User's Manual

6.3.3. Power Supply PCB layout Guidelines
As seen on the electrical design guidelines the power supply shall 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
working 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 Telit GE865 power input pads
or in the case the power supply is a switching type it can be placed close to the inductor
to cut the ripple provided the PCB trace from the capacitor to the GE865 is wide enough
to ensure a dropless 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 no voltage drops occur when the 2A current peaks are absorbed. Note that this
is not made in order to save power loss but especially 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, 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 power loss point of view, the same voltage drop may not be acceptable from the noise
point of view. If your application doesn't have audio interface but only uses the data
feature of the Telit GE865, then this noise is not so disturbing and power supply layout
design can be more forgiving.
 The PCB traces to the GE865 and the Bypass capacitor must be wide enough to ensure
no significant voltage drops occur when the 2A current peaks are absorbed. This is for
the same reason as previous point. Try to keep this trace 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 switching power supply). This is done in order to reduce
the radiated field (noise) at the switching frequency (100-500 kHz usually).
 The use of a good common ground plane is suggested.
 The placement of the power supply on the board should be done in such a way to
guarantee that the high current return paths in the ground plane are not overlapped to any
noise sensitive circuitry as the microphone amplifier/buffer or earphone amplifier.
 The power supply input cables should be kept separate from noise sensitive lines such as
microphone/earphone cables.