HE920 Hardware User Guide
1vv0301014 Rev.9 – 2014-01-23
Reproduction forbidden without written authorization from Telit Communications S.p.A. - All Rights
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Page 40 of 84
area as large as possible under HE920 which you will mount. You must
mount HE920 on the
large ground area of your application board and make many ground vias to dissipate the heat.
The peak current consumption in the GSM mode is higher than that in WCDMA. However,
considering the heat sink is more important in case of WCDMA.
As mentioned before, a GSM signal is bursty, thus, the temperature drift is more insensible
than WCDMA. Consequently, if you prescribe the heat dissipation in the WCDMA mode,
you don’t need to think more about the GSM mode.
5.2.3.
Power Supply PCB Layout Guidelines
As seen in the electrical design guidelines, the power supply must 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 HE920 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 if the PCB trace from the capacitor to
HE920 is wide enough to ensure a drop-less 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 to 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 (also
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 does not have audio interface but only uses the data feature of
the Telit HE920, then this noise is not so disturbing and power supply layout
design can be more forgiving.
The PCB traces to HE920 and the Bypass capacitor must be wide enough to
ensure no significant voltage drops to occur when the 2A current peaks are
absorbed. This is a must for the same above-mentioned reasons. 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 (usually 100-500 kHz).
The use of a good common ground plane is suggested.
The placement of the power supply on the board must be done in a way to
guarantee that the high current return paths in the ground plane are not
