DocID026803 Rev 1

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AN4569

2.1-channel application using one device

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2.1.2

Decoupling capacitors

Figure 2. Schematic for decoupling

The decoupling capacitors on high-voltage pins (C24, C25, C26 and C27) must be placed

as close as possible to the VCC pins. This is in order to avoid parasitic induction 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:



C23 is a part of the power supply and it is used as a far tank capacitor.



The 100 nF capacitors (C24 and C25) 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 the pins.

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

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

with a right star connection.

The decoupling capacitors on the regulators (C6 and C7) are also placed close to the

device.

All decoupling capacitors must be X7R. This dielectric is preferable because the X7R

capacitor is stable over temperature changes (±15%).

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

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

demo board using the bench power supply. The minimum THD is similar using10 μF or

1000 μF capacitors. The main difference is at medium-high power where the tank is not

enough to guarantee the peak current.