SNVSC36 December 2021 LP5866
PRODUCTION DATA
There are several methods to control the PWM duty cycle of each LED dot.
Every LED has an individual 8-bit or 16-bit PWM register that is used to change the LED brightness by PWM duty. The LP5866 uses an enhanced spectrum PWM (ES-PWM) algoithm to achieve 16-bit depth with high refresh rate and this can avoid flicker under high speed camera. Comparing with conventional 8-bit PWM, 16-bit PWM can help to achieve ultimate high dimming resolution in LED animation applications.
The group PWM Control is used to select LEDs into 1 to 3 groups where each group has a separate register for duty cycle control. Every LED has 2-bit selection in LED_DOT_GROUP Registers (x = 0, 1, … , 54.) to select whether it belongs to one of the three groups or not:
The Global PWM Control function affects all LEDs simultaneously.
The final PWM duty cycle can be calculated as below:
The LP5866 supports 125-kHz or 62.5-kHz PWM output frequency. The PWM frequency is selected by configuring the 'PWM_Fre' in Dev_initial register. An internal 31.2-MHz oscillator is used for generating PWM outputs. The oscillator’s high accuracy design (ƒOSC_ERR ≤ ± 2%) enables a better synchronization if multiple LP5866 devices are connected together.
A PWM phase-shifting scheme is implemented in each current sink to avoid the current overshot when turning on simultaneously. As the LED drivers are not activated simultaneously, the peak load current from the pre-stage power supply is significantly decreased. This scheme also reduces input-current ripple and ceramic-capacitor audible ringing. LED drivers are grouped into three different phases. By configuring the 'PWM_Phase_Shift' in Dev_config1 register, which is default off, the LP5866 supports tphase_shift = 125-ns shifting time shown in Figure 8-4.
To avoid high current sinks output ripple during line switching, current sinks can be configured to turn on with 1 clock delay (62.5 ns or 31.25 ns according to the PWM frequency) after lines turn on, as shown in Figure 8-3. This function can be configured by 'CS_ON_Shift' in Dev_config1 register.
The LP5866 allows users to configure the dimming scale either exponentially (Gamma Correction) or linearly through the 'PWM_Scale_Mode' in Dev_config1 register. If a human-eye-friendly dimming curve is desired, using the internal fixed exponential scale is an easy approach. If a special dimming curve is desired, TI recommends using the linear scale with software correction. The LP5866 supports both linear and exponential dimming curves under 8-bit and 16-bit PWM depth. Figure 8-5 is an example of 8-bit PWM depth.
In summary, the PWM control method is illustrated as Figure 8-6: