MAX5902/MAX5903
+72V, SOT23/TDFN, Simple Swapper Hot-Swap
Controllers
_______________________________________________________________________________________ 9
Selecting a Circuit-Breaker Threshold
The MAX5902A/MAX5903A and the MAX5902L/
MAX5903L offer a circuit-breaker function to protect the
external MOSFET and the load from the potentially
damaging effects of excessive current. As load current
flows through the external MOSFET, a voltage, V
SD
, is
generated from source to drain due to the MOSFET’s
on-resistance, R
DS(ON)
. The MAX5902A/MAX5903A
and MAX5902L/MAX5903L monitor V
SD
when the exter-
nal MOSFET is fully enhanced. If V
SD
exceeds the
circuit-breaker threshold the external MOSFET is turned
off, and PGOOD (PGOOD) is deasserted. To accom-
modate different MOSFETs and different load currents
the MAX5902/MAX5903 are available with circuit-break-
er threshold voltages of 300mV, 400mV, and 500mV. To
determine the proper circuit-breaker threshold for an
application use:
V
CB
> (R
DS(ON)
)
✕
(I
OUT(MAX)
)
where R
DS(ON)
is the on-resistance of the MOSFET and
I
OUT
(MAX) is the maximum expected output current.
The MAX5902N/MAX5903N have no circuit-breaker
function. For these parts choose an external MOSFET
which meets the load requirements.
The circuit-breaker function is intended to protect
against gross overcurrent or short-circuit conditions.
During a gross overcurrent or short-circuit condition,
the MAX5902/MAX5903 disconnect the load by dis-
abling the external MOSFET. For calculating the circuit-
breaker threshold use the MOSFET’s R
ON
at the worst
possible operating condition, and add a 20% overcur-
rent margin to the maximum circuit current. For
instance, if a MOSFET has an R
ON
of 0.06Ω at T
A
=
+25°C, and a normalized on-resistance factor of 1.75 at
T
A
= +130°C (from the MOSFET data sheet), the R
ON
used for calculation is the product of these two num-
bers, or (0.06Ω) x (1.75) = 0.105Ω. Then, if the maxi-
mum current is expected to be 2A, using a 20%
margin, the current for calculation is (2A) x (1.2) = 2.4A.
The resulting minimum circuit-breaker threshold is then
a product of these two results, or (0.105Ω) x (2.4A) =
0.252V. The next highest minimum available threshold
is 0.280V of the MAX5902ABEUT, which is an ideal
choice given these parameters. Using this method to
choose a circuit-breaker threshold allows the circuit to
operate under worst case conditions without causing a
circuit-breaker fault, but the circuit-breaker function will
still operate if a short circuit, or a gross overcurrent
condition exists.
Determining Inrush Current
Determining a circuit’s inrush current is necessary to
help choose the proper MOSFET. The MAX5902/
MAX5903 regulate the inrush current by means of con-
trolling the load voltage slew rate, but inrush current is
also a function of load capacitance. Determine inrush
current using:
where C is the load capacitance, and SR is the
MAX5902/MAX5903 Load Voltage Slew-Rate
Magnitude from the Electrical Characteristics table. For
example, assuming a load capacitance of 100µF, and
using the typical value of 9V/ms for the slew rate, the
inrush current is 0.9A typical.
If the maximum possible Load Voltage Slew Rate is
used, the maximum inrush current calculates to 1.8A.
Choose a MOSFET with a maximum pulsed current
specification that exceeds the maximum inrush current.
