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LM317 control by PWM

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iMo:
This is an LTspice rewrite of my LM317 PWM (see my link above).
How the output looks like when an R_LOAD is switched on/off, I=15mA/500mA, Vout=15V.
Still you may see some PWM feed-through at the output.
Simulation only.
Use at your own risk.. :)

Zero999:

--- Quote from: imo on February 01, 2019, 04:44:04 pm ---Not sure -15V rail is necessary, I would use an LM358 (or similar) with single 32.5V source.

--- End quote ---

--- Quote from: SiliconWizard on February 01, 2019, 06:21:25 pm ---I would get rid of the -15V supply and select an appropriate opamp. An LM358 would kinda work, but it doesn't have nulling offset pins (if that matters, since you'd probably get only a few mV offset), and whereas its output can get down to almost ground, it won't get high enough - due to the current it will have to sink - to get your full output voltage range here IMO.

--- End quote ---
You've got that backwards. The LM358 won't do without a negative supply because it can't sink enough current with a low enough saturation voltage for 1.25V out. The higher end of the voltage range is no problem.

I've done a quick search and there are no common, cheap op-amps, which can sink the minimum load current required by the LM317, with a low saturation voltage.

If a single supply is a requirement, then I'd add a current sink to the LM317's output and remove the resistor.

Here's how I'd do it. Q1 is a constant current sink which takes the LM317's minimum load.

U2 is configured as a Sallen-Key filter with a cut-off of 10Hz, a damping factor of 1.4 and a gain of 5.75. If a dual op-amp is being used, it might be better to separate the filter and gain stage, but I just wanted to demonstrate it can be done.

I cheated by using a calculator tool to select the component values, see the link below:
http://sim.okawa-denshi.jp/en/OPstool.php

Yansi:
So yet another reasons why NOT to use a damn 317!

//also you sure it will be stable at gain of almost 6?  I have had bad trouble making sallen-key filters to not oscillate above gain of 3.

Zero999:
Well I wouldn't agree with you about the LM317: the current limiting, thermal, safe operating area protection are very good reasons tio use it, in spite of the other minor inconveniences it causes.

Regarding the Salen-Key, you're probably right. I've not tried to design one with this much gain before. It would be a good idea to move the gain stage to after the filter.

David Hess:

--- Quote from: Zero999 on February 01, 2019, 09:39:08 pm ---I've done a quick search and there are no common, cheap op-amps, which can sink the minimum load current required by the LM317, with a low saturation voltage.
--- End quote ---

If the saturation voltage matters, then a different circuit topology should be used.  The LM317 is not suitable for outputs below 1.25 volts unless a negative control voltage and a minimum load current with a negative compliance is available but often this restriction is acceptable.

For performance and precision reasons, the minimum load or quiescent current of the integrated regulation should be buffered from the output of the operational amplifier with a transistor and old designs which use the LM317 or other integrated regulator do this.

Both of these problems are solved if the minimum load current is provided without the resistor between the output and adjustment pin of the LM317 despite how convenient it was.  Now no buffering is required and the load current on the operational amplifier is stable and low allowing for low saturation voltages.  But to operate below 1.25 volts, this still requires a negative supply which might as well be used to remove the saturation requirements on the operational amplifier also.


--- Quote from: Yansi on February 01, 2019, 06:03:36 pm ---One can protect for over-current and SOA with a single transistor. Even implement a fold-back current limit.
--- End quote ---

And thermal protection?  I really try to avoid adding discrete temperature sensors if I can avoid it.

It does not apply to this circuit but current boosted designs using the LM317 to drive a power transistor make the LM317 even more useful that just a current buffer because with a little attention to thermal resistance, the thermal protection of the LM317 can be extended to cover the power transistors.


--- Quote ---I see zero reason for LM317.   Current drive argument is irrelevant,  the opamp does not care if 10uA or 10mA needs to be supplied.
--- End quote ---

Precision applications (which this is not yet) require the operational amplifier's output current to be controlled and even better, low.  This makes the LM317 with the minimum load current returned to another point especially useful for simplified precision power applications.

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