MAX4995AF 查看數據表（PDF） - Maxim Integrated

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MAX4995AF

50mA to 600mA Programmable Current-Limit Switches

Maxim Integrated 

50mA to 600mA Programmable

Current-Limit Switches

Applications Information

Setting the Current Limit

A resistor from SETI to ground programs the current-limit

value for the MAX4995. Table 2 lists various current lim-

its set by different resistor values at SETI. Shorting SETI

to ground asserts FLAG.

Use the following formula to calculate the current limit:

RSETI (kΩ)

=

29042(V)

ILIM (mA)

−

2.48(kΩ)

Using an RSETI with a value smaller than 45.8kΩ results

in a higher current limit. A programmed output current

greater than 660mA can damage the device.

Connecting any capacitance larger than 20pF to SETI

can cause instability.

Table 2. Current Limit vs. Resistor Values

RSETI (kΩ)

45.8

55.6

70.6

94.2

143

191

287

576

∞ (Open)

TYPICAL CURRENT LIMIT (mA)

602

500

397

300

200

150

100

50

0

Input Capacitor

Connect a capacitor from IN to GND to limit the input

voltage drop during momentary output short-circuit

conditions. Use a 1µF minimum ceramic capacitor for

proper device operation. Larger capacitor values

reduce the voltage undershoot at the input.

Due to the very fast current-limit reaction time of the

MAX4995AF, a larger input capacitance might need to

be connected at the input to dampen oscillation due to

long wires. Choose a value large enough to ensure IN

doesn’t exceed the absolute maximum ratings.

Output Capacitor

For stable operation over the full temperature range

and over the full programmable current-limit range, use

a 1µF ceramic capacitor from OUT to ground.

If the load capacitance is too large, then current may not

have enough time to charge the capacitance and the

device assumes that there is a faulty load condition.

Calculate the maximum capacitive load (CMAX) value that

can be connected to OUT using the following formula:

CMAX

(μF )

=

ILIM

(mA)

×

t BLANK(MIN)

VIN (V)

(ms

)

For example, for VIN = 3.3V, tBLANK(MIN) = 10ms, and

ILIM = 300mA, CMAX equals 909µF.

Due to the very fast current-limit reaction time of the

MAX4995AF, a larger output capacitance might need to

be connected at the output to dampen oscillation due

to long wires. Choose a value large enough to ensure

OUT doesn’t exceed the absolute maximum ratings.

Layout and Thermal Dissipation

To optimize the switch response time to output short-

circuit conditions, it is very important to keep all traces

as short as possible to reduce the effect of undesirable

parasitic inductance. Place input and output capacitors

as close as possible to the device. IN and OUT must be

connected with wide, short traces to the power bus.

During normal operation, the power dissipation is small

and the package temperature change is minimal. If the

output is continuously shorted to ground at the maxi-

mum supply voltage, the operation of the switches with

the autoretry option does not cause problems because

the total power dissipated during the short is scaled by

the duty cycle:

PMAX

=

VIN(MAX) × IOUT(MAX) × tBLANK

tRETRY + tBLANK

Attention must be given to the MAX4995C continuous

current-limit version when the power dissipation during

a fault condition may cause the device to reach thermal

shutdown threshold.

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