Telit Wireless Solutions GM862-GPS - Manual (Page 18)

GM862-GPS Hardware User Guide
1vv0300728 Rev. 0 - 27/04/06
Reproduction forbidden without Telit Communications S.p.A. written authorization - All Right reserved page 18 of 55
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-GPS 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-GPS 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-GPS, then this noise is not so disturbing and power
supply layout design can be more forgiving.
• The PCB traces to the GM862-GPS 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.