retry timeout period.
Status Output
The status output is an open-drain output that goes low
under the following conditions:
? During the UVLO delay period
? In startup
? Forced off (ON <0.6V)
? In an overcurrent condition
? In the retry timeout period (or latched off, for the
latched parts)
STAT is high only if the part is in normal mode and no
faults are present (Table 1). Figure 5 shows the status
(STAT) output timing diagram.
Over/Undervoltage Lockouts
The UVLO prevents the MAX4271/MAX4272/MAX4273
from turning on the external MOSFET until V IN exceeds
the lockout threshold (2.25V min) for 150ms. The UVLO
protects the external MOSFET from insufficient gate
drive voltage. The 150ms timeout ensures that the
board is fully plugged into the backplane and that V IN
Table 1. Status Output Truth Table
IN
UVLO DELAY
PERIOD
Yes
X
X
X
X
No
PART IN
STARTUP
X
Yes
X
X
X
No
ON PIN
X
X
Low
X
X
High
OVERCURRENT
CONDITION
X
X
X
Yes
X
No
PART IN RETRY-TIMEOUT
PERIOD (OR LATCHED OFF DUE TO
OVERCURRENT CONDITION)
X
X
X
X
Yes
No
STAT PIN
(STATUS)
Low
Low
Low
Low
Low
High
14
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