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LM317 digital control
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spec:
Below is yet another schematic for a DAC controlled LM317 PSU which is scaled for 0V to 3V input to 0V to 30V output. Although the output goes down to 0V, a separate negative power supply is not required.

The scaling can easily be changed according to the formula Vout = Vin * (1+ R4/R8)

The three diodes must be 1N540x types and should be mounted in cool free air for heat dissipation.

The schematic is an outline, so decoupling compactors, frequency compensation (if required), etc are not shown.

Also there is no optimization, or fault analysis, and there is probably an issue if the LM317 goes into constant current, but nothing that can't be sorted.
SiliconWizard:
Technically, you're still generating a negative supply.
(It's going to make a nice heater...  ;D )

You can probably get away with a higher value for R2, so the opamp will have to sink a bit less current.

As a remark, we can see that whereas the opamp sinks ~7mA when the LM317 is not limiting current, when it starts limiting current, depending on your input voltage, the opamp could start sourcing current instead of sinking, and depending on the input/output voltage/and how much current the opamp is able to source, it could get pretty hot in this situation.


spec:

--- Quote from: SiliconWizard on February 04, 2019, 06:04:29 pm ---Technically, you're still generating a negative supply.
(It's going to make a nice heater...  ;D )
--- End quote ---
Very profound ^-^ The heat generated by the diodes, will be insignificant compared to the heat generated by the LM317 itself, not to mention the heat generated by the other four diodes in the bridge rectifier and the load itself.


--- Quote from: SiliconWizard on February 04, 2019, 06:04:29 pm ---You can probably get away with a higher value for R2, so the opamp will have to sink a bit less current.
--- End quote ---
The opposite is true. From the LM317 data sheet (link below), the minimum output current for an LM317 is 10mA, so R2 should be 125R maximum rather than 180R. A practical worst-case opamp dissipation would then be 36V x 10mA = 360mW, which is hardly a problem, especially if you use a decent opamp. Anyway, there are probably simple methods of reducing the opamp dissipation if absolutely necessary.


--- Quote from: SiliconWizard on February 04, 2019, 06:04:29 pm ---As a remark, we can see that whereas the opamp sinks ~7mA when the LM317 is not limiting current, when it starts limiting current, depending on your input voltage, the opamp could start sourcing current instead of sinking, and depending on the input/output voltage/and how much current the opamp is able to source, it could get pretty hot in this situation.
--- End quote ---
I already said it was an outline schematic and there could be an issue if the LM317 went into constant current. It is not exactly a difficult problem to sort though: a Schottky diode may do the job. The purpose of the outline schematic is illustrate the principle of the design: it is the architecture that counts.

http://www.ti.com/lit/ds/slvs044x/slvs044x.pdf
iMo:

--- Quote from: spec on February 04, 2019, 10:22:50 pm ---
--- Quote from: SiliconWizard on February 04, 2019, 06:04:29 pm ---You can probably get away with a higher value for R2, so the opamp will have to sink a bit less current.
--- End quote ---
The opposite is true. From the LM317 data sheet (link below), the minimum output current for an LM317 is 10mA, so R2 should be 125R maximum rather than 180R. A practical worst-case opamp dissipation would be 36V x 10mA =360mW, which is hardly a problem. Anyway, there are probably simple methods of reducing the opamp dissipation if required.

--- End quote ---
You may have 317' 10mA min current and 50uA opamp current (the ADJ current).
spec:

--- Quote from: SiliconWizard on February 04, 2019, 06:04:29 pm ---Technically, you're still generating a negative supply.
(It's going to make a nice heater...  ;D )
--- End quote ---
Very profound ^-^ The heat generated by the diodes, will be insignificant compared to the heat generated by the LM317 itself, not to mention the heat generated by the other four diodes in the bridge rectifier and the load itself. And then there would be the heat generated by the user: 100W. :-DD


--- Quote from: SiliconWizard on February 04, 2019, 06:04:29 pm ---You can probably get away with a higher value for R2, so the opamp will have to sink a bit less current.
--- End quote ---
The opposite is true. From the LM317 data sheet (link below), the minimum output current for an LM317 is 10mA, so R2 should be 125R maximum rather than 180R. A practical worst-case opamp dissipation would then be 36V x 10mA = 360mW, which is hardly a problem, especially if you use a decent opamp. Anyway, there are probably simple methods of reducing the opamp dissipation if absolutely necessary.

Or are you suggesting that the LM317 output has current drawn off it by another means, so that R2 can be an open circuit, when the tail current would be only 50uA or so.


--- Quote from: SiliconWizard on February 04, 2019, 06:04:29 pm ---As a remark, we can see that whereas the opamp sinks ~7mA when the LM317 is not limiting current, when it starts limiting current, depending on your input voltage, the opamp could start sourcing current instead of sinking, and depending on the input/output voltage/and how much current the opamp is able to source, it could get pretty hot in this situation.
--- End quote ---
I already said it was an outline schematic and there could be an issue if the LM317 went into constant current. It is not exactly a difficult problem to sort though: a Schottky diode may do the job. The purpose of the outline schematic is to illustrate the principle of the design: it is the architecture that counts.

http://www.ti.com/lit/ds/slvs044x/slvs044x.pdf
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