MAX4370
Current-Regulating Hot-Swap Controller with
DualSpeed/BiLevel Fault Protection
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__________Applications Information
Component Selection
N-Channel MOSFET
Select the external N-channel MOSFET according to
the application’s current level. The MOSFET’s R
DS(ON)
should be chosen low enough to have a minimum volt -
age drop at full load to limit the MOSFET power dissipa-
tion. High R
DS(ON)
can cause output ripple if the board
has pulsing loads, or it can trigger an external under-
voltage reset monitor at full load. Determine the
device’s power rating requirement to accommodate a
short-circuit condition on the board during start-up (see
MOSFET Thermal Considerations
).
MOSFETs can typically withstand single-shot pulses
with higher dissipation than the specified package rat-
ing. Also, since part of the inrush current limiting is
achieved by limiting the gate dV/dt, it is not necessary
to use a MOSFET with low gate capacitance. Table 2
lists some recommended manufacturers and compo -
nents.
Sense Resistor
The slow comparator threshold voltage is set at 50mV.
Select a sense resistor that causes a 50mV voltage
drop at a current level above the maximum normal
operating current. Typically, set the overload current at
1.2 to 1.5 times the nominal load current. The fast com-
parator threshold is set at 200mV. This sets the fault
current limit at four times the overload current limit.
Choose the sense-resistor power rating to accommo -
date the overload current (Table 3):
P
SENSE
= (I
OVERLOAD
)
2
· R
SENSE
Start-Up Timing Capacitor (C
TIM
)
The start-up period (t
START
) is determined by the capaci-
tor connected at CTIM. This determines the maximum
time allowed to completely turn on the MOSFET.
The default value for t
START
is chosen by leaving CTIM
floating and is approximately 5.5 µs. This is also the
minimum value (not controlled and dependent on stray
capacitance). Longer timings are determined by the
value of the capacitor, according to Figure 6, and can
be determined as follows:
t
START
(ms) = 0.31 · C
TIM
(nF)
Set the t
START
timer to allow the MOSFET to be
enhanced and the load capacitor to be completely
charged.
There are two methods of completing the start-up
sequences. Case A describes a start-up sequence that
does not use the current-limiting feature and slowly turns
on the MOSFET by limiting the gate dV/dt. Case B uses
the current-limiting feature and turns on the MOSFET as
fast as possible while still preventing high inrush current.
