LM2736: Not Regulating

Amir Eshraghi

Part Number: LM2736

I am using the LM2736XMK/NOPB to convert 15V to 6V but unfortunately below design is not regulating the output and I can not figure out what is going on.

I even try to simulate using Web Bench and I am getting correct voltage.

As I increase the input voltage the output is changing too. Can you take look at my schematic and see if you find issue.

L= 6.8uH NLCV32T-6R8M-EFR
Cout = Cin = 10uF

Diodes: D11=D12 PMEG3005EH,115

Cboost = 10nF

Let me know if you want me get some graph from output.

Thanks

over 2 years ago

+1 Nancy Zhang over 2 years ago

TI__Mastermind 47890 points

Hi Amir,

Your schematic is incorrect. You could not charge boot voltage with diode D11. Because the absolute max voltage for Boot to SW node is 8V. It is 15V in your case and it will damage the device. Vboot-Vsw should not exceed the max operating limit of 5.5V. Please refer to section 8.1.1 of datasheet. You need to use a zener diode from output to charge boot.

Hope this helps,

Thanks,

Nancy

0 Amir Eshraghi over 2 years ago in reply to Nancy Zhang

Expert 1270 points

Thanks for the quick answer, and it did work as you mentioned.

0 Shuai Fan over 2 years ago in reply to Amir Eshraghi

TI__Genius 13451 points

Nancy will reply you tomorrow!

Shuai

0 Nancy Zhang over 2 years ago in reply to Shuai Fan

TI__Mastermind 47890 points

Hi Shuai,

The issue was resolved.

Thanks,

Nancy

0 Amir Eshraghi over 2 years ago in reply to Nancy Zhang

Expert 1270 points

HI Nancy,

Sorry to ask another question, base on 8.2.5 of data sheet since VIn and Vout > 5V, how do we calculate the Zener diode voltage and power, also do I still need the Shockey diode.

Thanks

0 Nancy Zhang over 2 years ago in reply to Amir Eshraghi

TI__Mastermind 47890 points

Hi Amir,

Please follow Fig 25 in the datasheet. You need a zener diode and schockey diode in series. D2/D3 P/N are in table 5.

Thanks,

Nancy

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LM2736: Not Regulating