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.
CLOSED CONTROL LOOP WITH MICROCONTROLLER
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.
CLOSED CONTROL LOOP WITHOUT MICROCONTROLLER
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.
C1 is decoupling capacitor for VDD. R2 is the pull-down resistor for MLX75305.
R1 is used to change light level regulation emitted by LED: increase R1 to have a lower light level and decrease R1 to have a higher one. When changing R1, make sure to provide sufficient voltage head-room to allow the LED and PMOS to operate properly. R1 should be a precision resistor with low TC because it defines the light output value of the regulation system.
R3 sets the maximum LED current for LED protection. When applying a large LED current, voltage drop over R3 will limit the max LED current. Note: because R3 is included in the closed loop, its value does not change the gain of the system nor the settling point of the loop.
The product R3*(C2+C3) determines the dominant system pole, so it sets the maximum operating frequency or regulation speed of the system. The C2 and C3 capacitors define system response speed and system stability. Two capacitors are used in parallel to have good response over a wider frequency range. A larger R3*(C2+C3) makes the system slower, but more stable. Depending on system specifics, appropriate values of the R and C must be selected. These components' values can be modified to stabilize the loop depending on different applications.
Replacing the transistor and LED by other devices depends on users application. For transistor: PMOS, power transistor and Darlington pairs are all optional. If a different transistor or LED type is used, always check the current flowing through and choose the right R3 and R1 values to make sure the transistor will work properly. C2 and C3 values also should be chosen to ensure system stability.
The MLX75305 requires a 5 volt supply input. With an additional resistor and 4.7 V zener diode connected to VDD, the MLX75305 can be connected directly to the V-battery.
For high current LED applications, it is recommended to include the MLX10803 LED driver as shown in Figure 5.
In this example, requirements of higher current LED driver are considered. The circuit outside of the red box in Figure 5 is a standard implementation of the MLX10803 (high power LED Driver). The basic operating principle here is that the LED is driven by a switch mode power supply using an inductor as an energy storage element. Furthermore, for applications where thermal considerations are critical, PTC and NTC resistors are connected to the REF1 and REF2, respectively, for temperature compensation of the LED output.
The voltage output of MLX75305 Light-to-Voltage SensorEyeC, which is directly proportional to light intensity, is inverted via the PNP bipolar transistor (T1) into the VREF pin of the MLX10803. The VREF pin of the MLX10803 can be used to limit the peak current over R3 and determine the average current over the LEDs. In this way, the user can decide a target value for peak LED current by the sizing of R1.
Package choice is crucial for automotive optical sensors. The automotive environment is a harsh environment including mechanical stress, temperature cycles, pollutants and moisture. AEC-Q100 is the automotive standard for stress test qualification. This standard includes device qualification like high temperature operating lifetime (HTOL), latch-up and ESD testing. Package qualification includes:
- Preconditioning to simulate solder reflow 260 °C (3 times) at MSL3 conditions
- 1000 Temperature Cycles -50 °C to +150 °C
- Temperature and humidity bias: 1000 operating hours at 85 °C and 85% relative humidity
- Autoclave: 96 h at 121 °C and 100% humidity
- High Temp. Storage Life: 1000 hours at 150 °C
Cliff De Locht is currently product manager for the Opto Division at Melexis N.V. After obtaining his Engineering degree in Microelectronics at the University of Brussels, Belgium in 1992, De Locht specialized in Aeronautics and Astronautics Engineering (1994). His professional career started as R&D project manager for the largest telecommunications group in Belgium and evolved to system-level engineering management and product management.