subject: ProSLIC products are designed to continuously monitor the power dissipated [print this page] ProSLIC products are designed to continuously monitor the power dissipated
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.
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 in the corresponding indirect registers. If the power in any external transistor exceeds the programmed threshold (after passing through a user-programmable low pass filter which will be explained in the next section), a power alarm is triggered to indicate line fault condition. Unless the auto-open feature is disabled (direct register 67, bit 0), the ProSLIC automatically goes into the open state.
The value of the power threshold is calculated based on the characteristic of the transistors used. Transistor manufacturers provide this information in terms of thermal resistance for each transistor package. The relationship between the maximum junction temperature and the maximum power that can be dissipated by the transistor package is defined in the following equation:
TJMAX = TAMB + PMAX RTHJA
where TJMAX is the maximum junction temperature (usually 150 C), TAMB is the ambient temperature (70 C for commercial rating), and PMAX is the maximum power allowance on the transistor package. RTHJA is the junction to ambient thermal resistance of the transistor package.
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.
In practice, the transistors are normally mounted on a PCB with several square inches area, but for illustration purposes consider a model in which the transistor package is mounted on 1-inch square of FR4 PCB with 0.25-inch square of copper surface. This 1-inch square PCB model and the thermal resistance vs. PCB area charts provide the practical thermal resistances for the following transistor packages:
SOT23: RTHJA = 200 C/W
The thermal resistance can also be obtained from the transient thermal resistance curve with D = 1
SOT89: RTHJA = 82.5 C/W
SOT223: RTHJA = 62.5 C/W
Thermal Low Pass Filter
While the power threshold coefficient sets the absolute maximum dc power that the transistor can handle for an indefinite period of time, it only provides a static maximum dc trip point. In the Si321x circuit application, the transistors are subjected to complex power dissipation, which is comprised of dc biasing current and ac signaling. The ac part of the power dissipation may be limited to short times and with repeated pulse (ringing). A static maximum power threshold setting does not provide an adequate model for real operating conditions. In conjunction with the power threshold setting, the Si321x also provides the thermal low pass filter setting which models the operating condition more accurately.
Calculation of the thermal low pass filter is based on the characteristic of the transistor package. The heating process of the transistor package is an exponential phenomenon which can be described by the following equation:
T(t) = TDC (1 e t )
Where TDC is the final temperature and, is the thermal time constant. Thermal resistance () may replace the temperature (T) in this equation since they both represent the temperature of the transistor package allowing the setting of the thermal constants () to the registers.
Power Dissipation in the Si3201
The Si3201 is a line-side IC that replaces the discrete transistors in the Si321x schematic. Because the Si3201 circuitry differs from that of the discrete components, it is difficult to compute a maximum power threshold per transistor. Silicon Laboratories recommends SOT89 register settings when using the Si3201 linefeed IC.
