ACT4910 datasheet(20/37 Pages) QORVO

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

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ACT4910

Power Loss Protection with 6A eFuse

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.

��_��(��)=��(��)×��

0.9×��(��)

(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 ACT4910 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 320kHz to 1130

kHz via the I2C register BK_FREQ[2:0] which allows the

system to be optimized for different applications. Its

current limit is adjustable between 5A to 9A, allowing for

further system optimization. The output voltage is

externally programmable between 1.8V and 18V.

The buck converter generates a regulated output

voltage at the VOUT pin from the storage capacitors

when the IC enters supplement mode. This provides the

backup power when the system experiences fault

conditions. 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 3.6V.

Frequency Setting

Higher switching frequencies result in smaller solution

sizes at the cost of slightly lower efficiency. Lower

switching frequencies result in larger solution sizes with

higher efficiency. The maximum allowable switching

frequency for any given design is limited by the following

equation.

��_��< ��

��×180��

(5)

Where FSW_max is the maximum allowable frequency,

VSTR is the storage voltage, and VOUT is the output

voltage during supplement mode.

Output Voltage Setting

The buck converter output voltage is programmed by an

external resistor divider connected between the VOUT

pin and VSS, with the center tap connected to the FB

pin. The buck output voltage can be set above, below,

or equal to the input voltage supplement threshold.

When the input voltage drops below this threshold, the

IC enters supplement mode and regulates the output to

the programmed buck voltage. Although the buck

converter immediately starts up when the IC enters

supplement mode, the output voltage still has a small,

but finite drop in output voltage between the time the

eFuse turns off and the buck converter is fully on. This

voltage drop should be considered when setting the

output voltage. The following equation calculates the

correct resistor values to set the desired output voltage.

��1=��2×��−1�

(6)

Where R1 is the top feedback resistor, R2 is the bottom

feedback resistor, VOUT is the desired output voltage,

and VFB is the fixed 1.2V reference voltage on the FB

pin. Choose R2 in the range of 10kohm. Smaller resistor

values are acceptable, but larger values will affect

voltage accuracy due to bias currents into the FB pin.

Protection

The buck converter has several protection mechanisms

to insure safe operation. It stops operation when input

voltage from storage cap reaches STR_UVLO (3.6V) or

when the output voltage drops below the power good

threshold which is fixed at 93% of the output setpoint. It

also stops operating when the output voltage is above

the over voltage threshold which is fixed at 110% of the

output voltage setpoint. The output undervoltage

protection can be masked by the I2C register bit

Mask_BK_UV REG0x38 [0].

The buck converter provides overcurrent and short

circuit protection. Overcurrent protection is achieved

with cycle-by-cycle current limiting. The peak current

threshold is set between 5A and 9A by the BK_CLIM bits.

If the peak current reaches the programmed threshold,

the IC turns off the power FET. This condition typically

results in shutdown due to an output voltage UV

condition due to the shortened switching cycle.

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