+72V, SOT23/TDFN, Simple Swapper Hot-Swap

Controllers

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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

generated from source to drain due to the MOSFET’s

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:

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-

possible operating condition, and add a 20% overcur-

rent margin to the maximum circuit current. For

+25°C, and a normalized on-resistance factor of 1.75 at

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.

IC

dV

dt

CSR

==

×

GROUND

POWER IN

POWER

OUT

M1

SOT-223

SDG

U1

SOT23-6

MAX5902/MAX5903

Figure 7. Circuit Board Layout Example.

MAXIMUM

SUGGESTED

EXTERNAL

MOSFET

SUGGESTED MAXIM

PART

0.5

IRF9540NS

MAX5902AAEUT

1

IRF9540NS

MAX5902ABEUT

2

IRF5210S

MAX5902ABEUT

3

IRF5210S

MAX5902ACEUT

VIN = +9V to +72V