3V to 12V Current-Limiting Hot Swap Controllers

with Autoretry, DualSpeed/BiLevel Fault Protection

______________________________________________________________________________________

21

is discharged immediately, but note that the MOSFET is

power supply collapses below UVLO, the MAX4271/

MAX4272/MAX4273 will force the device to restart in

startup mode with a 150ms delay once the supply has

recovered. The main system power supply must be

able to deliver this fault current without excessive volt-

age drop.

The MOSFET is turned off in a very short time; there-

fore, the resulting dv/dt can be considerable. The back-

plane delivering the power to the external card must

have a fairly low inductance to limit the voltage tran-

sients caused by the removal of a fault. Bypassing the

input with a small capacitor alleviates false UVLO trips

due to these transients.

MOSFET Thermal Considerations

During normal operation, the MOSFET dissipates little

dissipated during the startup and turn-off transients.

The design must take into consideration the worst-case

scenario of a continuous short-circuit fault present on

the board. Two cases need to be considered:

1) The single turn-on with the device latched after a

fault (when using MAX4271 or MAX4273 in latched

mode)

2) The continuous automatic retry (when using the

MAX4272 or MAX4273 in retry mode)

Use the following equation to calculate the maximum

transient thermal resistance (in °C/W) required for an

output short to ground:

MOSFET manufacturers typically include curves for the

(Figure 17). Find the thermal impedance of the MOS-

choosing the single pulse curve for latched mode parts

or by choosing the duty cycle = 0.03 curve for the

required is less than that of the package, reduce

FET, or choose one with better thermal characteristics.

Figure 16. Using the MAX4271/MAX4272/MAX4273 on a

Backplane

IN

ON

BACKPLANE

MAX4271

MAX4272

MAX4273

SENSE

GATE

CTIM

REMOVABLE CARD

WITH NO HOT-INSERTION

PROTECTION

Figure 15. Adjustable Undervoltage Lockout and Output Voltage Reset Generator

OUTC

RESET

µP

INC

LLMON

ON

IN

SENSE

GATE

MAX4273

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100k