User's Manual
LE920 Hardware User Guide
1vv0301026 Rev.8 2015-01-03
Reproduction forbidden without written authorization from Telit Communications S.p.A. - All Rights
Reserved. Page 43 of 88
Even though peak current consumption in GSM mode is higher than in WCDMA,
consideration for the heat sink is more important in the case of WCDMA.
As mentioned before, a GSM signal is bursty, thus, the temperature drift is more insensitive
than WCDMA. Consequently, if you successfully manage heat dissipation in WCDMA mode,
you don’t need to think more about 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 LE920 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 as long as the PCB trace from the capacitor
to LE920 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 occur during the 2A current peaks. Note that
this is not done 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 LE920, then this noise is not
so disturbing and power supply layout design can be more forgiving.
The PCB traces to LE920 and the bypass capacitor must be wide enough to
ensure no significant voltage drops occur when the 2A current peaks are
absorbed. This is needed 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
overlapping any noise sensitive circuitry such as the microphone amplifier/buffer
or earphone amplifier.