Data Sheet Rev. C, August 2021 | Subject to change without notice

19 of 33

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ACT4911

Power Loss Protection with 6A eFuse

®

EFUSE

General Description

The ACT4911 eFuse is an “electronic” fuse that

disconnects

the

input

from

the

output.

It

has

considerable

advantages

over

mechanical

fuses

because it has adjustable current thresholds and it

resets after the fault condition is gone. The eFuse

consists of two back-to-back MOSFETS to provide

bi-directional

protection.

This

provides

protection

against both input and output short circuit conditions by

preventing current flow in both directions. Hot swap

functionality is provided by the eFuse softstart function

and current limiting circuitry. This prevents large inrush

currents from pulling down the input voltage. The eFuse

also provides protection against high input voltage

transients up to 24V by opening when the input voltage

goes above the programmed threshold.

The eFuse turns off in the following conditions.

1.

EN voltage below UV threshold

2.

EN voltage above the OV threshold

3.

Input voltage above the OV threshold

4.

eFuse current above the OC threshold

5.

VIN – VOUT > 560mV

Current Limit Setting

A resistor connected between the ISET pin and VSS

program the ACT4911 eFuse inrush and maximum DC

current limits. The input current limit is linearly

proportional to the resistor value on the ISET pin. The

following equation calculates the correct resistor value

to get the desired current limit threshold.

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(1)

Where RILIM is the current limit resistor in ohms and IILIM

is the desired current limit threshold in Amperes. Figure

5 shows the current limit setting vs RILIM value.

Figure 5: Current Limit Threshold vs RILIM

Note that the eFuse current can be measured using the

ILIM pin. If eFuse current is measured via the ILIM pin,

the measurement circuit must have a high impedance

input to minimize errors. The current flowing out of ILIM

is 1/50000 of the current through the eFuse. The

following equation calculates the eFuse current when

measured via the ILIM pin.

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������������������������������������������������������������

(2)

Where RILIM is the current limit resistor in ohms and VILIM

is the voltage on the ILIM pin in volts.

When the eFuse reaches the current limit threshold, it

clamps the output current. If the load tries to draw

additional current, the eFuse opens.

Soft-Start

The SS pin controls the eFuse start-up. The SS pin

drives a constant 5

μA internal current source into the

external soft-start capacitor to provide a linear ramping

voltage at the SS pin. The output voltage linearly ramps

with the SS pin voltage, resulting in a well-controlled

linear softstart ramp on VOUT, regardless of the load

conditions. The following equation calculates the

required soft-start capacitance.

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������������������������������������������������(������������)

(3)

Where the TSS is the soft-start time in ms and VOUT is the

output voltage.

The softstart current must be less than 90% of the

programmed eFuse current limit. If the softstart current

is greater than 90% of the eFuse current limit, the IC

enters the SHUTDOWN2 state, turns off the eFuse, and

then retries to softstart again. The following equation

calculates the minimum allowable softstart time.

0.000

2.000

4.000

6.000

8.000

10.000

12.000

14.000

0

10

20

30

40

50

60

RISET (kΩ)