Datasheet
ADM8845
Rev. C | Page 13 of 20
LED CONFIG.
FULL ON MAIN AND SUB OFF
CTRL1
CTRL2
V
OUT
I
LED
(SUB)
I
LED
(MAIN)
SUB DISPLAY
BRIGHTNESS
MAIN DISPLAY
BRIGHTNESS
37ms >
t
CP
> 15ms
SHDN
SHDN
50%
50%
100%
100%
80%
t
CP
04867-0-025
SUB AND MAIN 50%
DUTY CYCLE
MAIN 80% DUTY CYCLE,
SUB OFF
Figure 25. Application Timing
LED BRIGHTNESS CONTROL USING A
PWM SIGNAL APPLIED TO V
PWM
Adding two external resistors and a capacitor, as shown in
Figure 26, can also be used to control PWM brightness. This
PWM brightness control method can be used instead of CTRL1
and/or CTRL2 digital PWM brightness control. With this con-
figuration, the CTRL1 and CTRL2 digital logic pins can be used
to control shutdown of the white LEDs, while V
PWM
can be used
to control the brightness of all the white LEDs 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 ADM8845. A 0% PWM
duty cycle corresponds to 20 mA/LED, while a 100% PWM
duty cycle corresponds to a 0 mA/LED. At PWM frequencies
above 5 kHz, C5 may be reduced (see Figure 26). To have 20 mA
flowing in each LED, the amplitude of the PWM signal must be
0 V and 2.5 V only.
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
ADM8845
04867-0-026
100% = I
LED
= 0mA
0% = I
LED
= 20mA
R = 7.5kΩ
C5 = 1µF
Figure 26. PWM Brightness Control Using Filtered PWM Signal
LED BRIGHTNESS CONTROL USING A
DC VOLTAGE APPLIED TO V
BRIGHT
By adding one resistor, as in Figure 22, this configuration can
control the brightness of the white LEDs using a dc voltage
applied to the V
BRIGHT
node. Figure 27 shows an application
example of LED brightness control using a dc voltage with a
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 and V
SET
= voltage at I
SET
pin (1.18 V).
V
BRIGHT
04867-0-028
I
LED
2.5V
0mA
1.6V
7.2mA
0.8V
13.6mA
0V
20mA
Figure 27. PWM Brightness Control Application Diagram Using a
DC Voltage Applied to V
BRIGHT