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Zakładka z wyszukiwarką danych komponentów |
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LT3486 Arkusz danych(PDF) 13 Page - Linear Technology |
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LT3486 Arkusz danych(HTML) 13 Page - Linear Technology |
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13 / 20 page  LT3486 13 3486f The LED current can be set by: ILED ≈ (200mV/RFB), when VCTRL > 1.8V ILED ≈ (VCTRL/5 • RFB), when VCTRL < 1V Feedback voltage variation versus control voltage is given in the Typical Performance Characteristics graphs. (b) Using a Filtered PWM Signal A variable duty cycle PWM can be used to control the brightness of the LED string. The PWM signal is filtered (Figure 7) by an RC network and fed to the CTRL1, CTRL2 pins. 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 in the CTRL pins, which is 100kΩ. Pulse-Width Modulation (PWM) Adjusting the forward current flowing in the LEDs changes the intensity of the LEDs, as explained in the previous sec- tion. However, a change in forward current also changes the color of the LEDs. The chromaticity of the LEDs changes with the change in forward current. Many applications can- not tolerate any shift in the color of the LEDs. Controlling the intensity of the LEDs via applying a PWM signal allows dimming of the LEDs without changing the color. Dimming the LEDs via a PWM signal essentially involves turning the LEDs on and off at the PWM frequency. The human eye has a limit of 60 frames per second. By in- creasing the PWM frequency to say, 80Hz, the eye can be deceived into believing that the pulsed light source is continously on. Additionally by modulating the duty cycle (amount of “on-time”), the intensity of the LEDs can be controlled. The color of the LEDs remains unchanged in this scheme since the LED current value is either zero or a constant value. Figure 8(a) shows a 12V to 8/8 white LED driver. The PWM dimming control method requires an external NMOS tied to the cathode of the lowest LED in the string, as shown in 3486 F07 C1 1 µF PWM 10kHz TYP R1 10k LT3486 CTRL1,2 Figure 7. Dimming Control Using a Filtered PWM Signal APPLICATIO S I FOR ATIO 3486 TA10a 12V (TYP) 9V TO 15V COUT1 2.2 µF COUT2 2.2 µF 100mA PWM FREQ 1kHz RFB1 2 Ω RFB2 2 Ω 100mA OFF ON D1 Q1 100k 100k 3.65k 3.65k 21.5k Q2 5V D2 CIN 10 µF C1 1 µF L1 10 µH L2 10 µH VIN VIN DIMMING INPUT 1 DIMMING INPUT 2 2.2nF 22pF 2.2nF LUXEON LEDs LXCL-PWF1 LUXEON LEDs LXCL-PWF1 COUT1, COUT2: 35V, X5R OR X7R CIN: 25V, X5R OR X7R C1: 10V, X5R OR X7R CREF: 6.3V, X5R OR X7R CREF 0.1 µF VC1 VC2 RT SW1 SW2 VIN LT3486 OVP2 CTRL2 REF PWM2 FB2 OVP1 CTRL1 SHDN PWM1 FB1 PWM FREQ 1kHz D1, D2: ZETEX ZLLS1000 L1, L2: TOKO D53LC (TYPE A) Q1, Q2: FAIRCHILD FDN5630 IL 500mA/DIV ILED 200mA/DIV PWM 5V/DIV 0.2ms/DIV 3486 G18 VIN = 12V 8/8 LEDs PWM FREQ = 1kHz Figure 8a. 12V to 8/8 White LEDs Figure 8b. PWM Dimming Waveforms |
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