LP8866S-Q1: PWM DIMMING RESOLUTION 10bits with software 16bits impact Vout

Jimmy Gong

Part Number: LP8866S-Q1

Hi team,

I have some question here about PWM DIMMING RESOLUTION.

1. We use 124kohms R_PWM_FSET(10bits) then software's brightness set at 16bits(hear from my customer software team). the Vout will high to 27V output at low brightness(normal state is at 18V). then set to high brightness Vout will go to 18V. then when we replace R_PWM_FSET to 4.75kohms 16bits the Vout will be 18V then the issue is solved. My customer would like to explain this situation. Do you have any idea for this? or have any document talk about what's happen when our hardware PWM_FSET design at 10bits and software write 16bits?

2. If we use 124kohms R_PWM_FSET(10bits). How do we set the software's brightness at 10bits? Do we set offset 00h as below?

3. what's different between 10bits and 16bits? Output brightness resolution? is it at same max/min brightness, 10bits has few level can be adjusted?

BR,

Jimmy

11 months ago

0 Zhiqian Wu 11 months ago

TI__Mastermind 20855 points

Hi, Jimmy

For question1, what is the result if customer change the brightness from high to low after the output voltage is 18V.

For question2, if you don't enable the dither, some value will have the same brightness.

For question3, the minimum step of the PWM dimming is the same, so high dimming frequency means high percent change. That is what resolution means.

0 Hung Yu-chuan 11 months ago in reply to Zhiqian Wu

Prodigy 90 points

For question1,
Software and hardware settings: We use 124kohms R_PWM_FSET (10 bits), and then the software brightness is set to 16 bits.

Test method: When it is turned on, the brightness is the lowest brightness, and the output voltage is observed to be 27V. When the brightness is manually adjusted to the seventh-level brightness set by the software, the output voltage becomes 18V, and the voltage is still 18V when it is adjusted back to the lowest brightness. 18V, in line with the 18V boost that a string of 6 LEDs should have. Why does this happen?