ACT4921 datasheet(23/35 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

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 1/20000 of the current through the eFuse. The

following equation calculates the eFuse current when

measured via the ILIM pin.

20000

(2)

Where RILIM is the current limit resistor in ohms and VILIM

is the voltage measured 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 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.

(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 SHUTDOWN state, turns off the eFuse, and

then retries to softstart again. The following equation

calculates the minimum allowable softstart time.

0.9

(4)

Where COUT is the sum of the output capacitance and

storage capacitance in mF, VOUT is the output voltage in

volts, and IILIM is the eFuse current limit in Amperes

programmed by the ILIM resistor.

BUCK CONVERTER

General Description

The ACT4921 contains current-mode, synchronous

PWM

step-down

converter

that

achieves

peak

efficiencies of 95%. The buck converter minimizes noise

in sensitive applications and allows the use of small

external components. It is highly flexible with external

component selection and can be reconfigured via I2C

registers. External components set the output voltage

and compensation while I2C registers set the switching

frequency and current limit. The buck converter

operates in fixed frequency PWM mode. Its switching

frequency is programmable between 562kHz to 2250

kHz via the I2C register BK_FREQ[1:0] which allows the

system to be optimized for different applications. It has

two peak current limit options of 6A and 10A allowing for

further system optimization. Overcurrent is set by a non-

I2C bit BKILIM_OPT in register 0x0Eh. Refer to the CMI

section at the back of the datasheet for each IC’s

specific setting. The buck output voltage is externally

programmable between 1.8V and 6V.

The buck converter generates a regulated output

voltage at the VB pin from the storage capacitors when

the IC enters supplement mode. This provides the

backup power when the system experiences fault

conditions. Note that the buck output voltage goes into

the VB pin, through the blocking FET and out of the

VOUT pin to the system. After the IC exits the

SOFTSTART state, the buck converter is enabled but

remains turned off. It automatically turns on when the IC

enters supplement mode, and remains on until the

storage capacitors discharge to 3.0V. Note that ENB

must be pulled high to enable the buck converter. Most

systems wait until the system is up and running before

enabling the buck and boost with the ENB pin.

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