ACT4921 datasheet(22/35 Pages) QORVO

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Data Sheet Rev. B, January 2020 | Subject to change without notice

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ACT4921

Power Loss Protection with 6A eFuse

EFUSE

General Description

The ACT4921 eFuse is an “electronic” fuse that

disconnects

the

input

from

the

output.

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 7V by opening when the input voltage

goes above the programmed threshold.

The eFuse turns off in the following conditions.

VINS voltage below UV threshold

VINS voltage above the OV threshold

Input voltage above the OV threshold

eFuse current above the OC threshold

VIN – VOUT > 300mV

Blocking FET

The blocking FET is an internal MOSFET that provides

isolation between the system output voltage (VOUT)

and the storage capacitors. It provides system level fault

tolerance that allows the system to continue operating

normally in the event of a storage capacitor failure.

During the blocking FET softstart, it provides 300mA

constant current. The blocking FET stays in softstart

until the voltage across it (VOUT-VB) approaches 0V.

After softstart is complete, the blocking FET turns on

fully, and has a 1.8A current limit setting. The blocking

FET enters softstart again when VOUT-VB < 560mV.

During startup, the 300mA constant current source

linearly charges up the storage capacitors. If the storage

capacitors are not charged up to VIN within 1s, the

blocking FET turns off for 1s and then retries. With

extremely

large

storage

capacitors

such

supercapacitors, the blocking FET will turn on and off at

a 2s period. This is expected functionality and

minimizes the power dissipation in the IC.

In normal operation, when the blocking FET is fully

turned on, it applies power to the boost converter,

allowing the storage capacitors to charge up to their

final value. When the IC enters supplement mode, the

buck output power flows into the VB pin, through the

blocking FET, and out of VOUT to the system.

The blocking FET limits a storage capacitor overload

condition to 1.8A. This causes the voltage at VB to drop,

generates a VB undervoltage fault, and the IC pulls

nIRQ low. If VB voltage drops below VOUT-560mV, the

current limit drops from 1.8A to 300mA. During this

condition, the eFuse stays on and system continues to

operate normally.

If the overload condition remains for longer than 1s, the

blocking FET enters a hiccup mode where it turns on

and off for a 2s period. Hiccup mode applies to both

normal startup with very large storage capacitors and to

short circuit conditions. Note that the IC enters the

SUPPLEMENT-DISABLE state during both startup and

short circuit conditions.

The blocking FET can be manually turned on and off by

the ENB pin or the EN_BFET bit in register 0x06h.

When register EN_ENB_REG = 0, the ENB pin controls

the blocking FET. Pulling ENB low turns it off and pulling

ENB high turns it on. When register EN_ENB_REG = 1,

the EN_BFET bit pin controls the blocking FET. Setting

EN_BFET = 0 turns it off and setting EN_BFET high

turns it on.

Note that disabling the blocking FET also disables the

boost converter. This allows systems to startup faster by

keeping the boost disabled until the system is up and

running.

Current Limit Setting

A resistor connected between the ISET pin and VSS

programs the ACT4921 eFuse 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.

= 20000

(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. Note that

the maximum allowable eFuse current limit setting is 6A,

which corresponds to a 3.32kohm resistor.

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