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LM317 to step voltage up

Resistors values to use with LM317Simple LM317 variable voltage supply, does it limit current too?Why are the output voltages way too high on this stacked LM317 circuit?LM317 Voltage Regulator + ammeter = firecracker?LM317 µA Constant Current source possibility?How to improve load transient response of an LM317 voltage-regulator powering an audio amplifier?LM317 Maida Style regulatorLM317 maximum resistor/pot - Current regulator - Reference voltageLM317 - Input Voltage

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1

$begingroup$

I am sorry for questioning the "law of conservation of energy" first of all.

The expression for the output voltage of the LM317:

Vout=1.25*(1+R2/R1)

contains no any relation to the input voltage. It doesn't care what's the input voltage. It only cares about the two resistors R1 and R2 (in my simulation). Increasing the value of R2 increased the output. But once I get the output equal to the input, further increasing the value of R2 doesn't increase the output voltage. What is the factor that doesn't let the output voltage go beyond the input voltage. I know it won't go (law of conservation of energy) but why?

I have attached the snippets of the output voltage for two different values for the R2 resistors.(I don't know whether those large valued resistors are available. Please bear with me for this simulation)

Edit: So its all the matter of 'Minimum Dropout voltage' (Vin-Vout). But going through this datasheet shows the difference of -0.3 which indicates higher output voltage.

voltage-regulator lm317

share|improve this question

edited 9 hours ago

JuneStar_2918

asked 9 hours ago

JuneStar_2918JuneStar_2918

40022 silver badges1111 bronze badges

$endgroup$

• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  $begingroup$
  No, the -0.3V for the OnSemi part is not the recommended dropout voltage. That number is in the "Maximum Ratings" section and they are saying that if you ever allow the input voltage to be more than 0.3V below the output voltage then the device may be permanently damaged.
  $endgroup$
  – Elliot Alderson
  9 hours ago
  

  
  
  
  

add a comment | 

1

$begingroup$

I am sorry for questioning the "law of conservation of energy" first of all.

The expression for the output voltage of the LM317:

Vout=1.25*(1+R2/R1)

contains no any relation to the input voltage. It doesn't care what's the input voltage. It only cares about the two resistors R1 and R2 (in my simulation). Increasing the value of R2 increased the output. But once I get the output equal to the input, further increasing the value of R2 doesn't increase the output voltage. What is the factor that doesn't let the output voltage go beyond the input voltage. I know it won't go (law of conservation of energy) but why?

I have attached the snippets of the output voltage for two different values for the R2 resistors.(I don't know whether those large valued resistors are available. Please bear with me for this simulation)

Edit: So its all the matter of 'Minimum Dropout voltage' (Vin-Vout). But going through this datasheet shows the difference of -0.3 which indicates higher output voltage.

voltage-regulator lm317

share|improve this question

edited 9 hours ago

JuneStar_2918

asked 9 hours ago

JuneStar_2918JuneStar_2918

40022 silver badges1111 bronze badges

$endgroup$

• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  $begingroup$
  No, the -0.3V for the OnSemi part is not the recommended dropout voltage. That number is in the "Maximum Ratings" section and they are saying that if you ever allow the input voltage to be more than 0.3V below the output voltage then the device may be permanently damaged.
  $endgroup$
  – Elliot Alderson
  9 hours ago
  

  
  
  
  

add a comment | 

$begingroup$

I am sorry for questioning the "law of conservation of energy" first of all.

The expression for the output voltage of the LM317:

Vout=1.25*(1+R2/R1)

contains no any relation to the input voltage. It doesn't care what's the input voltage. It only cares about the two resistors R1 and R2 (in my simulation). Increasing the value of R2 increased the output. But once I get the output equal to the input, further increasing the value of R2 doesn't increase the output voltage. What is the factor that doesn't let the output voltage go beyond the input voltage. I know it won't go (law of conservation of energy) but why?

I have attached the snippets of the output voltage for two different values for the R2 resistors.(I don't know whether those large valued resistors are available. Please bear with me for this simulation)

Edit: So its all the matter of 'Minimum Dropout voltage' (Vin-Vout). But going through this datasheet shows the difference of -0.3 which indicates higher output voltage.

voltage-regulator lm317

share|improve this question

edited 9 hours ago

JuneStar_2918

asked 9 hours ago

JuneStar_2918JuneStar_2918

40022 silver badges1111 bronze badges

$endgroup$

share|improve this question

edited 9 hours ago

JuneStar_2918

asked 9 hours ago

JuneStar_2918JuneStar_2918

40022 silver badges1111 bronze badges

share|improve this question

edited 9 hours ago

JuneStar_2918

asked 9 hours ago

JuneStar_2918JuneStar_2918

40022 silver badges1111 bronze badges

asked 9 hours ago

JuneStar_2918JuneStar_2918

40022 silver badges1111 bronze badges

asked 9 hours ago

JuneStar_2918JuneStar_2918

40022 silver badges1111 bronze badges

2 Answers
2

active

oldest

votes

2

$begingroup$

All the LM317 can possibly do is act like a (controlled) variable resistor between the input and output, it cannot create energy out of thin air.

And it can't even do that, being a bipolar part it will have some significant voltage drop at best with even moderate currents (some CMOS-output LDO regulators have a few mV dropout at low currents such as a few mA).

In the case of the LM317 (which is not an LDO = Low Drop Out) part, there is a Darlington transistor pair between input and output, and the drive can't go all the way to the input voltage.

See this "representative" schematic from this datasheet:

In general you should have 3V or more between input and output, as specified under the "line regulation" parameter in the datasheet.

share|improve this answer

edited 9 hours ago

answered 9 hours ago

Spehro PefhanySpehro Pefhany

223k55 gold badges177177 silver badges467467 bronze badges

$endgroup$

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  $begingroup$
  Ahh! I can see why there is this limitation. And I think the datasheet I have attached is wrong I guess. Thank you @Spehro Pefhany Sir.
  $endgroup$
  – JuneStar_2918
  9 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  @JuneStar_2918 No, that data sheet is not wrong. Rather you are mistaken in only looking at the formula and ignoring the conditions the data sheet gives which bound when the formula is applicable. There may also be very slight difference between brands, both in the actual details and especially in how they are presented. Reading data sheets is a skill, generally when a manufacturer wants to provide something more tutorial in nature they create an Application Note or Reference Design, sometimes for novel parts there would be hints in a data sheet but not for something common like this.
  $endgroup$
  – Chris Stratton
  8 hours ago
  
  
  

  
  
  
  

add a comment | 

2

$begingroup$

An LM317 is defined by many parameters, not just the equation you provide. All of the required operating conditions must be satisfied, and one of the those is the "minimum dropout voltage". This is the minimum difference between the input and output voltage, where the input voltage must be greater than the output voltage. The datasheet for your specific LM317 will provide that number.

Edit: For the Texas Instrument's version of the LM317, the important parameter is in section 6.3 of the datasheet.

share|improve this answer

edited 9 hours ago

answered 9 hours ago

Elliot AldersonElliot Alderson

11.9k22 gold badges1212 silver badges2525 bronze badges

$endgroup$

• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  I have edited my question based on you answer. Thank you.
  $endgroup$
  – JuneStar_2918
  9 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  Why does it say recommended? Is it saying "if you need 40V at its output, then you must supply 43-80V at its input"?
  $endgroup$
  – JuneStar_2918
  9 hours ago
  

  
  
  
  

add a comment | 

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2

$begingroup$

All the LM317 can possibly do is act like a (controlled) variable resistor between the input and output, it cannot create energy out of thin air.

And it can't even do that, being a bipolar part it will have some significant voltage drop at best with even moderate currents (some CMOS-output LDO regulators have a few mV dropout at low currents such as a few mA).

In the case of the LM317 (which is not an LDO = Low Drop Out) part, there is a Darlington transistor pair between input and output, and the drive can't go all the way to the input voltage.

See this "representative" schematic from this datasheet:

In general you should have 3V or more between input and output, as specified under the "line regulation" parameter in the datasheet.

share|improve this answer

edited 9 hours ago

answered 9 hours ago

Spehro PefhanySpehro Pefhany

223k55 gold badges177177 silver badges467467 bronze badges

$endgroup$

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  $begingroup$
  Ahh! I can see why there is this limitation. And I think the datasheet I have attached is wrong I guess. Thank you @Spehro Pefhany Sir.
  $endgroup$
  – JuneStar_2918
  9 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  @JuneStar_2918 No, that data sheet is not wrong. Rather you are mistaken in only looking at the formula and ignoring the conditions the data sheet gives which bound when the formula is applicable. There may also be very slight difference between brands, both in the actual details and especially in how they are presented. Reading data sheets is a skill, generally when a manufacturer wants to provide something more tutorial in nature they create an Application Note or Reference Design, sometimes for novel parts there would be hints in a data sheet but not for something common like this.
  $endgroup$
  – Chris Stratton
  8 hours ago
  
  
  

  
  
  
  

add a comment | 

2

$begingroup$

All the LM317 can possibly do is act like a (controlled) variable resistor between the input and output, it cannot create energy out of thin air.

And it can't even do that, being a bipolar part it will have some significant voltage drop at best with even moderate currents (some CMOS-output LDO regulators have a few mV dropout at low currents such as a few mA).

In the case of the LM317 (which is not an LDO = Low Drop Out) part, there is a Darlington transistor pair between input and output, and the drive can't go all the way to the input voltage.

See this "representative" schematic from this datasheet:

In general you should have 3V or more between input and output, as specified under the "line regulation" parameter in the datasheet.

share|improve this answer

edited 9 hours ago

answered 9 hours ago

Spehro PefhanySpehro Pefhany

223k55 gold badges177177 silver badges467467 bronze badges

$endgroup$

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  $begingroup$
  Ahh! I can see why there is this limitation. And I think the datasheet I have attached is wrong I guess. Thank you @Spehro Pefhany Sir.
  $endgroup$
  – JuneStar_2918
  9 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  @JuneStar_2918 No, that data sheet is not wrong. Rather you are mistaken in only looking at the formula and ignoring the conditions the data sheet gives which bound when the formula is applicable. There may also be very slight difference between brands, both in the actual details and especially in how they are presented. Reading data sheets is a skill, generally when a manufacturer wants to provide something more tutorial in nature they create an Application Note or Reference Design, sometimes for novel parts there would be hints in a data sheet but not for something common like this.
  $endgroup$
  – Chris Stratton
  8 hours ago
  
  
  

  
  
  
  

add a comment | 

$begingroup$

All the LM317 can possibly do is act like a (controlled) variable resistor between the input and output, it cannot create energy out of thin air.

And it can't even do that, being a bipolar part it will have some significant voltage drop at best with even moderate currents (some CMOS-output LDO regulators have a few mV dropout at low currents such as a few mA).

In the case of the LM317 (which is not an LDO = Low Drop Out) part, there is a Darlington transistor pair between input and output, and the drive can't go all the way to the input voltage.

See this "representative" schematic from this datasheet:

In general you should have 3V or more between input and output, as specified under the "line regulation" parameter in the datasheet.

share|improve this answer

edited 9 hours ago

answered 9 hours ago

Spehro PefhanySpehro Pefhany

223k55 gold badges177177 silver badges467467 bronze badges

$endgroup$

share|improve this answer

edited 9 hours ago

answered 9 hours ago

Spehro PefhanySpehro Pefhany

223k55 gold badges177177 silver badges467467 bronze badges

answered 9 hours ago

Spehro PefhanySpehro Pefhany

223k55 gold badges177177 silver badges467467 bronze badges

answered 9 hours ago

Spehro PefhanySpehro Pefhany

223k55 gold badges177177 silver badges467467 bronze badges

2

$begingroup$

An LM317 is defined by many parameters, not just the equation you provide. All of the required operating conditions must be satisfied, and one of the those is the "minimum dropout voltage". This is the minimum difference between the input and output voltage, where the input voltage must be greater than the output voltage. The datasheet for your specific LM317 will provide that number.

Edit: For the Texas Instrument's version of the LM317, the important parameter is in section 6.3 of the datasheet.

share|improve this answer

edited 9 hours ago

answered 9 hours ago

Elliot AldersonElliot Alderson

11.9k22 gold badges1212 silver badges2525 bronze badges

$endgroup$

• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  I have edited my question based on you answer. Thank you.
  $endgroup$
  – JuneStar_2918
  9 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  Why does it say recommended? Is it saying "if you need 40V at its output, then you must supply 43-80V at its input"?
  $endgroup$
  – JuneStar_2918
  9 hours ago
  

  
  
  
  

add a comment | 

2

$begingroup$

An LM317 is defined by many parameters, not just the equation you provide. All of the required operating conditions must be satisfied, and one of the those is the "minimum dropout voltage". This is the minimum difference between the input and output voltage, where the input voltage must be greater than the output voltage. The datasheet for your specific LM317 will provide that number.

Edit: For the Texas Instrument's version of the LM317, the important parameter is in section 6.3 of the datasheet.

share|improve this answer

edited 9 hours ago

answered 9 hours ago

Elliot AldersonElliot Alderson

11.9k22 gold badges1212 silver badges2525 bronze badges

$endgroup$

• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  I have edited my question based on you answer. Thank you.
  $endgroup$
  – JuneStar_2918
  9 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  Why does it say recommended? Is it saying "if you need 40V at its output, then you must supply 43-80V at its input"?
  $endgroup$
  – JuneStar_2918
  9 hours ago
  

  
  
  
  

add a comment | 

$begingroup$

An LM317 is defined by many parameters, not just the equation you provide. All of the required operating conditions must be satisfied, and one of the those is the "minimum dropout voltage". This is the minimum difference between the input and output voltage, where the input voltage must be greater than the output voltage. The datasheet for your specific LM317 will provide that number.

Edit: For the Texas Instrument's version of the LM317, the important parameter is in section 6.3 of the datasheet.

share|improve this answer

edited 9 hours ago

answered 9 hours ago

Elliot AldersonElliot Alderson

11.9k22 gold badges1212 silver badges2525 bronze badges

$endgroup$

share|improve this answer

edited 9 hours ago

answered 9 hours ago

Elliot AldersonElliot Alderson

11.9k22 gold badges1212 silver badges2525 bronze badges

answered 9 hours ago

Elliot AldersonElliot Alderson

11.9k22 gold badges1212 silver badges2525 bronze badges

answered 9 hours ago

Elliot AldersonElliot Alderson

11.9k22 gold badges1212 silver badges2525 bronze badges