MAX5903 MAXIM [Maxim Integrated Products], MAX5903 Datasheet - Page 9

• Manufacturer Part Number: MAX5903
• Description: +72V SOT23 Simple Swapper Hot-Swap Controllers
• Manufacturer: MAXIM [Maxim Integrated Products]

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
generated from source to drain due to the MOSFET’s
on-resistance, R
and MAX5902L/MAX5903L monitor V
nal MOSFET is fully enhanced. If V
cuit 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:
where R
I
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
possible operating condition, and add a 20% overcur-
rent margin to the maximum circuit current. For
instance, if a MOSFET has an R
+25°C, and a normalized on-resistance factor of 1.75 at
T
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.
OUT
A
= +130°C (from the MOSFET data sheet), the R
(MAX) is the maximum expected output current.
Selecting a Circuit Breaker Threshold
DS(ON)
V
CB
is the on-resistance of the MOSFET and
> (R
DS(ON)
_______________________________________________________________________________________
+72V SOT23 Simple Swapper Hot-Swap
DS(ON)
. The MAX5902A/MAX5903A
)
✕
(I
OUT(MAX)
ON
SD
SD
of 0.06Ω at T
exceeds the cir-
ON
when the exter-
)
at the worst
SD
A
, is
ON
=
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 cur-
rent 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.
Figure 7. Circuit Board Layout Example.
V IN = +9V to +72V
MAXIMUM
I
LOAD
0.5
1
2
3
(A)
GROUND
POWER IN
I
Suggested External MOSFETs
=
SUGGESTED
EXTERNAL
IRF9540NS
IRF9540NS
IRF5210S
IRF5210S
C
MOSFET
Determining Inrush Current
SOT-223
dV
dt
SOT23-6
M1
=
U1
Controllers
C SR
S D G
×
MAX5902/MAX5903
SUGGESTED MAXIM
MAX5902ACEUT
MAX5902AAEUT
MAX5902ABEUT
MAX5902ABEUT
POWER
OUT
PART
9