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TL5209: Clarification on Low drop out

Vishal A1
Intellectual 540 points
Part Number: TL5209
Other Parts Discussed in Thread: LM317M

Hi,

We are using "TL5209DR" in our design and below is the design 

1.Vin =9V

2.Vout = 5V@500mA expected

3.Cout =10uF,0.1uF

4.Cin = 1uF,0.1uF

5.CBYP = 470pF

6.R1 = 100K ,R2 = 300K

Please confirm if the above design works as per our requirement of 5V output voltage @0.5A. It is mentioned in the datasheet that the dropout voltage at 500mA is 500mV.We are supplying input voltage of 9V and the expected output voltage is 5V and there is a headroom of 4 V between input and output voltage.If the system operates at full load(500mA) what is the expected output voltage.Is it 5V constant or will it be 4.5V(need to consider drop out?).Kindly clarify how the drop out should be considered? 

  • 0
    TI__Guru 50090 points

    Hi Vishal,

    Drop out would only be a concern if VIN approaches 5.6 to 5.6V.

    Your bigger concern here is thermals. The TL5209 has a TJA of 116.1C/W. So with VIN of 9V and VOUT of 5V at 500mA, you have (9-5)*0.5 or 2W of power dissipation. The die junction temperature would rise to TJ=TA+TJA*W or in this case TJ=25C+116.1C/W*2W= 257.2C!

    A device like the LM317M might be better in this application. In the SOT-223 package, the TJA is 60.2C/W and in the TO-220 package, it is 34.8C/W.

    So for the SOT-223 package, TJ would be 25+2*60.2 or 145.4C

    and the TO-220 TJ would be 25+2*34.8 or 94.6C

    Additionally, the LM317M has built-in thermal protection.

    I hope this helps to answer your question.