Telit Wireless Solutions UC864 manual Thermal Design Guidelines

UC864-E Hardware User Guide

1vv0300766a Rev.1 - 31/01/08

5.2.2 Thermal Design Guidelines

The thermal design for the power supply heat sink should be done with the following specifications:

• Average current consumption during WCDMA transmission @PWR level max: 620mA

•Average current consumption during class12 GPRS transmission @PWR level max: 930mA

NOTE: The average consumption during transmissions depends on the power level at which the device is requested to transmit by the network. The average current consumption hence varies significantly.

Considering the very low current during idle, especially if Power Saving function is enabled, it is possible to consider from the thermal point of view that the device absorbs current significantly only during calls.

If we assume that the device stays into transmission for short periods of time (let's say few minutes) and then remains for a quite long time in idle (let's say one hour), then the power supply has always the time to cool down between the calls and the heat sink could be smaller than the calculated one for 930mA maximum RMS current, or even could be the simple chip package (no heat sink).

Moreover in the average network conditions the device is requested to transmit at a lower power level than the maximum and hence the current consumption will be less than the 930mA, being usually around 150mA.

For these reasons the thermal design is rarely a concern and the simple ground plane where the power supply chip is placed can be enough to ensure a good thermal condition and avoid overheating. For the heat generated by the UC864-E, you can consider it to be during transmission 1W max during CSD/VOICE calls and 2W max during class12 GPRS upload.

This generated heat will be mostly conducted to the ground plane under the UC864-E; you must ensure that your application can dissipate it.

In the WCDMA mode, since UC864-E emits RF signals continuously during WCDMA transmission, you should pay special attention on how to dissipate the heat generated.

The current consumption will be up to about 620mA continuously at the maximum TX output power (24dBm). Thus, you should arrange the PCB area as large as possible under UC864-E, which you will mount. You can mount UC864-E on the large ground area of your application board and make many ground vias for heat sink.

The peak current consumption in the GSM mode is higher than that in WCDMA. However, considering the heat sink is more important with WCDMA.

As mentioned before, GSM signal is bursty. Thus, the temperature drift is more insensible than WCDMA. Consequently, if you prescribe the heat dissipation in the WCDMA mode, you don’t need to think more about the GSM mode

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