Adjusting the CCSI frequency is the effective way to solve the CLK_O duty cycle increase challenge and support a higher cascaded number. Because the increased time (∆t1+∆t2) over each LED driver can be assumed to be a fixed value if the slew rate from the host controller is the same, then a lower CCSI frequency allows for a longer logic-low period(T/2). A lower CCSI frequency must pass more daisy-chained LED drivers until the CLK low time reach the minimum limit.

The TLC696xx-Q1 family CCSI frequency is related to some system level performance including EMI and audio noise cancellation. In this application note, the focus is on the basic rule to calculate the minimum CCSI frequency required, which is to display and transmit the required brightness data in a single frame. Table 2-3 lists the 43 piece LED drivers the design requirements of the cascaded system, which is an example to calculate the required frequency.

Minimum frequency for PWM brightness display:

F_ Min_Display = (frame rate) × (2^brightness resolution) × (line number) = 0.98MHz

Minimum frequency for transmitting brightness (See the brightness write command in the data sheet):

F_Min_Transmission = (frame rate) × [16x3+16x (chip number) + 16 + (chip number) × 16x(line number) × 16+16] = 2.75MHz

The calculated minimum CCSI frequency for TLC696x4-Q1 to transmit and display the LED zone brightness data in one frame is 2.75MHz. But in most applications, a higher CCSI frequency must be selected to reserve more time for the TCON local dimming algorithm calculation and LED driver fault diagnostic. That's why TCON selected 8MHz as a starting point to bring up this system. In the following sections, how to further reduce the CCSI frequency required without sacrificing the system level requirements is discussed.