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KTZ8866

January 2022 - Revision 04b

Page 20 of 37

Company Confidential

I2C Brightness

Ramp

Generator

PWM Duty Detector

Map Generator:

Linear or Exponential

DAC

ISINK #1

ISINK #2

ISINK #3

ISINK #4

BOOST

MIN VHR

PWM INPUT

RAMP RATE

MAP MODE

VOUT

ISINK #5

ISINK #6

Figure 4. I2C and PWM Dimming Scheme

Backlight PWM Dimming

In backlight I2C x PWM Mode, the input PWM duty cycle is converted internally to produce a DC output sink

current (not pulsing). When PWM is enabled, it can be programmed as either active high or active low by register

0x02’s bit [2], with active high as default. When PWM dimming is enabled, KTZ8866 uses internal 20MHz

sampling clock to detect the PWM duty cycle. It is recommended to have the minimum PWM on time as 0.1

s.

For the example of 20kHz dimming frequency, the PWM duty cycle range can be 0.2%~100%. The PWM

dimming frequency range can be as wide as 100Hz to 100kHz.

PWM Dimming Step Response and Timeout

If the LED current is changed from one value to the other by PWM dimming duty cycle, the transition ramp

up/down time can be programmed by Register 0x15 bits [2:0]. For this transition ramp, its slope is fixed, so the

final transition ramp time is dependent on the change amount of the PWM duty cycle.

The KTZ8866 PWM timeout feature turns off the boost output when the PWM is enabled and there is no PWM

pulse detected.

PWM to Digital Code Readback

In I2C x PWM control mode, registers 0x12 and 0x13 contain the PWM duty cycle to the 11-bit code conversion

information. Register 0x12 contains the 8 LSBs of the brightness code and register 0x13 the 3 MSBs. They are

suggested to be read out in successive way to make sure the PWM duty result is correct. Too long delay between

reading them may cause incorrect returned result, since input PWM duty may change during the delay time. To

translate this reading to the actual LED current setting of the KTZ8866, convert it to the corresponding duty cycle

and multiply it by the brightness level setting in the brightness registers (0x04 and 0x05).