DocID024368 Rev 3

15/24

STPA002

General information

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5

General information

5.1

Operation

The STPA002's inputs are ground-compatible. If the standard value for the input capacitors

(0.22 μF) is adopted, the low frequency cut-off will amount to 16 Hz. The input capacitors

should have the same value of the capacitor connected to AC-GND pin for optimum pop

performances (see Figure 2: Standard test and application circuit (Flexiwatt25)).

Standby and mute pins are 3.3 V and 5 V compatible.

RC cells at both mute and stand-by pins have always to be used in order to smooth the

transitions for preventing any audible transient noise. A time constant slower than 2.5 V/ms

is suggested for the stand-by pin and 0.5 V/ms for the mute pin.

In case the standby function is not used, it could steadily be connected to Vs through a

470 kΩ resistor.

The capacitance on SVR sets the start-up and shut-down times and helps to have pop-noise

free transitions. Its minimum recommended value is 10 μF. However, to have a fast start-up

time, the internal resistor on SVR pin, used to set the time constant, is reduced from 100 kΩ

to 3 kΩ till voltage on SVR reaches VCC/4 -2VBE and then released. In this way the

capacitor on SVR is charged very quickly to VCC/4, as shown in the following figure.

Figure 20. SVR charge diagram

SVR pin accomplishes multiple functions:



it is used as a reference voltage for input pins (VCC/4)



the capacitor connected to SVR helps the supply voltage ripple rejection



it is used as a reference to generate the half supply voltage for the output

When the amplifier goes in stand-by mode or goes out from this condition, it is suggested to

put the amplifier in mute to ensure the absence of audible noise. Then the standby pin can

be set to the appropriate value (ground or > 2.6 V) and the capacitance on SVR pin is

discharged or charged consequently.