Datasheet

Application information A7986A
28/42 DocID022801 Rev 5
Figure 16. Switching losses
6.6 Layout considerations
The PC board layout of switching DC-DC regulator is very important to minimize the noise
injected in high impedance nodes and interference generated by the high switching current
loops.
In a step-down converter the input loop (including the input capacitor, the power MOSFET
and the freewheeling diode) is the most critical one. This is due to the fact that the high
value pulsed current is flowing through it. In order to minimize the EMI, this loop must be as
short as possible.
The feedback pin (FB) connection to the external resistor divider is a high impedance node,
so the interference can be minimized by placing the routing of the feedback node as far as
possible from the high current paths. To reduce the pick-up noise, the resistor divider must
be placed very close to the device.
To filter the high frequency noise, a small bypass capacitor (220 nF - 1 µF) can be added as
close as possible to the input voltage pin of the device.
Thanks to the exposed pad of the device, the ground plane helps to reduce the thermal
resistance junction to ambient; so a large ground plane enhances the thermal performance
of the converter allowing high power conversion.
In Figure 17, a layout example is shown.