In succession to the previous section on "Placement of Inductors", this time we explain "Placement of Output Capacitors", which are an important component. In conjunction with this discussion, it will be good to review the role played by output capacitors and properties required of them. Refer to the DC/DC Design Edition, “Selection of Output Capacitors”.
Differences in Currents in Output and Input Capacitors
As a review, we begin now by recalling that there are differences in the currents flowing in an input capacitor CIN and in an output capacitor CO. In the waveform diagram, ICO is the current waveform for an output capacitor, and ICIN below it is that for an input capacitor.
In the input capacitor, a comparatively large current repeatedly flows suddenly, but in the output capacitor, smooth charging and discharging is repeated, linked with an output ripple voltage and centered on the output voltage. This is because an inductor is inserted in series into the output line; the inductor L and CO serve as an output filter.
Placement of the Output Capacitor
The GND connection of the capacitor CO should be at a position 1 to 2 cm distant from the GND connection of CIN, and as close to the inductor as possible.
As stated above, in the input capacitor, a sharply rising/falling current flows repeatedly, and so a high frequency in the range of several hundred MHz flow into the GND pattern connected to CIN. Of course the GND pattern to which CO is connected is the same GND pattern, and so if Co is placed close to where CIN is connected, there is the possibility that the high-frequency noise in the input may pass through CO and be conducted to the output. This is shown schematically in the diagram on the lower right.
The reason for separating the GND of CO from the GND of CIN by 1 to 2 cm is that the inductance and resistance components of the thin film wiring can serve as a filter to reduce high-frequency noise. In other words, this design makes clever use of parasitic components
For an example of overall positional relationships, refer to the PCB pattern diagram above.
・An output capacitor should be placed as close as possible to an inductor.
・In order to reduce the propagation of high-frequency noise, the GND of CIN should be placed 1 to 2 cm distant from the GND of CO.