Telit Wireless Solutions UC864 manual Power Supply PCB layout Guidelines

UC864-E Hardware User Guide

1vv0300766a Rev.1 - 31/01/08

5.2.3 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 UC864-E power input pads or in the case of a switching power supply it can be placed close to the inductor to cut the ripple provided the PCB trace from the capacitor to UC864-E 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 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 UC864-E, then this noise is not so disturbing and power supply layout design can be more forgiving.

•The PCB traces to UC864-E and the Bypass capacitor must be big 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 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.

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