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Posted on 23 August 2019

Saving Energy in Portable Electronics

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Using Ambient Light Sensors (ALS)

In portable electronic products, reducing the power consumption to provide the user with increased battery life is one of today’s critical design considerations. The liquid crystal display (LCD) and its associated backlighting are among the more (and frequently the most) power-hungry loads in portable products. As a result, the use of an ambient light sensor (ALS) to optimize the operation of the backlight LEDs under a variety of environmental lighting situations is increasing while, at the same time, the preferred technology choices available to designers for sensing have shifted towards more integrated solutions.

By Steve Chutka, Field Application Engineer, ROHM Semiconductor

 

Ambient Light Sensor Background

Ambient light sensors are also called illuminance or illumination sensors, optical sensors, brightness sensors or simply light sensors. One very important application for ALS technology is cell phones. In a cell phone, the ALS enables automatic control of display backlight brightness over a wide range of illumination conditions from a dark environment to direct sunlight. With the ALS input, a microcontroller (MCU) or baseband processor increases or decreases the display brightness depending on the environment. This control improves visibility and dramatically reduces power consumption since LCD backlighting can draw as much as 51% of the power in the input standby mode. In addition, the ALS signal can be used to instruct the keypad LED driver to minimize keypad backlighting by reducing up to 30% of the power in the input standby power mode. In a bright environment, the LED keypad brightness is reduced for minimal power consumption.

Photo ICs, also referred to ALS ICs, are the newest technology, developed to address the shortcomings of discrete devices including photoelectric cells, photodiodes and phototransistors. In addition to increased functions possible with integration including amplification, logic control, and shutdown capability, the photodiode sensing has a relatively low dispersion. Low dispersion is one of the key criteria for selecting an ALS with detected wavelengths limited to the 380 to 780 nm range and visible essentially to the human eye. Both analog and digital photo ICs are available, each having advantages depending on the application. The photo IC has integrated functionality which eliminates the need for additional circuitry that takes up more board space and adds cost. As a result, many designers are making the transition to photo ICs from discrete devices.

Topology of ALS ICs

Analog and digital ALS devices are silicon monolithic circuits with an integrated light-sensitive semiconductor photodiode—a PN junction which converts light into an electrical signal. Both technologies are available in small surface mount technology packages. For example, ROHM offers both analog and digital ambient light sensor ICs in compact, surface-mount packages — the WSOF5 package (1.6 x 1.6 x 0.55 mm) as well as the WSOF6 package (3.0 x 1.6 x 0.7 mm).

Understanding the difference between analog and digital photo ICs is essential to selecting the proper ALS solution. The analog ambient light sensor IC has an analog current output proportional to the incident light level. As shown in Figure 1, the IC combines the photodiode, signal amplification and control logic. The current source output is typically converted to a voltage by means of a simple load resistor. This voltage output is typically applied to either the input of an analog-to-digital converter (ADC) interface on an MCU or directly as an input to an LED driver IC equipped with auto-luminous control (Figure 2).

The output of the analog ALS provides the control input to the system MCU

Fundamental design advantages for the analog ALS include an output current that is proportional to the brightness of the environment and spectrum sensitivity similar to the human eye.

The analog ALS output provides direct light level control

The typical digital output ambient light sensor (Figure 3) has a 16-bit digital I2C output. In addition to amplification for the photodiode, the IC’s integrated ADC converts the photosensor’s output to an I2C signal for direct connection to the I2C communication bus of an MCU or baseband processor. The I2C interface simplifies the circuitry in an application by removing the need for an external ADC. The digital ALS includes more integration than an analog ALS and can result in an overall cost and space savings on the printed circuit board (PCB).

In a digital ALS application, the controller communicates with the ALS and LED driver

In terms of power consumption, a digital ALS will likely draw more power in both the active mode (for example, 190 μA for the ROHM Semiconductor BH1750FVI) and power down mode (1.0 μA for the same digital ALS) due to the integration of the ADC when compared just to an analog ALS (97 μA and 0.4 μA, respectively, for the ROHM Semiconductor BH1620FVC). However, the total power consumption may be comparable when a separate ADC + MCU or broadband controller is taken into account. In either case, these values are quite low when compared to the power savings achieved by their ability to control the LED power consumption. Examples of specific analog and digital ALS applications/products can further help in the decision process.

Analog ALS Solutions

As an example of analog ALS capability, ROHM Semiconductor ALS ICs have an output current proportional to light (current sourcing) with a measurement range of 0 to 100,000+ lux (lx). These ICs have light sensing accuracy of ±15% based on a unique laser trimming technology that also ensures high output sensitivity. Each of these devices features an input voltage supply range of 2.4 to ~5.5V. A resistor connected to the output current (Iout) pin converts the current output to a linear voltage from 0V up to the supply voltage level for highly efficient component operation.

Figure 4 shows the relative spectral response of the analog ALS and luminosity versus output current. Since wavelengths outside of the range of human vision, such as ultraviolet and infrared, may cause inaccurate light sensor readings, it is important to choose a light sensor that has spectral sensitivity similar to the human eye. While the data in Figure 4 (a) is specifically for an analog ALS, this same performance is inherent in the digital designs as well. In addition to an Iout proportional to the luminosity in lux, ROHM’s analog ALS products have selectable high-gain and low-gain modes, a proprietary function. These gain control modes allow for direct control of the internal amplifier gain via the GC1 and GC2 input pins, providing designers even greater design options for trading off performance versus power consumption.

Spectral sensitivity and Luminosity vs. Iout

Digital ALS Solutions

Digital ALS ICs, such as ROHM’s BH1715, measure brightness and provide a 16-bit digital signal output over an I2C bus interface that supports both FAST mode (400 KHz) and 1.8V logic interface. The digital ALS ICs can detect a wide range of intensities (0 to ~65,535 lx). A unique internal shutdown function enables low current consumption. Several other features allow digital photosensor ICs to provide applications advantages as described below.

In the operating environment, it is important for a light sensor to generate a consistent output regardless of the light source. A stable output avoids generating different values depending on the light source, which could cause the system to turn on the backlighting when it is not needed. The stable output improves the battery life and also improves the end user’s experience.

The ability to operate in different spectral response or luminosity modes, depending on the required serial data, has a direct impact on the sensor’s performance. A comparison of a ROHM digital ALS with two resolution modes for improved lighting control is shown in Table 1. Note that in contrast to the analog ALS, different operating modes do not impact the power consumption in digital units.

In a digital ALS, switch operating modes depending on required measuring time

Conclusion

Based on their ability to provide extended battery life, Ambient Light Sensors are an important tool for enhancing performance in LEDbacklighted LCD displays. Depending on the application, either analog and/or digital units can provide an acceptable solution. Since ALS performance can provide a significant difference for portable and many other applications, characteristics such as spectral sensitivity, stability, selectable gain or data mode, and other factors should be reviewed carefully before making a final technology decision.

 

 

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