This idea describes a white-
LED (WLED) driver circuit
controlled by a simple ambient
light sensor (ALS). A typical
application would be a brightness
control for cell-phone backlighting.
On a very sunny day,
the brightness will increase to
improve the visibility.
The circuit uses the IA2505
four-channel WLED (Fig. 1).
Each LED can be driven up to 20
mA, with the drive current determined
by the current sourced
through the resistor (RSET) pin.
RSET is a constant voltage source
set at 1.2 V.
A resistor between the RSET pin and GND would set a fixed
current equally distributed through the four WLEDs irrespective
of the ambient light conditions. However, because ambient light
isn’t constant in the real world, replacing the RSET with the ALS
gives you automatic brightness control.
The ALS, which is basically a pseudo logarithmic amplifier,
employs an operational amplifier (IC2) and a signal diode (D3).
A log amp is used because of the large dynamic range of photo
current. The circuit is designed for 1µ A to 1 mA (60 dB) of photo
current. The photodiode, an infrared PIN photodiode, operates in
reverse-bias mode as shown. All ambient light sources (fluorescent
or incandescent bulbs and sunlight) have a significant amount of
IR light. Ideally, a photodiode sensitive to visible light would be
the best option.
The bias pot (R3) varies the sensitivity of the ALS depending
on the ambient light available and respective photo-current range.
For maximum brightness (80 mA total), RSET should source 66
A through the transistor (T1). The RSET source current is mirrored
with a gain of 1200 internally by the IA2505 to drive the four
LEDs. The table can be used as a guide for setting R3.
Figure 2 shows the transfer function between the photo current
and RSET source current for an expected photo current range (R3
= 45 kΩ and R3 = 2 kΩ).
