ACT4910QW
Rev 1.0, 15-Sept-2017
14.000
12.000
10.000
8.000
6.000
4.000
2.000
0.000
EFUSE
General Description
The ACT4910 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.
0
10
20
30
40
50
60
RISET (kΩ)
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.
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
ꢀꢋ
∗ꢆꢇꢇꢇꢇ
Equation (2)
ꢅꢀꢐꢂꢃꢂꢄ
ꢌꢍꢎꢏꢌ
ꢑ
ꢈꢉꢈꢊ
Where RILIM is the current limit resistor in ohms and VILIM
is the voltage on the ILIM pin in volts.
Current Limit Setting
When the eFuse reaches the current limit threshold, it
clamps the output current. If the load tries to draw
additional current, the eFuse opens.
A resistor connected between the ISET pin and VSS
program the ACT4910 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.
Soft-Start
The SS pin controls the eFuse start-up. The SS pin
drives a constant 5μA 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.
ꢅꢀꢆꢈꢈꢇꢉꢇꢈꢇꢊꢇ
ꢀꢁ
Equation (1)
ꢂꢃꢂꢄ
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.
ꢔ
ꢗ
ꢙꢚꢛꢜ ꢞꢓ
ꢝꢝ
ꢔ
ꢗ
ꢒꢓꢓ ꢕꢖ ꢘꢀ
ꢀ
Equation 2
ꢔ ꢗ
ꢟ
ꢟ
ꢠꢡꢢ
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
Innovative PowerTM
ActiveSwitcherTM is a trademark of Active-Semi
www.active-semi.com
Copyright © 2017 Active-Semi, Inc.
20
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