ADM8845 datasheet(13/20 Pages) AD

13 / 20 page

ADM8845

Rev. 0 | Page 13 of 20

LED CONFIG.

FULL ON

MAIN AND SUB OFF

CTRL1

CTRL2

VOUT

ILED

(SUB)

ILED

(MAIN)

SUB DISPLAY

BRIGHTNESS

MAIN DISPLAY

BRIGHTNESS

37ms >

tCP > 15ms

SHDN

50%

100%

80%

tCP

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 VPWM

Adding two external resistors and a capacitor, as shown on

Figure 26, also can 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 VPWM 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 ISET 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

)

(

120

Cycle

Duty

Voltage

SET

LED

−

ISET

RSET = 13.4kΩ

R = 7.5k

Ω

VPWM0V–2.5V

ADM8845

100% = ILED = 0mA

0% = ILED = 20mA

R = 7.5k

Ω

C5 = 1

µF

Figure 26. PWM Brightness Control Using Filtered-PWM Signal

LED BRIGHTNESS CONTROL USING A

DC VOLTAGE APPLIED TO VBRIGHT

Adding one resistor, as shown in Figure 22, this configuration

can also be used to control brightness the white LEDs by using

a dc voltage applied to the VBRIGHT 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 VBRIGHT.

The equation for ILED is

ISET = [(1/RSET + 1/R)(VSET)] – [(1/R)(VBRIGHT)]

ILED = 120 × ISET

where R = 15 kΩ and VSET = voltage at ISET pin (1.18 V).

VBRIGHT

ILED

2.5V

0mA

1.6V

7.2mA

0.8V

13.6mA

20mA

Figure 27. PWM Brightness Control Application Diagram Using a

DC Voltage Applied to VBRIGHT

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