LM317 PWM Output is not coming for Higher Amplitudes of PWM Signal

Thread Starter

hoyyoth

Joined Mar 21, 2020
70
Dear Team,

I am building an LM317 PWM Circuit. Please see the figure below.
1622120045301.png

My output is coming properly when VG1 output up to 3V when VG1 goes higher there is no output. Please see the output below.

1622120199543.png

1622120237716.png

I am attaching my simulation file along with this. please let me know where I went wrong.
 

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crutschow

Joined Mar 14, 2008
27,190
The LM317 is a voltage regulator with a low frequency response, designed to generate a DC output voltage.
Why are you using it to try to generate PWM?
 

Thread Starter

hoyyoth

Joined Mar 21, 2020
70
The LM317 is a voltage regulator with a low frequency response, designed to generate a DC output voltage.
Why are you using it to try to generate PWM?
Hi ,
I need to generate a variable output using LM317.In real board PWM will be coming from a microcontroller board.
Regards
HARI
 

Irving

Joined Jan 30, 2016
1,497
Hi ,
I need to generate a variable output using LM317.In real board PWM will be coming from a microcontroller board.
Regards
HARI
Then LM317 is the wrong device to use. ADJ input is a sample of output voltage for regulation purposes, not a voltage control input.

Now tell us what your requirement is, and maybe we can give a better suggestion.
 

Papabravo

Joined Feb 24, 2006
16,107
I think what you may be trying to do is use a Voltage Regulator as a Power Amplifier, because the LM741 does not have ANY substantial output Drive capability. If that is the case, I can tell you that it is a profoundly pointless approach to the problem. If you would like to have a suggestion or two on how to create a suitable PWM signal with appropriate voltage and current levels just state the requirement and we can help.
 

Papabravo

Joined Feb 24, 2006
16,107
@Irving How do you adjust the output voltage from a LM317?
To the TS: You sample the output voltage and use a fixed resistor divider to put the reference voltage of 1.25V ((?) consult datasheet for actual value), below the output voltage, on the adjust pin. What you have done in your misguided attempt to use the part in a way it was not designed for is caused it to detect a severe overvoltage on the output and shut the regulator down. It is doing just exactly what it is supposed to do!!
 
Last edited:

Irving

Joined Jan 30, 2016
1,497
@Irving How do you adjust the output voltage from a LM317?
As @Papabravo says, ADJ is held 1.25v below desired output but that has to be sampled from the output to keep it closed loop for regulation.

Now, there are ways to do this with an LM317 but they aren't straightforward. Also, from the TS setup he/she appears to be confused between PWM as a switching control signal in an SMPS, versus its use as a pseudo analog (DAC) output which is what I think was intended. In which case there are issues with switching noise/ripple to resolve.
 

Papabravo

Joined Feb 24, 2006
16,107
The TS really needs to clarify his intentions. It is absolutely pointless to speculate about what he is trying to do. We want to help, but can't without more information.
 

Thread Starter

hoyyoth

Joined Mar 21, 2020
70
Then LM317 is the wrong device to use. ADJ input is a sample of output voltage for regulation purposes, not a voltage control input.

Now tell us what your requirement is, and maybe we can give a better suggestion.
Hi Irving,

My intention is to control LM317 as given like this .EDN ARTICLE .
This is used to make a variable output regulator.Another Article

Regards
HARI
 

Papabravo

Joined Feb 24, 2006
16,107
Hello,

Have a look at the following article:
https://www.radiolocman.com/shem/schematics.html?di=98904

Bertus
This is exactly why a coherent statement of intention is so crucial to understanding what the TS was trying to do. Notice that in the TS's original circuit there is no DC path to ground for Vout. The circuit in the referenced article has the DC path to ground and is using the PWM signal to adjust the value of the output voltage. This intent was certainly not clear to me from his original post. Had the TS started with this circuit his intent would have been more clear.

I admit that my interpretation of what the TS was trying to do may have been flat out wrong, but we really won't know until he tells us.
EDIT: I posted just after he told us.
 

Papabravo

Joined Feb 24, 2006
16,107
Hi Irving,

My intention is to control LM317 as given like this .EDN ARTICLE .
This is used to make a variable output regulator.Another Article

Regards
HARI
So why didn't you implement the circuit from the EDN article, which is identical to the one Bertus posted, instead of the one you posted?
The one in the second article uses a different opamp (LM358) and does not show the power connections, and may or may not be operating in single supply mode. The drawback of the second circuit as I see it is the lack of a DC path to ground for Vout.
 
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