User's Manual

HE910 Hardware User Guide

1vv0300925 Rev.28 – 2015-06-24

5.3.3 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 HE910 power

input pads 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

HE910 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 HE910, then this noise is not so

disturbing and power supply layout design can be more forgiving.

• The PCB traces to the HE910 and the Bypass capacitor must be wide enough to

ensure no significant 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.