STA321 View Datasheet(PDF) - STMicroelectronics
Part Name
Description
MFG CO.
'STA321' PDF : 157 Pages View PDF
FFX
STA321
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
0000…000
Ramp
value
RAMP_THOLD
RAMP_INIT
1000…000
Output
0
PDM
PWM
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.
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Doc ID 15351 Rev 3
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