UL865-N3G Hardware User Guide
1VV0301177 Rev 2– 2015-04-20
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Page 31 of 70
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 performances.
The Bypass low ESR capacitor must be placed close to the Telit UL865-N3G V2
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 UL865-N3G V2 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 UL865-N3G V2,
then this noise is not so disturbing and power supply layout design can be more
forgiving.
The PCB traces to the UL865-N3G V2 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.
