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 http://onsemi.com 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. 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