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 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

LED

MOSFET ‘on’

I

V

R

I

LED MAX

LED

LED MIN

FB

()

(

)

=+

ZXLD1101

SEMICO NDUC TORS

ISSUE 4 - JULY 2004

9