ADM8845 Analog Devices, ADM8845 Datasheet - Page 13

• Manufacturer Part Number: ADM8845
• Description: Charge Pump Driver for 6 White LED LCD Backlights
• Manufacturer: Analog Devices

Specifications of ADM8845

• Vin Range: 2.6 to 5.5V
• Vout (v): Vin to 5.5
• Synchronous: No
• Package: 16-Lead LFCSP
• Led Configuration: Parallel
• Topology: Capacitive
• I2c Support: No
• Max Iout (ma): 30mA
• Brightness Control: PWM
• Peak Efficiency (%): 88%
• Switching Frequency: 1.5MHz

LED BRIGHTNESS CONTROL USING A
PWM SIGNAL APPLIED TO V
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
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
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.
I
LED
Figure 26. PWM Brightness Control Using Filtered PWM Signal
V
=
PWM
I
SET
0V–2.5V
R
R
SET
SET
LED CONFIG.
MAIN DISPLAY
SUB DISPLAY
_
BRIGHTNESS
BRIGHTNESS
Voltage
100% = I
×
+
0% = I
2
2
C5 = 1µF
R
R
R = 7.5kΩ
SET
CTRL1
CTRL2
(MAIN)
(SUB)
V
LED
I
I
LED
OUT
LED
LED
pin of the ADM8845. A 0% PWM
×
= 20mA
120
= 0mA
R = 7.5kΩ
×
1 (
FULL ON
PWM
−
Duty
I
100%
100%
ADM8845
SET
100
R
SET
Cycle
= 13.4kΩ
)
PWM
SUB AND MAIN 50%
can be used
DUTY CYCLE
50%
50%
Figure 25. Application Timing
Rev. C | Page 13 of 20
37ms >
t
CP
LED BRIGHTNESS CONTROL USING A
DC VOLTAGE APPLIED TO V
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
example of LED brightness control using a dc voltage with a
amplitude of 0 V to 2.5 V, applied to V
The equation for I
where R = 15 kΩ and V
MAIN AND SUB OFF
t
V
CP
BRIGHT
> 15ms
I
LED
I
I
Figure 27. PWM Brightness Control Application Diagram Using a
SET
LED
= [(1/R
= 120 × I
SHDN
SHDN
0mA
2.5V
SET
BRIGHT
SET
LED
+ 1/R)(V
DC Voltage Applied to V
is
node. Figure 27 shows an application
SET
7.2mA
MAIN 80% DUTY CYCLE,
1.6V
= voltage at I
SET
)] – [(1/R)(V
80%
SUB OFF
BRIGHT
BRIGHT
13.6mA
SET
0.8V
BRIGHT
pin (1.18 V).
BRIGHT
.
)]
ADM8845
20mA
0V