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TPS54062: Slow Start?

Nabeel Khan
Intellectual 1455 points
Part Number: TPS54062
Other Parts Discussed in Thread: LM5020,

Hi Team,

I have an issue with the TPS54062 and would appreciate your input. Please see the schematic below. This can be powered from a battery through D55 at about 7V.  Or it can be powered from POE through D30 from a flyback circuit (LM5020) that makes about 8V with limited current output.

Normally the load is 10-15mA and everything works OK.  However, some production units have a flaw in a circuit powered by 3.3V causing 3.3V current to be 60-70mA.  Even with the extra load, the circuit still works correctly when powered from the battery.  I think this is because the battery has no problem sourcing lots of current.

 

When powered by POE, the 3V3A output is steady at 2.5V instead of 3.3V and Vsense = 0.6V instead of 0.8V and f_switch = 50kHz instead of 100kHz.  This makes me think the TPS54062 is stuck in “slow start” mode, possibly because the flyback converter can’t supply current fast enough to escape slow start mode.

 

If I power by POE without the flaw (load = 10mA) and then add the flaw manually (load increases to 60mA), everything works OK.

 

Please let me know if you have any ideas on what is causing this.

 

Thanks,

Nabeel

TPS54062