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KTZ8866

January 2022 - Revision 04b

Page 19 of 37

Company Confidential

Backlight LED Current

The LED current is always a DC current (not PWM). It can be programmed for either exponential mapping mode

or linear mapping mode by Register 0x02 bit [3]. These two modes determine the transfer characteristic of

dimming code to LED current. It also has 11-bit control, including the 8-bit MSBs from register 0x05 bits [7:0]

and the 3-bit LSBs from register 0x04 bits [2:0]. If only 8-bit dimming is needed, the 3-bit LSBs should be kept

as ‘111’ while the 8-bit MSBs are programmed. If 11-bit dimming ratio is needed, the 3-bit LSBs should be

programmed first, then the 8-bit MSBs are programmed. Only programming the 3-

bit LSBs doesn’t change the

current ratio until the 8-bit MSBs are programmed.

In linear mapping 8-bit dimming mode, the LED current per channel can be calculated as:

������������������������_������������ = ������������������������_������������ ∗ ������������������������ ∗ (

3

2050

+

������������������������ ∗ 8 + 7

2050

) ,

(������������������������ = 0~255)

where ILED_FS is the backlight full-scale LED current which is programmed by 0x15 bits [7:3], ranges from 5.2mA

to 30mA with 0.8mA step, DPWM is the input PWM duty cycle if PWM dimming is enabled, otherwise DPWM = 1.

In linear mapping 11-bit dimming mode, the LED current per channel can be calculated as:

������������������������_������������ = ������������������������_������������ ∗ ������������������������ ∗ (

3

2050

+

������������������������

2050

) ,

(������������������������ = 1~2047)

For linear mapping 11-

bit dimming’s Code 0, current sink and boost converter will be disabled, LED will be turned

off.

In exponential mapping 8-bit dimming mode, the LED current per channel can be calculated as:

������������������������_������������ = ������������������������_������������ ∗ ������������������������ ∗

1.003040572(������������������������∗8+7)

500

(������������������������ = 0~255)

In exponential mapping 11-bit dimming mode, the LED current per channel can be calculated as:

������������������������_������������ = ������������������������_������������ ∗ ������������������������ ∗

1.003040572������������������������

500

(������������������������ = 1~2047)

For exponential mapping 11-

bit dimming’s Code 0, current sink and boost converter will be disabled, LED will

be turned off.

Backlight Brightness Control Mode

KTZ8866 has two brightness control mode, I2C Only Mode and I2C x PWM Mode, see Figure 4. In I2C Only

Mode,

register 0x02’s bit [0] PWM_ENABLE should be set to “0”, the LED brightness is controlled by registers

0x04 and 0x05. In I2C x

PWM Mode, register 0x02’s bit [0] PWM_ENABLE should be set to “1”, the LED

brightness will be controlled by I2C code and PWM duty together.

If the LED current is changed from one value to the other by I2C dimming Register 0x04 and Register 0x05, the

ramp time can help LED current transit smoothly from one brightness level to next one. Ramp time can be

adjusted from 1µs to 640ms via

0x03’s bits [6:3]. Ramp time applies both to ramp up and ramp down, it remains

same regardless the amount of change in brightness.