Operational amplifiers (OP-Amps) with cross-impedance or photodiodes are often used for backlight control throughout the vehicle to provide a comfortable viewing experience for the driver and to save power. Using operational amplifiers instead of more complex optical sensors allows for greater flexibility and customization. Automotive display system - Head unit, remote display, cluster or head-up display (HUD) - adjusts the backlight brightness according to the light intensity or time of day, usually via an analog-digital converter (ADC) or microcontroller unit (MCU), using the ambient light level detected by the circuit.

The goal of a photodiode amplifier circuit is to convert the low-level current from the photodiode into a useful voltage. For this purpose, op-amps are configured with photodiodes, as shown in the figure below. Photodiodes can detect many different light sources, including visible, infrared, and ultraviolet light. When photodiodes are exposed to more light, they generate more current, which increases the output voltage of the circuit. Choosing the best current-voltage operational amplifier is very important to the performance of the system. The mutual impedance amplifier isolates the photodiode from the output voltage of the operational amplifier and reduces the impedance seen by the photodiode. Typically, this design implements JFET or CMOS input operational amplifiers with low bias currents to reduce DC errors and reduce noise due to reduced input current noise.

The simplest cross-impedance amplifier consists only of a feedback resistor and a feedback capacitor, as shown in the figure below.The feedback resistor sets the gain of the operational amplifier, which can use the required maximum and minimum voltages as well as the maximum current seen from the photodiode.A feedback capacitor is always required to maintain circuit stability and thus compensate for the photodiode capacitance at the op amp's reverse phase input. The value of the capacitance used should be based on the selected feedback resistance value and the bandwidth required for the amplifier. Due to the combination of feedback resistance and feedback capacitance, a pole will be formed, and the amplification will decrease above the pole frequency. Therefore, it is necessary to measure the gain bandwidth of the op Amp to ensure the stability of the circuit. The gain-bandwidth depends on the junction capacitance of the photodiode, the differential input capacitance of the operational amplifier, the common-mode input capacitance of the inverting input, and the selected values of the feedback resistance and capacitance.

 

For automotive functions, a small bias voltage must be applied to the op-AMP's non-inverting input so that the output does not saturate the negative power rail when there is no input current, as in the dark.In general, the resistance divider of a positive voltage supply is used as the in-phase input on the offset, as shown in the figure below.In general, a bias voltage of 0.1V is considered acceptable, and the bias network resistance can be designed to achieve the voltage at the input of a non-inverting operational amplifier.In addition, an additional capacitor is required to filter out the Vref voltage and to be placed in parallel with the resistor directly connected to the ground.The value of the capacitor directly affects the angular frequency and should be designed low enough to prevent power noise from reaching the output.This reverse bias will result in a reduced photodiode junction capacitance from zero reverse bias, but the circuit will respond faster.In addition, reverse bias improves high frequency performance.Other methods for the correct circuit design can be found in the circuit design of the photodiode amplifier.The previous photodiode amplifier circuit can also be analyzed in depth.

 

Photodiode amplifiers allow the designer to customize the circuit as needed.By strategically selecting any vehicle class operational amplifier or modifying elements in the circuit, the designer can control the gain, ensure the correct input bias current, or increase compensation to solve any unique stability problem.For example, a designer can customize the gain of a photodiode amplifier circuit to extend the ADC's input range.In addition, there are a number of automotive operational amplifiers for each specific application.For example, TI's tlv6001-q1 is designed for extremely low level photodiode currents of 75 A and can operate over an extended bandwidth of 1MHz.In addition, TI's LM294-Q1 is composed of two independent frequency-compensated operational amplifiers with the gain bandwidth product of 0.7mhz.The use of operational amplifiers provides an economical solution for automatic tuning: these operational amplifiers cost less than $0.25 per $1,000, while integrated circuits that perform the same function cost $1.14 per kilogram, with little performance gain.TI offers a variety of operational amplifier solutions to meet a variety of specific needs.Photodiode amplifier circuits are a simple and flexible way to automatically adjust the backlight of many automotive systems.TI's extensive library of operational amplifiers and resources contributes to correct circuit design, enabling you to tailor perfect solutions for backlight control.The cross-impedance configuration in the vehicle display system will result in automatic backlight control, which is simple, inexpensive and effective.


What about the backlight control of the car display module