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LTC3785 Burst Mode


I have designed a 5V@4A boost-buck voltage regulator using the LTC3785 - I found the Excel Spreadsheet calculator tool and Applications Information provided in the datasheet to be most helpful in this process, thank you.

However, I am seeing some unexpected behaviour when operating in Burst Mode, as load current becomes significant, when supplying a nominal input voltage of 6.4V. 

At light loads, switching Burst Mode on and off changes the quiescent current draw, as expected.

If Burst Mode is left on and I increase the load current above around 0.5A, it appears that the current limiting action takes effect as the efficiency and output voltage falls and the C&D FETs (Buck) heat up rapidly.

If I disable Burst Mode, the regulator supplies current all the way up to the 4A limit before feedback into the error amp starts to reduce the output voltage.

Is this behaviour expected?  I had the impression that I could simply leave Burst Mode on permanently and that it's function would be disabled by the action of the (separate) Undervoltage Protection.

Please note that I have used two separate voltage divider networks (albeit with same resistor values) for the FB and VSENSE control pins.

I could use our host controller to monitor load current and engage Burst Mode as required, but this will complicate the software and I'm not sure how quickly it would need to respond to avoid damaging the controller or FETs in the event of a large and sudden increase in load current.

I'd appreciate any advice you may have in relation to this.

Thank you,