LED headlights in cars require precise intensity control over lifetime and over temperature. Automotive headlight control levels typically require multiple nested closed control loops to compensate for ambient temperature variations and LED aging. Control parameters include LED current and forward biasing voltage. Creating a closed feedback loop with an optical sensor would simplify much of the feedback loop control circuitry. However, selecting the automotive qualified optical device that best fits this application from hundreds in not trivial.

This article recommends using an integrated light sensor that includes transimpedance amplifier and output transistor with the photodiode in one single package. Because integrated sensor packs photodiode and electronics on the same die, it offers lower noise and better EMC protection than circuits based on discrete solution. In addition, it also features high linearity and low temperature coefficient to take the complexity out of the current closed loop control designs. One such sensor that comes close is the Melexis' automotive grade light-to-frequency SensorEyeC (MLX75304).

As shown in Figure 1, the SensorEyeC is an integrated light sensor that includes photodiode, transimpedance amplifier and output transistor on one chip to minimize use of external discrete components. It features linearity over the full temperature range and light range deviates less than +/- 2%. Temperature coefficient (TC) for operations below +85 °C is -0.2%/°C (Figure 2). For higher operating temperatures up to +125 °C the TC stays under +2%/°C.

As part of its AEC-Q100 Grade 1 automotive qualification, the SensorEyeC series exceed an operating lifetime of over 100000 hours. The standard compound open cavity package is highly robust with a proven track record of more than 20 million pieces shipped. The sensor is compatible with pick-and-placing and 260 °C reflow soldering to optimize production costs. And matches well with the temperature coefficient of silicon to avoid thermal stress.

In fact, the package for the SensorEyeC series is unique (Figure 3). In this package type the photodiode is exposed to the environment while bond wires and all other sensor electronics are overmoulded and thus well protected. A special passivation layer protects the chip to the degree that all automotive qualification tests are passed. Early trials with glass lids lead to the conclusion that glass lids cause more problems than they solve. Additionaly, exposing the photodiode to the environment avoids sensitivity loss, reflection, refraction, possible glass fogging and leads to improved yield.

The MLX75304 features a square wave 50% duty cycle frequency output to combine high precision with direct low-cost microcontroller interfacing. The sensor's output can be connected directly to the timer input of a microcontroller or counter input of a microcontroller. For high light conditions, the sensor output is directly connected to the timer input of a microcontroller. The elapsed time between two consecutive rising or falling edges is a direct measure of the frequency and hence of light intensity.

When there is no light input on the sensor, the output frequency drops below 10 Hz range, potentially causing a 16-bit timer to overflow. Consequently, the counter input detects whether rising or falling edges have been detected. If required, the counter input is able to give an exact readout of the low-light value.

The MLX75305 light-to-voltage output varies linearly with incident light intensity. For closed loop applications, as the output of the MLX75305 increases with light, a sign inversion of the output voltage is needed for negative feedback. This can be done with an external PNP-transistor or PMOS. Figure 4 shows the feedback loop for low current applications (without LED driver). Later in this article we extend this circuit with the MLX10803 LED driver for high current applications like LED headlights.

This circuit will regulate the LED current to a stable value. When LED light on the MLX75305 increases, output voltage of the MLX75305 (Vout) increases as well. This diminishes the PNP LED current (Iout). A stable state will be occur on the crossing of Vout and Iout.

Vout ≈ VDD - Iout×R1 - threshold voltage of PMOS

Case1: When there is no light, Vout will be 0. This switches the PMOS completely “ON” and thus increases the LED current Iout and generates more light. As a consequence, the Vout will start to rise.

Case2: If the LED is completely “ON”, Vout will be at full scale which turns the PMOS “OFF” and so, blocking the LED current Iout.

In steady-state, the system will regulate the LED current to a stable position, depending on the values of the passive components.