User Manual

GM862-QUAD / PY Hardware User Guide

1vv0300748 Rev. 5 - 20/09/07

6.2.3 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 / PY 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 / PY 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 / PY, then this noise is not so disturbing and

power supply layout design can be more forgiving.

• The PCB traces to the GM862-QUAD / PY 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.

• The placement of the power supply on the board should be done in such a way to guarantee that

the high current return paths in the ground plane are not overlapped to any noise sensitive circuitry

as the microphone amplifier/buffer or earphone amplifier.

• The power supply input cables should be kept separate from noise sensitive lines such as

microphone/earphone cables.