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TPS61150A: question about regulation

Other Parts Discussed in Thread: TPS61150A

I am seeing some unexpected behavior from a TPS61150A design with output ripple.  The chip is driving 2 strings of WLEDs at 15mA ea. and approx. 20V  output.

schematic is copied from the typical application on the front page of the datasheet  with 75K substituted for the 56.5K shown and a PTC between the chip and the external LED strings.  Part is full on, no PWM running.  Vin is approx. 4.2V

What I see is about a volt of ~80kHz AC ripple riding on top of the DC along with the expected switching noise at 1.2MHz

the scope shot below has one of the IFB pins on the top trace and AC coupled trace of the Iout pin on the bottom.

I wasn't expecting to see such large excursions on the IFB pins.  Any suggestions as to what I should be looking at?