User guide

UC864-E/G/WD/WDU Hardware User Guide

1vv0300766a Rev.4

- 03/02/09

5.2.3. 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 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 UC864-

E/G/WD/WDU 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 if the

PCB trace from the capacitor to UC864-E/G/WD/WDU is wide enough to

ensure a drop-less 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 to 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 (also 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 does not

have audio interface but only uses the data feature of the Telit UC864-

E/G/WD/WDU, then this noise is not so disturbing and power supply

layout design can be more forgiving.

• The PCB traces to UC864-E/G/WD/WDU and the Bypass capacitor must

be wide enough to ensure no significant voltage drops to occur when the

2A current peaks are absorbed. This is a must for the same above-

mentoned 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 to any noise sensitive circuitry as the microphone

amplifier/buffer or earphone amplifier.

• The power supply input cables must be kept separately from noise

sensitive lines such as microphone/earphone cables.