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TPS62260: TPS62260 Low Amplitude Output Oscillation

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Part Number:TPS62260

We have a new design which uses four TPS62260DRV devices to produce four DC power rails +2.8V, +2.5V, +1.8V, +1.2V. The circuits are more ore less identical apart from the resistor from the lower half of the feedback voltage divider. At this stage the regulators are not feeding any loads, these are being simulated by fixed resistors.The pcb tracking for all four is identical.. All four share a common stablised +5V input and the DC level out of all four regulators is correct. All four circuit have local 10uV, X5R capacitors on input and outputs. The Mode pins are strapped to +5V in all four.

The +2.5V & +1.2V are fine, output noise and ripple is less than 10 mV.

However, the +2.8V and +1.8V are exhibiting a low frequency saw-tooth shaped ripple of around 100mV pk-pk at around 15 KHz. If I change the lower feedback resistor on the +1.8V to the same value as that of the 1.2V (365K) it still shows the low frequency ripple. Varying the load from about 10 mA up to 400 mA  has limited effects.

A second assembly shows the same effects on the same power rails.

Has anybody got any suggestions about how to fix this please?

Robert Phillips


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