ZHCSGN5B March 2017 – July 2018 LP8863-Q1
PRODUCTION DATA.
The number of brightness steps when using LED output PWM dimming is equal to the 20-MHz oscillator frequency divided by the LED PWM frequency (set by PWM FSET resistor). The PWM duty cycle dither is a function the LP8863-Q1 uses to increase the number of brightness dimming steps beyond this oscillator clock limitation. The dither function modulates the LED driver output duty cycle over time to create more possible average brightness levels. The DITHER_SELECT[2:0] register bits control the level of dither, disabled, 1, 2, or up to 5 bits using the I2C or SPI interface. By default the dither has 3 bits enabled (default 011b).
When the 1-bit dither is selected, the width of every second PWM pulse is increased by one LSB (one 20-MHz clock period). When the 3-bit dither is selected, within a sequence of 8 PWM periods the number of pulses with increased length varies: dither value 000 - all 8 pulses at default length; 001 - one of the 8 pulses is longer; 010 - two of the 8 pulses are longer, etc., until at 111 seven of the 8 pulses have increased length. Figure 20 shows one example of PWM output dither.
The dither block also has an additional mode at low brightness levels when LED PWM duty cycle is less than the minimum pulse width (that is, less than the LED driver rise time). In this mode the dither block skips full PWM pulses to reduce the brightness further enabling very high dimming modes. The end result is the LED PWM frequency is reduced as more and more minimum pulses are skipped or dithered out. This function can be enabled using the I2C or SPI interface by programming the EN_MIN_PWM_LIMIT bit to 1. Figure 21 shows how the 2-bit minimum brightness dithering works.