3V to 12V Current-Limiting Hot Swap Controllers

with Autoretry, DualSpeed/BiLevel Fault Protection

14

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Table 1. Status Output Truth Table

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

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

IN

UVLO DELAY

PERIOD

PART IN

STARTUP

ON PIN

OVERCURRENT

CONDITION

PART IN RETRY-TIMEOUT

PERIOD (OR LATCHED OFF DUE TO

OVERCURRENT CONDITION)

STAT PIN

(STATUS)

Yes

X

X

X

X

Low

X

Yes

X

X

X

Low

X

X

Low

X

X

Low

X

X

X

Yes

X

Low

X

X

X

X

Yes

Low

No

No

High

No

No

High