GE866-QUAD Hardware User Guide
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Page 35 of 82
Mod. 0805 2011-07 Rev.2
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 performance.
•
The Bypass low ESR capacitor must be placed close to the Telit GE866-QUAD
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 GE866-QUAD 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
approx. 217 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
GE866-QUAD, then this noise is not so disturbing and power supply layout design
can be more forgiving.
•
The PCB traces to the GE866-QUAD 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.
