User's Manual

GE866-DUAL Hardware User Guide
1VV0301051 Rev. 2 – 2014-04-07 7
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Reserved. Page 32 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.