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ZXSC400E6TA Datasheet(PDF) 8 Page - Diodes Incorporated |
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ZXSC400E6TA Datasheet(HTML) 8 Page - Diodes Incorporated |
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8 / 16 page  Dimming Control using a DC voltage For applications where the shutdown pin is not available a DC voltage can be used to control dimming. By adding resistors R2 and R3 and applying a DC voltage, the LED current can be adjusted from 100% to 0%. As the DC voltage increases, the voltage drop across R2 increases and the voltage drop across R1 decreases, thus reducing the current through the LEDs. Selection of R2 and R3 should ensure that the current from the DC voltage is much less than the LED current and much larger than the feedback current. The component values in the diagram below represent 0% to 100% dimming control from a 0 to 2V DC voltage. Dimming Control using a filtered PWM signal The filtered PWM signal can be considered as an adjustable DC voltage by applying a RC filter. The values shown in the diagram below are configured to give 0% to 100% dimming for a 1kHz to 100kHz PWM signal with a 2V amplitude. e.g. a 50% duty cycle will give 50% dimming. Dimming Control using a logic signal For applications where the LED current needs to be adjusted in discrete steps a logic signal can be applied as shown in the diagram below. When Q1 os ‘off’,R1 sets the minimum LED current. When Q1 is ‘on’, R2 sets the LED current that will be added to the minimum LED current. The formula for selecting values for R1 and R2 are given below: MOSFET ‘off’ I V R LED(MIN) FB LEB = MOSFET ‘on’ I V R I LED(MAX) FLB LEB LED(MIN) =+ Where VFB = 300mV ZXSC400 ISSUE 1 - JANUARY 2003 8 R3 67k R2 10k R1 ZXSC400 VFB VDC R3 67k R2 10k R1 ZXSC400 PWM R4 10k C1 0.1µF VFB R2 R1 ZXSC400 VFB LOGIC SIGNAL Q1 |
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