MAX5902 查看數據表（PDF） - Maxim Integrated

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MAX5902

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

Maxim Integrated 

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

Controllers

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, VSD, is

generated from source to drain due to the MOSFET’s

on-resistance, RDS(ON). The MAX5902A/MAX5903A

and MAX5902L/MAX5903L monitor VSD when the exter-

nal MOSFET is fully enhanced. If VSD 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:

VCB > (RDS(ON)) ✕ (IOUT(MAX))

where RDS(ON) is the on-resistance of the MOSFET and

IOUT(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 RON 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 RON of 0.06Ω at TA =

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

TA = +130°C (from the MOSFET data sheet), the RON

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.

GROUND

POWER IN

U1

SOT23-6

MAX5902/MAX5903

M1 S D G

SOT-223

POWER

OUT

Figure 7. Circuit Board Layout Example.

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:

I = C dV = C × SR

dt

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.

MAXIMUM

ILOAD (A)

SUGGESTED

EXTERNAL

MOSFET

0.5

IRF9540NS

1

IRF9540NS

2

IRF5210S

3

IRF5210S

VIN = +9V to +72V

SUGGESTED MAXIM

PART

MAX5902AAEUT

MAX5902ABEUT

MAX5902ABEUT

MAX5902ACEUT

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