Datasheet

MAX4271/MAX4272/MAX4273
3V to 12V Current-Limiting Hot-Swap Controllers
with Autoretry, DualSpeed/BiLevel Fault Protection
______________________________________________________________________________________ 21
is discharged immediately, but note that the MOSFET is
not completely off until V
GS
< V
TH
. If the main system
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
power, it is fully turned on, and its R
DS(ON)
is minimal.
The power dissipated in normal operation is P
D
=
I
LOAD
2
x R
DS(ON)
. A considerable amount of power is
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:
Z
θJA
(max) = (T
JMAX
- T
A
) / (V
IN
✕
I
FAST, SET
)
MOSFET manufacturers typically include curves for the
transient thermal resistance, Z
θJA
, of the package
(Figure 17). Find the thermal impedance of the MOS-
FET by using t
START
as the pulse duration and by
choosing the single pulse curve for latched mode parts
or by choosing the duty cycle = 0.03 curve for the
MAX4272 (the duty cycle is fixed at 32:1). If the Z
θJA
required is less than that of the package, reduce
t
START
, reduce I
FAST,SET
, use a heatsink on the MOS-
FET, or choose one with better thermal characteristics.
Figure 16. Using the MAX4271/MAX4272/MAX4273 on a
Backplane
IN
V
IN
ON
BACKPLANE
MAX4271
MAX4272
MAX4273
SENSE
GATE
CTIM
C
BOARD
V
OUT
REMOVABLE CARD
WITH NO HOT-INSERTION
PROTECTION
Figure 15. Adjustable Undervoltage Lockout and Output Voltage Reset Generator
OUTC RESET
μP
INC
LLMONON
IN SENSE GATE
V
IN
MAX4273
•
•
•
•
•
•
100k