LT3590 APPLICATIONS INFORMATIONProgramming LED CurrentUsing a DC Voltage The feedback resistor (R1 in Figure 1) and the sense volt- For some applications, the preferred method of brightness age (VIN - VLED) control the LED current. control is a variable DC voltage to adjust the LED current. The CTRL pin voltage can be modulated to set the dim- V − V I IN LED ming of the LED string. As the voltage on the CTRL pin LED = R1 increases from 0V to 1.5V, the LED current increases from 0 to I The CTRL pin controls the sense reference voltage as LED. As the CTRL pin voltage increases beyond 1.5V, it has no effect on the LED current. shown in the Typical Performance Characteristics. For CTRL higher than 1.5V, the sense reference is 200mV, The LED current can be set by: which results in full LED current. In order to have accu- rate LED current, precision resistors are preferred (1% ILED = 200mV ,when V R1 CTRL > 1.5V is recommended). The formula and table for R1 selection are shown below. ILED = VCTRL ,when VCTRL < 1.25V mV 200 6.25 • R1 R1= I LED Feedback voltage variation versus control voltage is Table 3. R1 Theoretical Value for 200mV Sense shown in Figure 4. ILED (mA)R1 (Ω) 10 20 Using a Filtered PWM Signal 20 10 A variable duty cycle PWM can be used to control the 30 6.8 brightness of the LED string. The PWM signal is fi ltered 40 5.0 (Figure 5) by a RC network and fed to the CTRL pin. 50 4.0 The corner frequency of R1, C1 should be much lower than the frequency of the PWM signal. R1 needs to be Dimming Control much smaller than the internal impedance in the CTRL There are three different types of dimming control circuits. pin which is 100kΩ. The LED current can be set by modulating the CTRL pin with a DC voltage, a fi ltered PWM signal or directly with LT3590 a PWM signal. R1 10k PWM CTRL kHz TYP C1 0.25 1μF 3590 F05 0.20 Figure 5. Dimming Control Using a Filtered PWM Signal (V) 0.15 LED-VINV 0.10 0.05 0 0 0.5 1.0 1.5 2.0 VCTRL (V) 3590 F04 Figure 4. Dimming and Shutdown Using CTRL Pin 3590f 9
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