GE865 Hardware User Guide
1vv0300799 Rev.9 – 27-07-2009
Reproduction forbidden without Telit Communications S.p.A. written authorization - All Rights Reserved
page 31 of 70
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.
