Detailed explanation of the functions of the μPD168891 control IC for LED headlights

Renesas Electronics has developed the "μPD168891" IC that controls the lighting and extinguishing of automotive LED headlights , and has started supplying samples since August 30, 2010 (Figure 1). The sample price is 500 yen per unit. It is scheduled to enter mass production at a scale of 50,000 units per month from 2012, and gradually expand production from the following year. The company has been producing LED control ICs for consumer devices since 2009. Considering that the use of LEDs as headlight light sources will increase in the future for energy saving, the company has developed LED control IC business for automotive use. The LED control IC business for headlights is scheduled to be launched globally. Renesas Electronics expects that LED headlights will be popularized in the Japanese market earlier than in the European and American markets, so it is expected that more Japanese manufacturers will contact them.

Figure 1: The μPD168891 IC that turns LED headlights on and off

The μPD168891 is designed to drive and control up to 12 high-output white LEDs in series at rated current (Figure 2). The output is single-channel. The LED drive current ranges from 0.35 to 1A. The power supply voltage ranges from 6 to 18V, which is boosted and supplied to the LED. Renesas Electronics expects that in practice, eight high-output white LEDs are often connected in series, using a current of around 700mA. The switching frequency of the boost control is a maximum of 500kHz. Although the higher the switching frequency, the smaller the external inductor and capacitor can be, thus reducing the circuit area, this will cause interference with the AM band, so the maximum switching frequency is reduced to a level slightly lower than the frequency of the AM band.

Figure 2: Example of a headlight unit using an LED control IC

Built-in multiple protection functions

In order to reduce the size of the LED control circuit, the peripheral components are integrated. First, the IC has built-in boost drive and pre-drive circuits for two power MOSFETs that block the current path when an abnormality occurs. Then, various protection circuits required for LED control are integrated. The protection functions include: overcurrent protection and overvoltage protection for LED and power MOSFET, power supply short circuit (short circuit with battery system) and ground fault (short circuit with ground) protection for LED, overheat protection, power supply voltage drop and rise protection (low Vcc protection/high Vcc protection) and LED short circuit detection. When using a general-purpose LED control IC, if these circuits are constructed as external components, there will be a problem of increasing the printed circuit board area of ​​the LED control circuit. However, using this LED control IC, the area of ​​the printed circuit board may be reduced to about half of the original area depending on different situations.

In the above protection circuit, low Vcc protection is a function that prevents the LED from turning off immediately even if the power supply voltage suddenly drops. When the power supply voltage is 9V or above, the current is normally supplied to the LED. When it is lower than 9V and higher than 6V, the output current supplied to the LED is reduced, and the LED continues to light up in a way that reduces the brightness (Figure 3). Recently, the number of models that stop and restart the engine when the car is stopped at a traffic light or other intermittent stop (Stop and Go) is increasing. The power supply voltage may drop when starting the engine. For example, if you start the car while waiting for a nighttime traffic light, the headlights may turn off. Low Vcc protection is a function that prevents this problem from occurring.

Figure 3: Low Vcc protection

High Vcc protection is also based on the consideration of maintaining the lighting function due to the change of power supply voltage. When the power supply voltage rises, if the power supply voltage remains at a high level for a shorter time than the set time, the current will continue to be supplied to the LED. If it is longer than the set time, the boost action will stop and the current will stop being supplied to the LED (Figure 4). However, when the power supply voltage rises to more than 28V, the boost action will stop instantly. In addition, in any case, if the power supply voltage returns to normal level, the current will continue to be supplied to the LED.

Figure 3: High Vcc protection

The LED control IC is packaged in a small V QFN package with low thermal resistance . The package has a mounting area of ​​7mm×7mm and 48 terminals.

A second product aimed at improving LED performance will also be planned

Renesas Electronics will develop a plan to expand its automotive LED control IC products. It will plan a product for DRL (day-time running light) and a second product for headlights after 2011. The DRL product will be developed with the goal of implementing DRL in Europe starting in 2012.

The second product for headlights is expected to increase the switching frequency based on the buck-boost switch. It will meet the requirements of higher performance of high-output white LEDs and miniaturization of LED control circuits. The luminous efficiency of high-output white LEDs will be significantly improved, and the same brightness can be maintained even if the number of LEDs is reduced in the future. From the perspective of reducing the cost of LED headlights, the number of LEDs installed is expected to be reduced. By reducing the number of LEDs connected in series, the voltage output from the LED control circuit to the LED will be reduced. In addition, the forward voltage of the LED itself will also be reduced. In this way, if the number of LEDs is reduced to 4, the output voltage to the LED may be lower than the power supply voltage - the voltage of the battery (12V). In this case, it is estimated that it will be difficult to meet the needs using only the current boost type, so the buck-boost type will be required. The switching frequency may also reach more than 2MHz.