CC864-DUAL Hardware User Guide
1vv0300791 Rev 4.4 – 2011-01-26
Reproduction forbidden without Telit Communications S.p.A’s. written authorization - All Rights Reserved.
Page 30 of 63
The placement of these components is crucial for the correct operation of the circuitry
and application.
A misplaced component can be ineffective or even decrease the power supply
performance. Therefore, the following guidelines are offered:
The Bypass low ESR capacitor must be placed close to the module power
input pads. If the power supply is of the switching variety it can be placed
close to the inductor to cut the ripple provided the PCB trace from the
capacitor to the module is wide enough to ensure no voltage drops during the
transmission 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 transmission
current peaks.
Note
: (GSM/UMTS specific consideration): This recommendation is not
made to save power but instead 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 a noise floor at the burst
base frequency. For this reason, while a voltage drop of 300-400 mV may be
acceptable for power loss, it may not be acceptable for noise considerations.
If the application does not have an audio interface but only uses GSM/UMTS
data, then this noise may not be so disturbing and power supply layout design
can be more forgiving.
For the reasons outlined above, the PCB traces to the module and the bypass
capacitor must be wide enough to ensure no significant voltage drops occur
during the GSM 2A/CDMA 1A current peaks. This trace should be 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 will 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 should guarantee that the
high current return paths in the ground plane are not overlapped with any
noise sensitive circuitry such as the microphone amplifier/buffer or earphone
amplifier.
