Active antenna driver for car infotainment systems
AM/FM, DAB and satellite signals are received by active antenna systems with stubby, shark fin or windshield antennas. As car infotainment units have becoming more complex, the number of active antenna systems has grown as well. Sensitive circuitry like LNAs in these active antenna systems requires a stable power supply, protection and isolation from the harsh automotive environment as well as monitoring and diagnostic functions to provide antenna status. The new TLF4277-2 (figure 1) is designed to reliable supply active antennas in car infotainment applications.
Fig. 1: TLE4277-2 – second generation of single-chip ICs for active antenna supply with integrated precise current monitoring and enhanced protection features
An active antenna is an antenna that contains active electronic components. This allows – different to passive antennas – the construction of antennas of limited size and/or wide frequency range. Power for the active components may be supplied by batteries, a filtered power supply, or through the signal feeder itself (phantom power). The TLF4277-2 low drop out voltage regulator provides the related voltage to the active antenna. The adjustable output voltage (5 V to 12 V) makes the new voltage regulator capable of supplying the majority of standard active antennas, such as FM/AM, DAB, XM or SIRIUS.
The TLF4277-2 protects both the antenna and itself in this demanding electrical environment, while providing a stable output voltage. The IC also protects active antenna supply circuitry in the event of a short circuit within the antenna supply itself with an adjustable current limitation. Thermal conditions in automotive environments are equally challenging, requiring the supply to be stable over the automotive temperature range of -40 to 150°C with robust over temperature protection. In addition to these protection requirements, the TLF4277-2 simplifies the gathering of diagnostic information required to report antenna status.
New generation with enhanced features
The TLE4277-2 is the second generation of the TLE4277 family which was successfully implemented in various active antenna applications and shipped in high volume. The new voltage regulator provides several enhancements like better accuracy (of VQ & current monitor), higher output current and less external components. The TLF4277-2 is a monolithic integrated low drop out voltage regulator (figure 2) capable of supplying loads up to 300 mA. For an input voltage up to 40 V the TLF4277-2 provides an adjustable output voltage in a range from 5 V up to 12 V. The integrated current monitor function is a unique feature that provides diagnosis and system protection functionality.
Fig. 2: Simplified block and application diagram of the TLF4277-2
Fault conditions such as over-temperature and output overvoltage are monitored and indicated at the current sense output. The maximum output current limit of the device is adjustable to provide additional protection to the connected load. Via the enable function the IC can be disabled to lower the power consumption. Other features are a low dropout voltage (typ. 100 mV) and very low power consumption (typ. 150 µA at 25°C).
Accurate antenna status monitoring is provided by very accurate current sense, with a continuous monitoring of the load. Compared to other solutions with current sense accuracy in the range of 10 to 30 percent, the TLF4277-2 provides accuracy (current monitor factor) as high as 5 % across the full range of output current. Compared to previous solutions the output voltage tolerance is also improved (2 % compared to 3 – 5%). The adjustable maximum current limitation in the range of 10 mA to 300 mA improves amplifier lifetime. The digital status output signal shows if one or more of the monitored protection functions (over current, over voltage or temperature shutdown) is activated.
The TLF4277-2 needs less external components reducing the related PCB space. No external shunt is needed, which results in optimized systems costs. Reduced component count is a critical aspect as most infotainment applications in cars are implemented on expensive multilayer boards. 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 typical 1.19 V in an error amplifier. It controls the output voltage.
Current and Protection Monitor
The TLF4277-2 provides a set of advanced monitor functionality (figure 3). The current flowing out of the power stage can be monitored at the CSO output. In addition the current limitation can be adjusted via external resistor. Events of the implemented protection functions are reported through dedicated voltage levels at the CSO output.
Fig. 3: Block diagram of the integrated current and protection monitor
This information can be processed by an external microcontroller for system analysis and failure identification. The monitored events are over-current, overvoltage, and temperature shutdown. In addition all three fault conditions are routed also to the digital output pin ST.
The new voltage regulator is automotive certified according to AECQ100. In addition the TLF4277-2 is a green product, compliant to the RoHS and WEE standards. It comes in a PG-SSOP-14 EP package which provides an enhanced thermal performance within a SO-8 body size. The single chip solution is ideal for active antenna supply, providing improved performance and reliability at less system costs. Besides the power supply of active antennas for AM/FM, DAB, GPS or satellite applications the TLF4277-2 may be also used in microphones for hands free car kits, ATV camera systems , various industrial applications or LNA systems (Digital TV, AM FM, etc.). Detailed information about the new active antenna driver is available at: www.infineon.com/TLF4277-2.
About the author:
Bernd Stein is Product Manager, Linear Voltage Regulators Infineon Technologies, Neubiberg, Germany. He can be reached under email@example.com