Datasheet

ADM8843
Rev. C | Page 12 of 16
LED BRIGHTNESS CONTROL USING A
PWM SIGNAL APPLIED TO V
PWM
Adding two external resistors and a capacitor, as shown in
Figure 21, can also be used for PWM brightness control. This
PWM brightness control method can be used instead of CTRL1
and/or CTRL2 digital PWM brightness control. With this con-
figuration, CTRL1 and CTRL2 digital logic pins can control
shutdown of the white LEDs, and V
PWM
can control the bright-
ness of all the white LEDs. This is done by applying a high
frequency PWM signal (amplitude 0 V to 2.5 V) to drive an
R-C-R filter on the I
SET
pin of the ADM8843. A 0% PWM duty
cycle corresponds to 20 mA/LED, and a 100% PWM duty cycle
corresponds to a 0 mA/LED. At PWM frequencies above 5 kHz,
C5 may be reduced (see Figure 21). The amplitude of the PWM
signal must only be 0 V and 2.5 V to result in 20 mA flowing in
each LED.
100
)1(
120
2
2
_
CycleDuty
RR
RR
VoltageI
I
SET
SET
SET
LED
−
××
+
×
=
I
SET
R
SET
= 13.4kΩ
R = 7.5kΩ
V
PWM
0V–2.5V
ADM8843
05050-021
100% = I
LED
= 0mA
0% = I
LED
= 20mA
R = 7.5kΩ
C5 = 1μF
Figure 21. PWM Brightness Control Using
Filter-PWM Signal
LED BRIGHTNESS CONTROL USING A DC
VOLTAGE APPLIED TO V
BRIGHT
By adding one resistor, as shown in Figure 17, this configuration
can also be used for brightness control of the white LEDs by
using a dc voltage applied to the V
BRIGHT
node. Figure 22 shows
an application example of LED brightness control using a dc
voltage with an amplitude of 0 V to 2.5 V applied to V
BRIGHT
.
The equation for I
LED
is
I
SET
= [(1/R
SET
+ 1/R)(V
SET
)] – [(1/R)(V
BRIGHT
)]
I
LED
= 120 × I
SET
where:
R = 15 kΩ
V
SET
= voltage at I
SET
pin (1.18 V)
V
BRIGHT
05050-023
I
LED
2.5V
0mA
1.6V
7.2mA
0.8V
13.6mA
0V
20mA
Figure 22. PWM Brightness Control Application Diagram Using a
DC Voltage Applied to V
BRIGHT