ACT4921 datasheet(25/35 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

Inductor Selection

The Buck regulator utilizes current-mode control and a

proprietary

internal

compensation

scheme

simultaneously simplify external component selection

and optimize transient performance over their full

operating range. The ACT4921 is optimized for

operation with 1uH to 3.3uH inductors. Choose an

inductor with a low DC-resistance, and avoid inductor

saturation by choosing inductors with DC ratings that

exceed the maximum output current by at least 30%.

Due to the requirement for the buck converter to start

up as quickly as possible, the inductor should be

designed to give a maximum ripple current, ΔIL, of 50%

to 60% of the maximum output current. The following

equation calculates the recommended inductor value.

.1 − ' +,

- /

' +,

∆

(7)

Where L is the inductor value in µH, VBUCK is the output

voltage, VSTR is the maximum storage voltage, FSW is

the switching frequency in Hz, and ΔIL is the desired

ripple current in Amperes.

Output Capacitor Selection

The buck converter is designed to take advantage of the

benefits of ceramic capacitors, namely small size and

very-low ESR. The buck converter is designed to

operate with 44µF output capacitor over most of its

operating ranges, although more capacitance may be

desired depending on the duty cycle and load step

requirements.

Choose

ripple

voltage

that

approximately 1% of the output voltage setpoint. Note

that the output capacitance must be placed at the output

the

buck

converter.

Additional

downstream

capacitance will be placed at the loads, but this

capacitance should not be considered when calculating

the buck output capacitance. However, the downstream

capacitance should be considered when compensating

the power supply. The following equation calculates the

output voltage ripple as a function of output capacitance.

Note that the worst case ripple voltage occurs at the

beginning of supplement mode when the storage

capacitors are fully charged.

∆

1 223

(8)

Where Vripple is the desired output ripple voltage, FSW is

the switching frequency in Hz, and ΔIL is the maximum

ripple current in Amperes.

As with the input capacitor selection, use X5R or X7R

dielectrics and be sure to consider the capacitor’s DC

bias effects.

STORAGE BOOST CONVERTER

General Description

The ACT4921 contains an integrated peak current-

mode, synchronous boost converter. It minimizes

system level costs by using the same components as

the buck converter. The peak current is adjustable

between 250mA and 950mA, allowing for system

optimization. The output voltage is adjustable between

5V and 28V via I2C registers and an external resistor.

The peak current-mode control topology eliminates the

need for compensation. The boost automatically

charges up the storage capacitors from the input

voltage so they are ready to provide backup power in

the event of a system fault.

Startup

The boost converter automatically starts when the IC

exits the SOFTSTART state and the ENB pin is pulled

high. Note that the IC exits the SOFTSTART state 10ms

after the output voltage is within 200mV of the input

voltage. Most systems wait until the system is up and

running before enabling the buck and boost with the

ENB pin.

When the storage voltage reaches regulation, the boost

enters standby mode and monitors the storage voltage.

It automatically turns back on and “tops off” the storage

capacitors when the storage voltage drops below 95%

of the programmed voltage. The boost converter

automatically turns off if it is “topping off” the storage

capacitors when the IC enters supplement mode.

The average input current when the boost charges the

storage capacitors is approximately ½ of the peak

switching current. The peak switch current is programed

by register BST_CLIM[1:0].

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