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TPS61120 startup stability

Other Parts Discussed in Thread: TPS61120

Hello,

I'm having some intermittent issues with the TPS61120, this is set up to provide 5.3V from a Li-Ion battery. On most boards this works, but on some boards the switching never starts. The FB pin voltage is < 0.5V, so it should be switching, but it is not. If I probe around the SW connection to the inductor a lot, , or touch it with a soldering iron (i.e. ground) I can get it to start switching. Once an IC has started switching, it always seems to work on subsequent power-ons.

Any ideas why this could be ? I am using a ceramic output capacitor, but also have 4.7pF in parallel with the upper feedback resistor to make sure of small-signal stability. 

Thanks,

Kevin