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ADM8843
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.
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)
2.5V
I LED =
I SET _ Voltage
R SET × 2 R
R SET + 2 R
× 120 ×
(1 ? Duty Cycle )
100
V BRIGHT
1.6V
0.8V
0V
20mA
13.6mA
I LED
7.2mA
100% = I LED = 0mA
0% = I LED = 20mA
R = 7.5k Ω R = 7.5k Ω
V PWM
ADM8843
I SET
0mA
Figure 22. PWM Brightness Control Application Diagram Using a
DC Voltage Applied to V BRIGHT
0V–2.5V
C5 = 1 μ F
R SET = 13.4k Ω
Figure 21. PWM Brightness Control Using
Filter-PWM Signal
Rev. C | Page 12 of 16
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