Active antenna driver for car infotainment systems: Page 3 of 4

June 12, 2014 //By Bernd Stein, Infineon
Active antenna driver for car infotainment systems
The days of simple car radios using large telescope antennas are over. Nowadays cars are equipped with full featured infotainment or multimedia systems. While the AM/FM radio is still a part of the system, features like digital audio broadcasting (DAB), HDTV, satellite radio, integrated cell phones, CD/DVD/MP3 players, GPS navigation or video game systems are added.
Furthermore this leads to higher reliability (better FIT rate).

The TLF4277-2 enables a faster reaction time for the microcontroller at error cases compared to previous solutions. The immediate error flag for the microcontroller improves the system reliability.

Principal function of the voltage regulator

The output voltage VQ is controlled by comparing the feedback voltage ( VADJ) to an internal reference voltage and driving a PNP pass transistor accordingly. The control loop stability depends on the output capacitor CQ, the output series resistivity ESR, the load current and the chip temperature. For a stable operation a ceramic output capacitor with an ESR range from 1mΩ to 10Ω and only 1 μF is needed. The output capacitor may need to be sized larger to buffer load transients. An input capacitor is not needed for the control loop stability, but recommended to buffer line influences.

Protection circuitry prevents the IC as well as the application from destruction in case of catastrophic events. The integrated safeguards consist of output current limitation, reverse polarity and short circuit protection as well as thermal shutdown in case of over temperature. The thermal shutdown circuit prevents the IC from immediate destruction under fault conditions (e.g. output continuously short-circuited) by switching off the power stage. After the chip has cooled down, the regulator restarts. This leads to a cycling behavior of the output voltage until the fault is removed. The TLF4277-2 allows a negative supply voltage. However, several small currents are flowing into the IC increasing its junction temperature. This reverse current has to be considered for the thermal design, respecting that the thermal protection circuit is not operating during reverse polarity condition.

The output voltage of the TLF4277-2 can be adjusted between 5 V and 12 V by an external output voltage divider, closing the control loop to the voltage adjust pin ADJ.

The voltage at pin ADJ is compared to the internal reference of

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