LED

Driver

BRT

Register

ALS Select

1

0

VOUT

7 bits

7 bits

LED Ramp

Rate Control

3 bits

Ramp Rate

Increasing

7 bits

DAC

Active Zone

Target

Register

7 bits

Dig Code

3 bits

Ramp Rate

Decreasing

Full Scale

Current

PWM

EN_PWM bit

PWM Polarity Bit

(0 = active high,

1 = active low)

Note 1

Note 2

Note 3

For EN_PWM bit = 1

For EN_PWM bit = 0

Note 3:

DPWM Is a Scaler between 0 and 1 and corresponds to the duty cycle of the PWM input signal

Note 2:

ACODE Is a Scaler between 0 and 1 based on the Brightness Data or Zone Target Data Depending on the ALS Select Bit

Note 1:

IFS = 25 mA

ILED

ILED = IFS x ACODE x DPWM

ILED = IFS x ACODE

ACODE

DPWM

Zone0

Zone1

Zone2

Zone3

Zone4

1

3

4

2

2

1

1

Averager

Output

1 Ave

Period

tBRGT-CHANGE =

2.75 Average

Time

tBRGT-CHANGE =

1.75 Average

Time

24

LM3535

SNVS598B – AUGUST 2010 – REVISED MARCH 2018

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Copyright © 2010–2018, Texas Instruments Incorporated

When using the ALS averaging functionality, it is important to remember that the averaging cycle is free running

and is not synchronized with changing ambient lighting conditions. Due to the nature of the averager round down,

an increase in brightness can take between 2 and 3 averaging cycles to change zones while a decrease in

brightness can take between 1 and 2 averaging cycles to change. See Figure 25 for a list of possible averager

periods. Figure 35 shows an example of how the perceived brightness change time can vary.

Figure 35. Perceived Brightness Change Time

8.2.2.1.4

Ambient Light Current Control + PWM

The ambient light current control can also be a function of the PWM input duty cycle. Assume the LM3535 is

configured as described in the previous example, but this time the Enable PWM bit set to 1 (Configuration

Register bit [0]). Figure 36 shows how the different blocks (PWM and ALS) influence the LED current.

Figure 36. Current Control Block Diagram