ZXLD1101
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:
I
LED
= V
FB
/R1
where V
FB
=100mV
The table below gives recommended resistor values
for required LED currents:
LED Current
10mA
15mA
20mA
30mA
R1 Value
10
6.8
5
3.3
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:
I
OUTavg
»
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 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 V
REF
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 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.
ISSUE 4 - JULY 2004
SEMICONDUCTORS
8
ZETEX [ ZETEX SEMICONDUCTORS ]
