LM317A: Differential voltage and Input voltage max for LM317AT/NOPB

Logesh Padmanabhan

Part Number: LM317A

Dear sir,

Please clarify the below content given in datasheet of LM317AT/NOPB.

" LM317A regulator is floating and detects only the input-to-output differential voltage, supplies of several hundred volts can be regulated as long as the maximum input-to-output differential is not exceeded. Exceeding the maximum input-to-output deferential will result in short-circuiting the output. By connecting a fixed resistor between the adjustment pin and output, the LM317A can be also used as a precision current regulator."

Shall we consider differential voltage between input and output.?

Example - input voltage 53V & Voltage output 23V. so available differential voltage will be 20V

Please help us to resolve this issue. We ready to cascade two regulators to reduce the differential voltage.

Our requirement is 20mA output at Input voltage 53V max and output 39V, operating temperature -40 to 85 Deg C or better with TO-220 package.

over 3 years ago

0 Kyle Van Renterghem over 3 years ago

TI__Genius 13085 points

Hi Logesh,

You are correct the differential voltage the datasheet is referring to is the input to output differential. So your application appears to be within the stated parameters however if you think there is a chance that the output voltage will be shorted to GND (0V) you may want to consider using the cascaded design you referred to in order to stay below the 40V limit.

Below is a quick screenshot of the Abs Max table from the datasheet for reference.

0 Logesh Padmanabhan over 3 years ago in reply to Kyle Van Renterghem

Prodigy 70 points

Hi Kyle Van Renterghem,

Thanks for your quick reply.

My question is "can i apply 53V voltage input to regulator to get a 25V output"

Regards,

Logesh.P

0 Kyle Van Renterghem over 3 years ago in reply to Logesh Padmanabhan

TI__Genius 13085 points

Hi Logesh,

Yes since the differential is less than 40V, 53V to 25V is fine.

0 Logesh Padmanabhan over 3 years ago in reply to Kyle Van Renterghem

Prodigy 70 points

Hi Kyle Van Renterghem,

Thanks for your reply.

could you please send application note for LM317AT/NOPB.

it will help to review our product with management, since the datasheet not having any content as you told.

at present specifications are conflicting between pages.

Regards,

Logesh.P

0 Kyle Van Renterghem over 3 years ago in reply to Logesh Padmanabhan

TI__Genius 13085 points

Hi Logesh,

I do not have a specific App Note on this topic and I don't see any conflicting information in the datasheet. The section you quoted early is supported through out the datasheet so can you use the highlighted sections of the datasheet's front page which describes the LDO as a floating regulator and then defines the maximum Vin to Vout voltage?

Also the EC Table conditions also specify all the input voltages relative to Vout (instead of GND like a normal LDO).

Hopefully this will help convince them.

0 Logesh Padmanabhan over 3 years ago in reply to Kyle Van Renterghem

Prodigy 70 points

Hi Kyle Van Renterghem,

Thank you so much for your reply.

Please see the below data, I am trying to show the same. what will be the maximum input voltage..? since the differential is less than 40V,

if Is it fine 75Vdc(Vi) to 35Vdc (Vo) also.?

Could you give us some more details on the same. we would appreciate your help in this matter.

Regards,

Logesh.P

0 Kyle Van Renterghem over 3 years ago in reply to Logesh Padmanabhan

TI__Genius 13085 points

Hi Logesh,

I can't see the picture you attached can you try again?

And yes, if you have Vout=35V then the maximum acceptable input would be Vin=75V.

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LM317A: Differential voltage and Input voltage max for LM317AT/NOPB