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ZXLD1100 Datasheet(PDF) 8 Page - Diodes Incorporated |
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ZXLD1100 Datasheet(HTML) 8 Page - Diodes Incorporated |
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8 / 16 page  APPLICATIONS Programming the maximum LED current The maximum LED current is programmed by adding a single resistor in series with the LED chain. The current is determined by the resistor value and feedback voltage and is given by: ILED = VFB/R1 where VFB=100mV The table below gives recommended resistor values for required LED currents: LED Current R1 Value 10mA 10 15mA 6.8 20mA 5 30mA 3.3 Dimming Control via a PWM signal on the EN pin A Pulse Width Modulated (PWM) signal can be applied to the EN pin in order to adjust the output current to a value below the maximum LED current. Two modes of adjustment are possible as described below. True Analog Dimming - Filtered ‘DC’ mode If a PWM signal of 10kHz or higher is applied to the EN pin, the device will remain active when the EN pin is low. However, the input to the internal low pass filter will be switched alternately from VREF to ground, with a duty cycle (D) corresponding to that of the PWM signal. This will present a filtered dc voltage equal to the duty cycle multiplied by VREF to the control loop and will produce a dc output current lower than the maximum set value. This current is given by: IOUTdc = 0.1D/R1 This mode of adjustment minimizes flicker in the light output and system noise. Pulsed Dimming - Gated Mode If a lower frequency of 1kHz or less is applied to the EN pin, the device will be gated ‘on’ and ‘off’ at a duty cycle (D) corresponding to that of the input signal. The average output current is then given by: IOUTavg » 0.1D/R1 This mode may be preferred over dc current control if the purest white output is required. However, note the 120 s nominal turn-off delay of the device, when using the device in this mode. Dimming Control using a DC voltage For applications where the EN 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 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 LED () MOSFET ‘on’ I V R I LED MAX LED LED MIN FB () ( ) where VFB = 100mV ZXLD1100 SEMICO NDUC TORS ISSUE 4 - JULY 2004 8 |
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