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T6309B-AX Datasheet(PDF) 7 Page - Taiwan Memory Technology |
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T6309B-AX Datasheet(HTML) 7 Page - Taiwan Memory Technology |
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7 / 11 page  TE CH tm T6309B TM Technology Inc. reserves the right P. 7 Publication Date: SEP. 2005 to change products or specifications without notice. Revision: A Applications Information Dimming Control There are several types of dimming control circuit as follows: 1. Using a PWM signal to CE pin When using a PWM signal at CE pin, the T6309B is turned on or off by the PWM signal. The average LED current increases proportionally with the duty cycle of the PWM signal. The typical frequency range of the PWM signal is 150Hz to 250Hz. The magnitude of the PWM signal should be greater than the threshold voltage of CE voltage high. 2. Using a DC voltage A DC voltage signal can be used as well to control the LED brightness. The dimming control method is shown in the figure 2. When the DC voltage is zero, the LED is fully turned on. As the DC voltage increases, the voltage drop on R2 increases and the voltage drop on R1 decreases. Thus the LED current decreases. The selection of R1 and R2 should make the current from the variable DC source much smaller than the LED current and much larger than the FB pin bias current. For VDC range from 0V to 2V, the selection of resistors in this figure gives dimming control of LED current from 0mA to 20mA. Following steps are used to select the resistor values: • Select the voltage VMAX to turn the LEDs off. (e.g. 2.0V) • Select 0V as default to turn the LEDs fully on. • Select the maximum LED current IMAX (e.g. 20mA) and 0mA as the minimum LED current. • Calculate R2 to achieve a feedback current in the range of I1 = 3uA to 10uA as the LEDs are fully turned on: R2 = VREF / I1 (e.g. 0.4V/4uA =100KOhm ) • Calculate R1 to meet the equation: R1 / (R1+R2) = VREF / VMAX (e.g. VREF=0.4V, VMAX=2.0V, R2=100KOhm, the calculated R1 is 25KOhm) • Calculate the sense voltage Vs at maximum LED current: Vs = VREF x (1 + R1/R2) (e.g. Vs = 0.4V x (1 + 25K/100K) = 0.5V) • Calculate the sense resistor Rs: Rs = Vs / IMAX (e.g. 0.5V/20mA = 25 Ohm) 3. Using a Filtered PWM signal The filtered PWM signal can be considered as an adjustable DC voltage. It can be used to replace the DC voltage source if adjustable analog signal is not available in the system. The circuit is shown in the figure below: |
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