User's Manual

UE910 3G HARDWARE USER GUIDE 1vv0301171 Rev.3 • 2015-04-17 26 of 78
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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 on the input to protect the supply from spikes The placement of this
component 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 UE910 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 UE910 is wide enough to ensure a
dropless connection even during an 1A current peak.
 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 an 1A current peak is absorbed.
 The PCB traces to the UE910 and the Bypass capacitor must be wide enough to ensure no
significant voltage drops occur. This is for the same reason as previous point. Try to keep this
trace as short as possible.
 To reduce the EMI due to switching, it is important to keep very small the mesh involved; thus
the input capacitor, the output diode (if not embodied in the IC) and the regulator have to form
a very small loop.This is done in order to reduce the radiated field (noise) at the switching
frequency (100-500 kHz usually).
 A dedicated ground for the Switching regulator separated by the 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.
 The insertion of EMI filter on VBATT pins is suggested in those designs where antenna is
placed close to battery or supply lines.
A ferrite bead like Murata BLM18EG101TN1 or Taiyo Yuden P/N FBMH1608HM101 can be
used for this purpose.
The below figure shows the recommended circuit: