HE910 Hardware User Guide
1VV0300925 Rev. 32 Page 48 of 111 2019-11-25
5.3.3. 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 HE910 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 HE910 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
HE910, then this noise is not so disturbing and power supply layout design can be
more forgiving.
• The PCB traces to the HE910 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.
• 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.
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