User's Manual

CL865-DUAL Hardware User Guide
1vv0301104 Rev.1 – 2014-03-07
Reproduction forbidden without written authorization from Telit Communications S.p.A. - All Rights
Reserved. Page 30 of 70
Mod. 0805 2011-07 Rev.2
6.3.6. 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 operation of the circuitry. A misplaced component can be useless or can even decrease
the power supply performance.
 The bypass low ESR capacitor must be placed close to the Telit CL865 power
input pads, or if the power supply is a switching type, the capacitor can be placed
close to the inductor to cut the ripple if the PCB trace from the capacitor to
CL865 is wide enough to ensure a drop-less connection even during the 1A
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 1A current peaks are absorbed.
While a voltage drop of hundreds of 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 the application does not have an audio interface but only uses the data
feature of the Telit CL865, then this noise is not as disruptive and the power
supply layout design can be more forgiving.
 The PCB traces to CL865 and the Bypass capacitor must be wide enough to
ensure no significant voltage drops occur when the 1A 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
overlapped with any noise sensitive circuitry such as the microphone
amplifier/buffer or earphone amplifier.
 The power supply input cables must be kept separate from noise sensitive lines
such as microphone/earphone cables.