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TPS61169: Not responding to higher freq PWM

Part Number: TPS61169

 There are another 11 LEDs in series (12 toral - set to 50mA max wire 3R9)

The LIGHTS net is direct from a 1.8V CPU, and voltage goes rail to rail (checked on scope).  When we run the PWM at around 500Hz (way below the spec) , the LEDs work and it appears to function ok.

HOwever winding up PWM  to 1000Hz its starts only working at the lower PWMs, going to max output at around 50% 

Winding up PWM to 5KHz and abive as recommended in datasheet and  the device doesn't appear to respond at all.

This trace shows the PWM when runnig at around 800Hz