ZXLD1101
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
LED
(
MIN
)
=
MOSFET ‘on’
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.
V
FB
R
LED
V
FB
R
LED
+
I
LED
(
MIN
)
I
LED
(
MAX
)
=
where V
FB
= 100mV
ISSUE 4 - JULY 2004
9
SEMICONDUCTORS
ZETEX [ ZETEX SEMICONDUCTORS ]
