Digital Potentiometer (POT) to Control LED Brightness

AND8412/D
Former Catalyst Document Number AB6
Digital Potentiometer (POT)
to Control LED Brightness
Light-emitting diodes (LEDs) require a regulated current,
and their brightness is proportional to the current that flows
through them. Some LED drivers use an external resistor to
set the LED current. A digital POT can replace a discrete
resistor with the advantage of providing an adjustable value
allowing the LED brightness to dynamically change. Most
digital POT circuits have the ability to store permanently the
resistor value in non-volatile memory.
This application brief shows a digital POT circuit used in
combination with the CAT32 white LED driver. The digital
POT is connected as a 2-terminal variable resistor, as shown
in Figure 1. The potentiometer Wiper-to-Low resistor
(between W and L pins) is connected in series with the RSET
resistor to adjust the LED brightness. This example uses a
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APPLICATION NOTE
32-tap linear potentiometer, the CAT5114, with an
end-to-end resistance of 50 kW. The wiper terminal (W)
position is controlled through a 3-wire interface. The INC
input increments the wiper in the direction which is
determined by the logic state of the Up/Down input. The CS
input is used to select the device and also store the wiper
position prior to power down.
D1
6.8 mH
2.7 V to 4.2 V
4.7 mF
VCC
VIN
SHDN
VCC
RSET GND
0.1 mF
VLED
15 mA
H
VCC
Controller
LED
1 mF
White
LEDs
SW
CAT32
VOUT
CAT5114
Up/Down
INC
Control
W
CS
RSET
1.1 k
L
Figure 1. CAT32 LED Driver with CAT5114 Potentiometer Circuit Diagram
 Semiconductor Components Industries, LLC, 2013
July, 2013 − Rev. 1
1
Publication Order Number:
AND8412/D
AND8412/D
LED Current
The LED current is a function of the total resistance
connected to the RSET pin, and is calculated as follows:
I LED +
obtained with the largest RW−L of 50 kW, which results in an
LED current of about 1 mA.
The number of taps define the resolution of the
potentiometer or the minimum increment in the resistance.
A larger number of taps gives more resolution. Table 1 lists
a selection of ON Semiconductor digital POTs with their
characteristics.
22.5
R SET ) R W−L
RSET defines the minimum resistance and therefore the
maximum LED current, for example with RSET = 1.13 kW,
the LED current is 20 mA. The lowest LED current is
Table 1. DIGITAL POT SELECTION
Part Number
Number of Taps
Resistor Options (kW)
Digital Interface
Resistor Scale
Wiper Memory
CAT5114
32
10, 50, 100
Increment
Linear
1 position
CAT5113
100
10, 50, 100
Increment
Linear
1 position
CAT5116
100
32
Increment
Log
1 position
CAT5115
32
10, 50, 100
Increment
Linear
Volatile
Information on the digital POT products and the LED
drivers are available on the ON Semiconductor web site at
http://www.onsemi.com.
In applications where the LEDs backlight an LCD, such
as in cellular phones or PDAs, the LEDs are either turned
ON or OFF. The potentiometer can be used to generate a soft
fading between those two states.
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