User's Guide

LE910Cx HW User Guide
Doc#: 1VV0301298
Rev. 13.0 Page 59 of 124 2019-07-15
6.3.3. Power Supply PCB Layout Guidelines
As seen in the electrical design guidelines, the power supply must have a low ESR
capacitor on the output to cut the current peaks and a protection diode on the input to
protect the supply from spikes and polarity inversion. The placement of these components
is crucial for the correct operation of the circuitry. A misplaced component can be useless
or can even decrease the power supply performances.
• The bypass low ESR capacitor must be placed close to the LE910Cx power input
pads, or if the power supply is of a switching type, it can be placed close to the
inductor to cut the ripple, as long as the PCB trace from the capacitor to LE910Cx
is wide enough to ensure a drop-less connection even during the 2A current
peaks.
• The protection diode must be placed close to the input connector where the power
source is drained.
• The PCB traces from the input connector to the power regulator IC must be wide
enough to ensure that no voltage drops occur during the 2A current peaks.
Note that this is not done in order to avoid RF power loss but to avoid the voltage
drops on the power line at the current peaks frequency of 216 Hz that will reflect
on all the components connected to that supply (also introducing the noise floor at
the burst base frequency)
For this reason, while a voltage drop of 300-400 mV may be acceptable from the
RF power loss point of view, the same voltage drop may not be acceptable from
the noise point of view. If your application does not have an audio interface but
only uses the data feature of the LE910Cx, this noise is not so disturbing, and the
power supply layout design can be more forgiving.
• The PCB traces to LE910Cx and the bypass capacitor must be wide enough to
ensure that no significant voltage drops occur when the 2A current peaks are
absorbed. This is needed for the same above-mentioned reasons. Try to keep
these traces as short as possible.
• The PCB traces connecting the switching output to the inductor and the switching
diode must be kept as short as possible by placing the inductor and the diode very
close to the power switching IC (only for the switching power supply). This is done
to reduce the radiated field (noise) at the switching frequency (usually 100-
500 kHz).
• Use a good common ground plane.
• Place the power supply on the board in a way to guarantee that the high current
return paths in the ground plane do not overlap any noise sensitive circuitry, such
as the microphone amplifier/buffer or earphone amplifier.
• The power supply input cables must be kept separate from noise sensitive lines,
such as microphone/earphone cables.