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LT3471 Datasheet(PDF) 11 Page - Linear Technology |
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LT3471 Datasheet(HTML) 11 Page - Linear Technology |
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11 / 24 page  LT3498 11 3498f The LED current can be set by: I mV R V I V LED SENSE LED CT ≈> ≈ 200 15 1 ,. when VCTRL1 R RL SENSE R V 1 1 625 125 .• ,. when VCTRL1< Feedback voltage variation versus control voltage is given in the Typical Performance Characteristics. Using a Filtered PWM Signal A filtered PWM signal can be used to control the bright- ness of the LED string. The PWM signal is filtered (Figure 4) by a RC network and fed to the CTRL1 pin. The corner frequency of R1, C1 should be much lower than the frequency of the PWM signal. R1 needs to be much smaller than the internal impedance of the CTRL1 pin which is 10MΩ (typ). APPLICATIONS INFORMATION— LED DRIVER Programming LED Current The feedback resistor (RSENSE1) and the sense voltage (VCAP1 – VLED1) control the LED current. The CTRL1 pin controls the sense reference voltage as shown in the Typical Performance Characteristics. For CTRL1 higher than 1.5V, the sense reference is 200mV, which results in full LED current. To have accurate LED current, precision resistors are preferred (1% is recommended). The formula and table for RSENSE selection are shown below. R mV I SENSE LED 1 200 = Table 4: RSENSE1 Value Selection for 200mV Sense ILED (mA) RSENSE1 (Ω) 540 10 20 15 13.3 20 10 Dimming Control There are three different types of dimming control circuits. The LED current can be set by modulating the CTRL1 pin with a DC voltage, a filtered PWM signal or directly with a PWM signal. Using a DC Voltage For some applications, the preferred method of brightness control is a variable DC voltage to adjust the LED current. The CTRL1 pin voltage can be modulated to set the dim- ming of the LED string. As the voltage on the CTRL1 pin increases from 0V to 1.5V, the LED current increases from 0 to ILED. As the CTRL1 pin voltage increases beyond 1.5V, it has no effect on the LED current. Figure 4. Dimming Control Using a Filtered PWM Signal LT3498 CTRL1 C1 0.1 µF PWM 10kHz TYP 3498 F04 R1 100k Ω |
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