APPLICATIONS
Programming the maximum LED current
ThemaximumLEDcurrentis programmed by adding a
single resistor in series withthe LED chain. The current
is determined by the resistor value and feedback
voltage and is given by:
I
LED
= V
FB
/R1
where V
FB
=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
willbeswitched alternately from V
REF
toground,with a
dutycycle(D) corresponding to that of the PWMsignal.
This will present a filtered dc voltage equal to the duty
cycle multiplied by V
REF
to the control loop and will
produce a dc output current lower than the maximum
set value. This current is given by:
I
OUTdc
= 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,thedevicewillbe gated ‘on’ and ‘off’ at adutycycle
(D) corresponding to that of the input signal. The
average output current is then given by:
I
OUTavg
» 0.1D/R1
This mode may be preferred over dc current control if
the purest white output is required. However, note the
120snominal 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 aDC
voltage can be used to control dimming. By adding
resistors R2 and R3 and applying aDC voltage, the LED
current can be adjusted from 100% to 0%. As the DC
voltageincreases, thevoltagedropacrossR2increases
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
setsthe minimumLEDcurrent.When Q1is‘on’,R2 sets
the LED currentthat will beadded to theminimum 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 V
FB
= 100mV
ZXLD1100
SEMICONDUCTORS
ISSUE 4 - JULY 2004
8
LAST TIME BUY
Closest Alternative is AP5724FDCG-7
