The following data is obtained using the LM369xx according to the 11-bit exponential dimming formula in Equation 1 and Equation 2:

Equation 1 shows the LM3697 Dimming Equation:

Equation 1.

Equation 2 shows the LM36922_3 Dimming Equation:

Equation 2.

Where:

• Code is the 11-bit backlight code in the Control Brightness MSB and LSB registers, or 11-bit brightness dimming data
• I_{LED_FULLSCALE} is the full-scale LED current setting
• D_PWM is the PWM Duty Cycle; 50 µA is the minimum LED current that the LM36922_3 can handle

Code Configuration sample: Control Brightness MSB and LSB registers. Full format of 11-bit brightness dimming code: 8-bit MSB(10:3) + 3Bit LSB(2:0) = 11 bit, in an actual design, the 11-bit value should be translated to decimal format .

Figure 2-1 LM3697 LSB+MSB 11Bit Brightness Dimming Code Format

Figure 2-2 LM36922_3 LSB+MSB 11Bit Brightness Dimming Code Format

Assumes target full brightness LED current of 30 mA, total 2048 step, or 11 bit:

Each step of the LED current in linear format is I_{LED_Step} = 30 mA / 2048 = 14.6 µA.

• For a design using the LM3697:

Changing Equation 1 to I_{LED_FullScale} = 30 mA, assumes D_PWM = 1.

Equation 3.

Then, it translates it from decimal code to Hex code.

Make a linear format excel table (Exponential Data to Linear Format Dimming Lookup Table) with the previous equations for quick calculation:

• For a design with the LM36922 and LM36923
  
  
  
  

Because the minimum LED current that the LM3692x can support is 50 µA in exponential mode, keep all of the value that is less than 50 µA to 50 µA, then make the following change to Equation 2:

Equation 4.

Then, translate decimal code to Hex code.

Make a linear format excel table (Exponential Data to Linear Format Dimming Lookup Table) with the previous equations for quick calculation:

When dimming begins, the AP(Application Processor) can lookup this table according to linear dimming steps to easily get the target exponential mode brightness code. An example is made to show its working process:

1. The Software Dimming Module in a smartphone OS sends out linear orders from 1 to 2048 as the Linear Format Dimming Step column of Table 2-1 and Table 2-2 show to AP.
2. The AP will map linearity dimming orders to exponential dimming code that has been preset in the Exponential Dimming Code Based on Linear Format, Decimal column in Table 2-1 and Table 2-2.
3. Next, the AP gets this exponential code from Table 2-1 and Table 2-2 sends it to the DPU.
4. The DPU executes commands from the AP and outputs the target brightness.

In some user cases, the AP cannot handle exponential dimming processes directly for Software Dimming Module limits, so this method helps designers keep the AP dimming code linearity and take the use of smoother dimming performance in expo dimming. Furthermore, this Lookup Table could be integrated into software platform, it could be used as a general tool in all projects in which LM369xx family is designed-in. This solution is acceptable from all sides: easy dimming compatibility and better performance.