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LM5021: Standby Functions Consult

Stanley Dai
Expert 5810 points
Part Number: LM5021

Hi Team,

Greeting! Recently, customers are evaluating our LM5021’s standby functions. And they found one strange phenomenon needing your comments here.

They disconnect OUT pin and continually monitor the voltage of VIN pin, shown below. And they find out the voltage of VIN pin will always drop from 25V to 7.3V around no matter how they adjust the charging RC value.  They expect the VIN voltage should be in cycle voltage(25V-->7.5v--->25V-->7.5V..) under 100V voltage input condition. They now are worried about our LM5021 cannot restart itself if they disconnect OUT pin. Sometimes they will do it for saving power. 

Can you please comment on above phenomenon and any solutions here? Many thanks.

 

Best Regards,

Stanley

  • 0
    TI__Expert 7080 points

    Hi  Stanley,

    If OUT pin is disconnected , that means no current will flow in the power stage , Current signal detect from CS pin is always low , the controller think fault on CS pin happens . then it go into fault mode. that is why you see VDD drop from 25V to 7.3V . this is a normal behavior for LM5021.

    Disconnect OUT pin is not a good choice for disabled the controller . I recommend customer use a MOSFET to pull down SS pin when they want to disable the part . and once SS pin is released , the controller will start soft start.

    Thanks.