MAX44009 ambient light sensor LCD backlight brightness control application 2

The second part of the control scheme is to adjust the backlight brightness of the display. This can be done in a variety of ways, depending on the display module in the device. The two simplest ways are direct regulation via a pulse width modulation (PWM) scheme and indirect regulation using a display controller.

Many display modules now have an integrated controller that allows the user to set the backlight brightness directly by sending serial commands to the controller. If the display module does not have an integrated controller, a simple backlight control actuator can be installed to control the input current to the white LED lamps behind the display for backlighting. A simple way to implement this control is to connect a field-effect transistor (FET) directly in series with the LED and use a PWM signal to quickly turn the FET on and off (Figure 5). However, it can also be accurately and reliably regulated using a single chip (the MAX1698 boost converter for LED control) (Figure 6). Refer to application note 3866, "Low-power PWM output controls LED brightness," for more information.

Figure 5. Simple PMW control circuit

Figure 6. LED brightness adjuster based on the MAX1698.

Backlight Control: Making the Connection

The final step is to establish the connection between the sensor and the actuator, which is implemented by the microcontroller. One might first ask, “How is ambient light intensity mapped to backlight brightness?” In fact, there are some literature that specifically describes relevant schemes. One of the mapping methods is proposed by Microsoft® for computers running Windows® 7¹ operating system. The curve shown in Figure 7 is provided by Microsoft and can map ambient light intensity to display brightness (expressed as a percentage of full brightness).

Figure 7. Example of a curve that maps ambient light intensity to optimal display brightness.

This special curve can be represented by the following function:

If the device uses an LCD control chip with integrated brightness control function, the backlight brightness can be easily set by sending instructions to the chip. If the device uses PWM to directly control the brightness, it is necessary to consider how to map the proportional signal to the display brightness.

In the MAX1698 example, according to its data sheet, the drive current can be mapped to a voltage. In this example, we can assume that the LED current intensity is almost linearly related to its current. In this way, we can multiply a coefficient in the above equation to calculate the effective voltage mapped by PWM, which is then mapped to the LED current and finally converted to the display brightness.

Implementation

It is better not to jump directly from one brightness level to another, but to smoothly adjust the backlight brightness up and down to ensure seamless transitions between different brightness levels. To achieve this goal, it is best to use a timer interrupt with fixed or different brightness steps to gradually adjust the brightness, or a timer interrupt with a PWM value that can control the LED input current, or a timer interrupt with a serial command that can be sent to the display controller. Figure 8 provides an example of such an algorithm.

Figure 8. Algorithm example for step-by-step brightness adjustment

Another issue is how quickly the system responds to changes in ambient light intensity. We should try to avoid changing the brightness level too quickly. This is because instantaneous changes in light intensity (such as a window opening or a light beam sweeping across the room) can cause unnecessary changes in backlight brightness, which often makes the user feel uncomfortable. In addition, a longer response time helps reduce the number of times the microcontroller checks the light sensor, thereby freeing up some microcontroller resources.

The most basic approach would be to check the light sensor every one or two seconds and adjust the backlight brightness accordingly. A better approach would be to adjust the backlight brightness only when the light intensity deviates from a certain range for a certain period of time. For example, if the normal light intensity is 200 lux, we might adjust the brightness only when the light intensity drops below 180 lux or rises above 220 lux for more than a few seconds. Fortunately, both the MAX9635 and MAX44009 have integrated interrupt pins and threshold registers to make this easy. See application note 4786, "Interface Procedures for the MAX9635 Ambient Light Sensor," for more details.