In-car multimedia switches regulators
01 August 2006
At low output voltages and even with moderate current levels above a few hundred mA, a linear regulator generates too much heat, prompting the replacement of linear regulators with switching ones.
Significant growth is expected for DVD/HDD-based infotainment systems. The navigation system, AM/FM radio, CD player are increasingly adding DVD players with multiple outputs, satellite radio and even satellite TV. Complexity is added with rear-facing automotive cameras, distress communication systems and Bluetooth compatibility. Unit sales are expected to exceed 15million units worldwide in 2006.
Despite increasing component count, available space requirements continue to shrink. Practical heat sinking is too large, so power conversion efficiency becomes critical. At low output voltages and even with moderate current levels above a few hundred mA, it is no longer practical to use a linear regulator to generate these system voltages as they generate too much heat. A switcher has increased efficiency and smaller footprint, outweigh the additional complexity and EMI considerations.
A typical infotainment system (see figure 1) can have eight or more different supplies, including 8.5V (in Europe, 9.5V in Japan), 5V, 3.3V, 2.5V, 1.5V, and 1.2V. The 8V is used to supply power to the DVD motor, which spins up the disc, usually requiring peak currents of up to 2A. The five and 3.3V rails are typically the system buses and usually required to deliver two to 3A each. The 2.5V rail is for both memory and I/O; one to 2A is sufficient. The 1.5 and 1.2V rails power the CPU- and DSP-core voltages, respectively. The power levels of these two rails are usually between three and 5W each. Smaller power levels (less than 1W) are needed for biasing of the main TFTLCD (typically seven to 10in) as well as the video camera CCD. Another five to 10W are required for white LED backlighting of the TFT-LCD.
While switching regulators from handheld applications can be used in benign electrical environments, if the system being powered requires an always-on state, minimal quiescent current will be most important. Similarly, switching regulator ICs that have a switching frequency close to AM/RF/GPS circuitry must not create interference in these respective bands. LED driver ICs for backlighting the TFT-LCD must maintain a wide dimming range to compensate for even slight changes in ambient light. Finally, all of these DC/DC converters need to fit into compact formats as well as being efficient to eliminate heat sinking.
Battery power
Initial power must be derived from the car’s battery and charging system. Unfortunately, this is where the harshest electrical environment lies.
Load-dump transients are voltage spikes caused by disconnecting the battery being charged (see figure 2). This can produce transient voltage spikes as high 80V as the alternator is attempting a full charge. Transorbs on the alternator usually clamp the bus voltage somewhere between 36 and 60V. However, primary DC/DC converters downstream from the alternator can still be subjected to 60V transient voltage spikes. Converters and the sub-systems they power, are expected to operate during and after this transient event so DC/DC converters must be able to deal with 60 to 100V(MAX) transients. There are various protection circuits (usually transorbs), which can be implemented externally, but they add cost and take up space. High voltage switching regulators, such as Linear Technology’s LT3433, LT3434/5, LT1976/7 and LT3437/8, are capable of withstanding transient voltages as high as 80V while maintaining output regulation with no effect on system performance or reliability. Typically, these are step-down or buck-boost converters, which are exposed to the car’s battery and alternater voltage permutations caused by interruptions in the charging/battery system.
Cold crank, when a car’s engine is subjected to low temperatures for a period of time, causes the engine oil to become viscous. The starter motor has to deliver more torque, which needs more current from the battery. This large current load can pull the battery/primary bus voltage as low as 4V during start-up, after which it typically returns to a nominal 13.8V. Certain subsystems require a constant, well-regulated output throughout this cold crank condition. These applications typically operate from 3.3V and include engine control units (ECUs), environmental and emergency systems.
Any switching regulators that are connected directly to the battery need to work over an input range of three to 60V. This also gives a good margin for 14V systems that are usually clamped from 36 to 40V.
Always-on operation
GPS receivers, IR systems and system microprocessors require always-on operation. As the ignition is turned off they must be connected directly to the battery to operate in low-power mode. This requires using minimal quiescent current and withstanding the voltage variations encountered in the battery and charging
system when the ignition is turned back on.
Collectively, several hundreds of mA of supply current are required to maintain always-on processor voltages, which could completely drain a battery in a matter of weeks. Automotive manufacturers seek a maximum quiescent current target of 100uA for each always-on DC/DC converter. Until recently, the requirement of high input voltage capability and low quiescent currents were mutually exclusive parameters for a converter. Linear Technology’s high voltage, 100uA step-down DC/DC meet both of these constraints with maximum input from 60 to 80V and output currents range from 450mA (LT3437) to 2.5A (LT3434). Quiescent current is under 100uA. The LT3437 employs a BurstMode design which enables it to operate from only 100ìA of supply current, for always-on applications. Similar parts such as the LT3434 and LT1976 are available for higher current applications, such as 2.5A and 1.25A of output currents respectively.
After power has been delivered into the infotainment box, at 3.3 or 5V, it can directly drive many of the system’s internal electronics as many of them can operate from inputs of 2.7 to 5.5V. However, the CD/DVD drive motor needs 8.5V; some of the CPUs and DSPs require 1.2 to 1.5V; the CCD for the rear facing camera requires a biasing supply (15 and -8V) and the TFT-LCD requires biasing voltage (eight, 15V and -8V) as well as white LED backlighting (up to 36V). The primary power conversion from the battery has protected these rails from transients and over-voltage variations eliminating this requirement in secondary power conversions. Minimising switching noise becomes important as many power converters can conversions are close to sensitive AM, RF or GPS receivers. If a switching regulator switches at a constant frequency in normal mode and the switching edges are clean and predictable with no overshoot or high frequency ringing, then EMI is minimised. If the regulator can be used with low-ESR ceramic capacitors, both input and output voltage ripple can be minimised, which are potential sources of noise in the system.
Power demands
The DVD/CD drive motors require up to 2A of current at 8.5V. Power is generally taken directly from the battery and stepped-down to 8.5V. This enables a higher-efficiency solution compared to boosting the 8.5V from a lower voltage rail. Delivering power directly from the battery requires a converter with robust input voltage capability to protect the motor drive from high voltage transients.
Microprocessors in an infotainment system require 1.2 to 2.5V for their cores, as well as 2.5 or 3V for memory and I/O. They need to be stepped down from a main five or 3.3V intermediate rail. Efficiency is important, there is no room in the subsystems for heat sinking. A regulator, such as the LTC3407-2 with synchronous rectification topology offers up to 96 per cent efficiency. Its constant switching frequency is 2.25MHz, keeping noise out of the sensitive AM band of the adjoining audio receiver.
TFT panels generally require small biasing supplies such as 16V at 10mA and 8V at 10mA, in addition to an intermediate voltage (usually called AVDD) around 8V that requires up to 450mA of current. Devices with minimal footprint are most suitable. Rear-facing cameras will also require some type of biasing power for CCDs, generally they require voltages of 15V at 20mA and - 8V at 50mA.
TFT-LCDs backlighting traditionally uses CCFL lighting. However, smaller white LEDs are replacing CCFL in backlighting applications. Light variations in car interiors combined with the human eye’s sensitivity to changes in light intensity mean that dimming ranges of up to 1000:1 are required for backlighting. The LT3486 (see figure 4) is a dual step-up DC/DC converter specifically designed to drive up to 16 white 100mA LEDs (eight LEDs in series per channel) from a 12V input voltage. It acts as a constant current source to maintain constant brightness of wide LEDs even as their forward voltage varies. The LED driver uses True Color PWM dimming, which delivers constant full current to the LEDs with dimming controlled by reducing the PWM driver duty cycle.
JEFF GRUETTER is product marketing engineer, Linear Technology Corporation.
Contact Details and Archive...
Related Articles...
Most Viewed Articles...