3V to 12V Current-Limiting Hot Swap Controllers

with Autoretry, DualSpeed/BiLevel Fault Protection

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13

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 1k

Ω 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

2.7V TO 6V

STAT

ON

Figure 4. Response to a Fault Condition

STAT

2.7V TO 6V