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TPS40057: TPS40057 , UVLO setting and Vo drop issue at DC-DC converter

Part Number: TPS40057

Dear Sir ,

Use the TPS40057 and find the issue as below , could you plase help it ? Thanks.

-  Adjusting the RKFF (R5 of schematic) , RA (R12 of schematic) and CA (C16 of schematic) parameter value for setting the UVLO point.

   But , UVLO point don’t change => Whether can adjusting what components or parameter ?

-  Vo will drop when Vin is increasing => Found the RKFF (R5 of schematic) will influence it => How to set it for avoid the Vo drop issue ?

   Vin < 20Vdc_in that Vo is 4.9Vdc when Io = 2.1A

   Vin > 20Vdc_in taht Vo is 4.5Vdc when Io = 2.1A

UVLO table from datasheet as below. Whether the UVLO set point must between 6.95V and 7.95V ?

Ex: IC will occur error information when UVLO more than 7.95V , Right ? 

Schemati portion :