Telit GM862-QUAD/-QUAD-PY
Hardware User Guide
1vv0300692, Rev. ISSUE#0, - 04/10/2005
Reproduction forbidden without DAI Telecom written authorization – All Right reserved – Right of modification reserved
page 15 of 69
If we assume that the device stays into transmission for short periods of time (let's say few minutes)
and then remains for a quite long time in idle (let's say one hour), then the power supply has always
the time to cool down between the calls and the heat sink could be smaller than the calculated one
for 350mA maximum RMS current, or even could be the simple chip package (no heat sink).
Moreover in the average network conditions the device is requested to transmit at a lower power
level than the maximum and hence the current consumption will be less than the 350mA, being
usually around 150mA.
For these reasons the thermal design is rarely a concern and the simple ground plane where the
power supply chip is placed can be enough to ensure a good thermal condition and avoid
overheating.
3.2.3 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 GM862-QUAD power input
pins 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 GM862-QUAD 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 GM862-QUAD, then this noise is not so
disturbing and power supply layout design can be more forgiving.
•
The PCB traces to the GM862-QUAD and the Bypass capacitor must be wide enough to ensure
no 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.
