Optical LED Intensity Control 808AE

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by Cliff De Locht Melexis N.V.

USING OPTICAL FEEDBACK FOR PRECISE INTENSITY CONTROL OF LED HEADLIGHTSED 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

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Implementing optical feedback using automotive-grade light-to-voltage sensor for precisely controlling the intensity of lED headlights.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 is not trivial. This article recommends using an integrated light sensor that includes a transimpedance amplifier and an output transistor with the photodiode in one single package. Because an 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

Figure 1. a high dynamic range scene and its representation with two different sets of camera parameters; radiance map of the scene showing a dynamic range of about 88 dB. HDR image courtesy of laurence Meylan, EPFl; LDR images and radiance map courtesy of Paul Debevec.

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out of the present closed loop control designs. Two such sensors that come close are the Melexis automotive-grade light-to-frequency SensorEyeC (MLX75304) and light-to-voltage SensorEyeC (MLZ75305). As shown in Figure 1, SensorEyeC are integrated light sensors that includes photodiode, transimpedance amplifier and output transistor on one chip to minimize use of external discrete components. They feature nonlinearity over the full temperature range and light range deviations of better than +/- 2%. As part of its AEC-Q100 Grade 1 automotive qualification, the SensorEyeC series exceed an operating lifetime of more than 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 standard pick and place equipment and 260 C reflow soldering to optimize production costs. The package uses standard, filled epoxy mold compounds to minimize thermal stress on the silicon chip.. 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. Additionally, exposing the photodiode to the environment avoids sensitivity loss, reflection, refraction, possible glass fogging

Figure 2. Integrated light sensors feature low temperature coefcient for medium to high light levels.

and leads to improved yield.

CLOSED CONTROL LOOP WITH MICROCONTROLLERThe MLX75304 features a square wave 50% duty cycle frequency output to combine high precision with direct low-cost microcontroller interfacing for its more than 100 dB dynamical range. (Figure 1 shows a high dynamical range scene). The sensors output can be connected directly to the timer input 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 the 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 MLX75304 features a TC of 0 ppm at 540 nm light and 300 ppm at 850 nm light. Figure 2 shows responsivity versus light intensities at 25 C and at 125 C. High temperatures have an impact on sensor responsivity through the increased dark current (see upper graph). Medium and high light levels are not disturbed by high temperatures (see responsivity curves output frequencies higher than 2 kHz).

CLOSED CONTROL LOOP WITHOUT MICROCONTROLLERThe 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 PNPtransistor 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

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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.

COMPONENT DESCRIPTIONC1 is a 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, 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 C2 and C3 define system response speed and system stability. Two capacitors are used in parallel to have good response

Figure

3.

automotive

open

cavity

packages.

(Courtesy

of

Melexis).

Figure 4. Closed control loop without microcontroller.

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 - IoutR1 - threshold voltage of PMOS Case 1: 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. Case 2: If the LED is completely

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Fastest Insightover 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 plications 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

to Automotive Networks

Agilent J8130A Vehicle Protocol Tester Series 1000 (VPT1000) Single-click Signal Insight 2 FlexRay+ 2 CAN Interfaces 4/4 Analog + Digital I/O 4 GB Standalone Logging

Integrated light sensor layout (courtesy of Melexis).

stabilize the loop depending on different applications. Replacing the transistor and LED by other devices depends on users application. For the 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. In this example, requirements of higher current LED driver are considered. The circuit outside of the red box in Figure 5 is a standard implemen-tation 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 ap-

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.

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AUTOMOTIVE QUALIFICATIONPackage 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 an

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