ADM8843EB-EVALZ Ver la hoja de datos (PDF) - Analog Devices
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ADM8843EB-EVALZ Datasheet PDF : 16 Pages
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ADM8843
LED BRIGHTNESS CONTROL USING A
PWM SIGNAL APPLIED TO VPWM
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 VPWM 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 ISET 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.
I LED
=
I SET _ Voltage
RSET × 2R
× 120 ×
(1− Duty Cycle)
100
RSET + 2R
100% = ILED = 0mA
0% = ILED = 20mA
ADM8843
ISET
VPWM
0V–2.5V
R = 7.5kΩ R = 7.5kΩ
C5 = 1µF
RSET = 13.4kΩ
Figure 21. PWM Brightness Control Using
Filter-PWM Signal
Data Sheet
LED BRIGHTNESS CONTROL USING A DC
VOLTAGE APPLIED TO VBRIGHT
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 VBRIGHT 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 VBRIGHT.
The equation for ILED is
ISET = [(1/RSET + 1/R)(VSET)] – [(1/R)(VBRIGHT)]
ILED = 120 × ISET
where:
R = 15 kΩ
VSET = voltage at ISET pin (1.18 V)
VBRIGHT
2.5V
1.6V
0.8V
0V
ILED
0mA
7.2mA
13.6mA
20mA
Figure 22. PWM Brightness Control Application Diagram Using a
DC Voltage Applied to VBRIGHT
Rev. E | Page 12 of 16
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