DocID026803 Rev 1

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AN4569

2.1-channel applications using two devices

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inductance with the copper wires on the PC board. For each VCC pin, two capacitors are

needed (100 nF and 1 μF). The meaning of each decoupling component is as follows:



C23S and C24M are part of the power supply and they are used as a far tank capacitor.



The 100 nF capacitors (C24S, C24M, C25S and C25M) smooth the high-frequency

spikes



The 1 μF capacitors (C26 and C27) smooth the low-frequency spikes and they are also

used as a close tank for the 100 nF capacitor.

The decoupling capacitors on the VDD_DIG and PLL_VDD pins (C1, C2 and C3) are placed

close to the pins.

The reason why the VDD_DIG and PLL_VDD are separated with beads is to avoid high

frequencies on VDD_DIG. It is possible to avoid these components if the layout is done with

the right star connections.

Decoupling capacitors on regulators (C6 and C7) are placed close to the device.

All decoupling capacitors must be X7R. This is because the X7R capacitor is stable over

temperature (±15%).

For example, the Y5V dielectric is very unstable (specs are +22% -82%).

The next figure shows THD at different values of the tank capacitor (C23) in the ST demo

board using the bench power supply. The minimum THD is similar using 10 μF or 1000 μF

capacitors. The main difference is at medium-high power where the tank is not sufficient to

guarantee the peak current.

Figure 13. THD+Noise ratio with different values of the tank capacitor