AN47

www.DataSheet4U.com AN47 S i321 X L I N E F E E D P O W E R M O N I T O R I N G A N D P R O T E C T I O N Introduction The Silicon Laboratories’ ProSLIC products are designed to continuously monitor the power dissipated in each of the six external bipolar transistors in the linefeed circuit. These power measurement results are available to the user in software registers and are also used by the ProSLIC to protect linefeed transistors from damage due to overpower conditions. Using proper power threshold and thermal low-pass filter settings, the ProSLIC will either alert the user or automatically transition the open state in the event of an overpower condition. The thermal resistance (RTHJA) of the transistor is improved when it is mounted on a PCB board. This improvement depends on the PCB size, the material it is made of, and the amount of the copper surface on the PCB board. Figure 1 illustrates how the board material, available board area, and the amount of copper present influence the thermal resistance of the transistors. This chart can be obtained from the transistor manufacturer if not included in the transistor data sheet. As the dissipated power in linefeed transistors increases, so does the junction temperature of the transistor die. The maximum admissible junction temperature must not be exceeded because this could damage or destroy the transistor die. In the Si321x, the measured power consumed in each of the transistors is compared to the power threshold values ...