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LM3432 Datasheet(PDF) 10 Page - Texas Instruments |
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LM3432 Datasheet(HTML) 10 Page - Texas Instruments |
10 / 27 page CMODE = 1.65 x 10 -5 fPWM where CMODE is in Farads fPWM = 1.65 x 10 -5 CMODE where fPWM is in Hz + - + - - 3V - Comp2 1V 1V 3V Ramp Generator VDIM 1V to 3V MODE DIM CMODE VCC PWM Modulator 34 PA PWM Comp1 + - LM3432, LM3432B SNVS498D – APRIL 2007 – REVISED MAY 2013 www.ti.com Figure 14. Analog Dimming of LEDs An internal current source of 34 µA (typical) will charge the external capacitor (CMODE) linearly until it reaches 3V. The comparator (Comp1) then forces CMODE to be discharged to 1V very quickly. By repeating the cycle, a saw- tooth waveform as shown in Figure 14 is generated. Comparator (Comp2) compares this ramp waveform with the external dc voltage at the DIM pin generating the desired PWM control signal. When VDIM ≤ 1V, ON duty factor, DON = 0% and when VDIM ≥ 3V, DON = 100%. The frequency of the PWM control signal can be calculated as shown below. (2) Or (3) PWM DIGITAL DIMMING of LED STRINGS Alternatively, the dimming control can be implemented by direct application of a digital signal to the device. With the MODE pin connected to ground, an externally applied PWM dimming signal is applied to the DIM pin. The peak amplitude of the externally applied PWM signal should be greater than 1.5V to ensure clean PWM switching. During PWM dimming, channels are not switched simultaneously in an effort to minimize large surge currents from being drawn from the LED supply rail. Each channel will have 0.5 µs phase delay with respect to the preceding channel. As a consequence of the phase delay, for a control pulse width less than 0.5 µs, the duty cycle will be rounded down to 0% and for a control pulse width less than 0.5 µs off time, the duty cycle will be rounded up to 100%. Therefore, 0.5µs becomes the finest pulse width resolution that can be realized. The PWM switching timing for all six channels is shown in Figure 15. 10 Submit Documentation Feedback Copyright © 2007–2013, Texas Instruments Incorporated Product Folder Links: LM3432 LM3432B |
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