UCC28070A: UCC28070A one leg is only working

Hello

The UCC28070 does not automatically turn off one of the two phases at light loads, although this function can be added externally by pulling down the CAOx (current amplifier output) pin. This will set the current demand for the phase to zero and the duty cycle will go to zero due to the 0.7V PWM ramp offset.  As this is a current limited transconductance amplifier, its can only provide a 50uA pull up. Here is a sketch of how this could be implemented -  /cfs-file/__key/communityserver-discussions-components-files/196/2514.PR780ASCH.pdf  

This means that at low power both phases should be running. It may be the case that at very low powers the input offsets of the current amplifiers or the gain of the current sensing networks are different enough to drive one phase to zero duty cycle and the other is running normally.

I'd suggest you probe the GDA and GDB outputs and slowly increase the load. I'd expect to see something on both outputs before the load gets to 10%. If not, then I'd double check the current sensing networks and the CAO outputs.

Please let me know how you get on

Regards

Colin