MAX44007/09: Maxim's digital ambient-light sensors cut power consumption by over 100x

The MAX44007/MAX44009 consume 100x less power than the nearest competitive product, significantly extending battery life. They offer a unique interrupt function that constantly measures the amount of light and reports to the microcontroller when the measurement passes the threshold. This functionality extends power savings by reducing the frequency of I2C communications.

The MAX44007/MAX44009 are well suited for applications such as tablet and notebook PCs, smartphones, TVs, digital lighting-management systems, and light-intensity monitoring applications.

Thinking beyond the sensor

In light-sensing systems, any variance along the path that the light travels results in different measurements. These variances are caused by where the sensor is placed on the board, the distance tolerance between the top of the sensor and the glass surface, the transmission characteristics of the glass covering the sensor, and many others. If the part-to-part variation of the sensor is also added to this equation, the reading error can be as much as 50%. This causes false triggers and eventually poor yield at production.

With the all-in-one integrated solution provided by the MAX44007/MAX44009, the maximum total gain error is as little as 15%. In addition to this very precise light measurement, the digital communication of Maxim’s light sensors is immune to parasitics on the communication interface. Both these features provide robust and reliable light measurement to obtain good yield at production.

Seeing light as the human eye sees it

Replicating the optical response of the human eye with electronic components is difficult. Traditional light sensors measure the amount of light in an environment regardless of wavelength. These designs are unduly influenced by ultraviolet and infrared light, which are not perceptible by the human eye. This leads to inaccurate ambient-light readings and, therefore, widely varying brightness adjustments. This problem is exacerbated by the fact that light sources can have different spectra; for instance, incandescent light has more infrared content than fluorescent light.

Maxim’s BiCMOS technology enables the integration of two photodiodes along with an optical filter to reject ultraviolet and infrared light. This allows the MAX44007/MAX44009 to replicate the optical response of the human eye and accurately measure visible light in a variety of environmental settings. Advanced algorithms correct for any spectra variations between light sources, ensuring an extremely accurate lux response.

Moreover, the ADC integration time of the sensors can be adjusted from 6.25ms to 800ms. A default integration time of 100ms ensures excellent 50Hz/60Hz rejection.

Improving black-glass operation

Handheld devices frequently employ tinted or black glass on top of the sensor. This causes many challenges for the light sensor, since black glass shifts the response to various light sources, all of which have different light emission spectra. Measurement errors typically result when these different spectra are combined with the spectrum profile of the glass.

The MAX44007 provides access to two different optical sensors: a visible-plus-infrared photodiode and an infrared photodiode. Once the designer knows the response of the black glass, it can be factored into the light sensor’s measurement for superior accuracy and sensitivity (down to 0.025 lux).

Increasing dynamic range and precision

Ambient-light sensors require the widest dynamic range possible to support a broad spectrum of light conditions, from direct sunlight to complete darkness with every variation in between. Even in digital light sensors, the gain has to be set to a certain value to address different modes of operation in multiple light scenarios. This added configuration requirement increases both design complexity and time to market.

The MAX44007/MAX44009 feature an integrated adaptive-gain block that automatically selects the optimum gain range. This capability relieves designers of the time-consuming process of manually programming the device, and it offloads work from the application processor or microcontroller. Additionally, it provides an extremely wide dynamic range. The MAX44009 can measure light levels from 0.045 lux to 188,000 lux, yielding a dynamic range of more than 4,000,000 to 1. Simply stated, this technology provides the most precise lux measurements over the widest range of ambient-light conditions.

Reducing power consumption and design complexity

The MAX44007/MAX44009 are designed to meet the tightest of power budgets. The ICs boast an operating current (0.65microamps) that is lower than the power-down current of many competitive products, and they operate from an industry-low 1.7V to 3.6V supply voltage. Consuming as little as 1.1microwatts of operating power, Maxim’s solutions use 100x less power than the nearest competitor (124microwatts).

As an added benefit, the devices’ low supply voltage allows them to use the same 1.8V supply for both the supply voltage and the I2C interface. This reduces power consumption and design complexity by only requiring a single voltage rail for the part. An address pin allows the use of two sensors on the same I2C bus.