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Maxim > Design Support > Technical Documents > Application Notes > Amplifier and Comparator Circuits > APP 1862
Maxim > Design Support > Technical Documents > Application Notes > Display Drivers > APP 1862
Maxim > Design Support > Technical Documents > Application Notes > Power-Supply Circuits > APP 1862
Keywords: WLED, White LED, active ballast, current mirror, LED, boost, LED driver, transistor
APPLICATION NOTE 1862
Circuit Drives Mixed Types and Quantities of LEDs
Jan 24, 2003
Abstract: This application note presents a technique that allows you to mix LED quantities and types, and
requires just one power-supply voltage.
A similar version of this article appeared in the October 3, 2002 issue of EDN magazine.
Portable systems often use LEDs of different colors-white for the display backlight, green for keypad
illumination, red for power, etc., and in varying quantities of each. Typically the LEDs are driven by at
least two power supplies, one for "standard" LEDs (red and green) and one for white LEDs (white LEDs
require a higher forward voltage.) The keypad and other indicator LEDs each have current-limiting
resistors.
The circuit in Figure 1 drives the LED strings via transistors Q1-Q4, which operate as current mirrors.
This technique offers the following advantages:
eliminates the current-limiting resistors
drives groups of dissimilar LEDs
requires just one power supply voltage
allows each string to operate at a different current
and allows the brightness of all the LEDs to be adjusted with one control point (U1's ADJ pin).
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