Hi,
I have an LCD with 4 string LED backlight, 70mA per string, 37.4 Max Vf, 315kHz switching frequency. Got the ok from a local TI FAE that my circuit is setup ok.
When driving the backlight, I get a 1.4V delta pulse with about a 3.26 second period on the drive signal, which causes the backlight to alternate between two levels of brightness. This is shown quite readily at any PWN frequency/duty cycle, but seems to minimize around a 10kHz PWM rate.
I have disabled spread spectrum with no change.
FYI, I also added more capacitance directly at pins 1 and 20, tried changing the switching freq to 635kHz (and associated inductor change), changed enable/VDDIO logic levels to 3.3V from 1.8V, with no effect on this issue. Also not specific to one board, I have five of these built up, all exhibiting the same.
Any thoughts? Scope shot attached. Thank you.