STA321 datasheet(58/157 Pages) STMICROELECTRONICS

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FFX

STA321

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Doc ID 15351 Rev 3

7.7

pfStart™ operation

In order to avoid pop noise the bypass capacitor, situated between the filtered amplifier

output and the load in single-ended applications, needs to be pre-charged to half of the

power supply voltage. This is usually done by connecting a resistive partition to the output

and then disconnecting it at the end of the charging phase (see the analog pop free

description in section 9).

In the STA321 the FFX digital pop-free feature allows the digital pre-charging of the bypass

capacitor using the amplifier instead of a resistive partition. This active pre-charge is also

faster than the resistive partition method. The digital pop-free function can be independently

set on both power stages, that is, the CMOS bridge stage using bit CB_PFDIG and the

embedded amplifier stage using bit EA_PFDIG in register FFXCFG2 on page 83.

Registers CB_PFRAMP1-6 beginning on page 97 and EA_PFRAMP1-6 beginning on page

99 control the charging function. The register usage is given in the following description.

The capacitor is charged from zero to half the supply voltage with the PWM signal. By

applying a suitable ramp to the input of the modulator the PWM signal begins from near 0%

duty cycle to 50% duty cycle.

The method is based on a slow ramp signal (from ground to VCC / 2), implemented using

both pulse density modulation (PDM) and pulse width modulation (PWM). At the beginning

of the ramp PDM is used starting from an initial value set by bits CBRMPINI and EARMPINI,

and then switching to PWM when reaching a threshold value set by bits CBRMPTH and

EARMPTH.

The total ramp time can be modified via bits EATIM_RMP and CBTIM_RMP.

Figure 35.

Digital pop-free ramp implementation

The PDM is realized with a noise shaper circuit, where the sampling time (Td) of the noise

shaper is equal to the minimum pulse size set by bits CBRMP_MP and EARMP_MP.

1000…000

RAMP_INIT

RAMP_THOLD

0000…000

Output

PDM

PWM

Ramp

value

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