To increase the refresh rate in time-multiplexing display system, an DS-PWM (Dynamic Spectrum- Pulse Width Modulation) algorithm is proposed in this device. One frame is divided into many segments shown as below. Note that one frame is divided into n sub-periods, n is set by SUBP_NUM (FC0 register bit24-22), and each sub-period is divided into 32 segments for 32 scan lines. Each segment contains GS GCLKs time for grayscale data display and T_SW GCLKs time for switching lines. GS is configured by the SEG_LENGTH (FC1 register bit9-0 in Table 8-8) , and T_SW is the line switch time, which is configured by the LINE_SWT (see FC1 register bit 40-37 in Table 8-8).

Figure 8-28 DS-PWM Algorithm with 32 Scan Lines

The DS-PWM can not only increase the refresh rate meanwhile keep the same frame rate, but also decrease the brightness loss in low grayscale, which can smoothly increase the sub-period number when the grayscale data increases.

To achieve ultra-low luminance, the LED driver must have the ability to output a very short current pulse (1 GCLK time). However, because of the parasitic capacitor of the LEDs, such pulse can not turn on the LEDs. And the larger GCLK frequency is, the harder to turn on LEDs.

The DS-PWM algorithm has a parameter called sub-period threshold, which is used to calculate when to change sub-period number according to the giving grayscale data. Sub-period threshold defines the LED minimum turn-on time, so as to conquer the current loss caused by LED parasitic capacitor. Sub-period threshold is configured by the SUBP_TH_R/G/B (FC1 register bit24-10 in Table 8-8).

With DS-PWM algorithm, the brightness has smoothly increased with the gradient grayscale data.