Phototransistor takes control of backlight levels

Edited by the Electronicstalk editorial team Jul 26, 2002

A new miniature phototransistor that is sensitive to visible light can reduce power consumption in display, keypad and control panel backlights.

A new miniature phototransistor that is sensitive to visible light can reduce power consumption in display, keypad and control panel backlights.

By automatically adjusting the intensity of panel backlighting to match ambient light conditions, the Rohm RPM-075PT from Flint substantially reduces the average backlight current consumption.

Since panel illumination represents a significant percentage of the total power budget for a mobile product, precise control can dramatically extend the battery life of mobile appliances and can enable size and weight savings.

The sensitivity of the Rohm RPM-075PT phototransistor peaks at 600nm, allowing backlight control circuitry to detect when ambient lighting conditions are good enough for the user to view the keypad, control panel or LCD display with reduced backlighting.

The backlight can be automatically adjusted accordingly - or turned off altogether - to save battery power.

The majority of phototransistors are general purpose devices tuned for maximum response in the near-infra-red region of the visible spectrum, around 880nm wavelength.

These are not the ideal characteristics for an automatic backlight sensor, which should peak in the visible light range.

Further application-optimised characteristics include sustained sensitivity throughout a half angle of 60 degrees.

This contrasts with a half angle of around 20 degrees for use in applications such as remote controls or stroboscopic devices, which aim to exclude ambient effects.

The RPM-075PT is designed for low voltage operation at 3V, and displays a linear collector current characteristic under incandescent or fluorescent lighting over the illuminance range 10 to 10,000 lux.

This eases the design of associated backlighting control circuitry.

The new device is also packaged in the standard 2125 low profile, surface mount transparent epoxy resin case just 0.8mm thick and weighing 3mg.

Mounting dimensions are only 2.0 x 1.25mm.

This new technology allows unobtrusive backlight controls to be quickly added to products that have until now defaulted to user-activated or continuous backlight modes, which are unnecessarily power-hungry.

The RPM-075PT can be supplied in tape-and-reel format for automated assembly.

The package can withstand reflow soldering and, as a part of Rohm's environmentally compliant component portfolio, is fully compatible with lead-free processes.