1. You MAY be able to modulate the MC34063 Vout fast enough to not
need output PWM, depending on application.

2. If you drive the LM317 adj terminal with a voltage which is stiff
wrt the pot load then Vout will be 1.25V above the drive voltage. You
could drive it with a less stiff voltage and lift the pot ground end
with the same result. Getting the changeover timing right is a good
idea.

3. For interest, why are there two identical but parallel HV PWM channels?

4. If a pot is setting VPWM source then adjusting the 34063 output
seems MUCH wiser unless you hav an unusual application. ie at 1.25V
out you get about 96% of your smps power dissipated in the LM317.

5. Even as shown, shutting down the smps below 12V and using direct
feed seems liable to be useful.

6. Dropout of LM317 is such that you cannot guarantee to reach the Vmax sho=
wn.

7. Telling us what you are actually trying to do may result in (even)
better answers ;-)

8. You will have trouble reaching the high side current limit as shown :-)



On 21 March 2011 20:30, IVP <joecolquitt@clear.net.nz> wrote:
> Hi all,
>
> could I have suggestions for controlling an LM317 please
>
> In the attached diagram, the 10k pot is used to set the voltage
> of the HV PWM outputs
>
> What I need to add is a modulation control, which simulates
> the pot being rotated left and right. The pot will stay, modulation
> is just another mode
>
> The frequency will be 10Hz at most, sinusoidal. That's not a
> problem. Reading the LM317 datasheet, I see an example of
> a Power Follower, using an LM195, but I don't think it's going
> to be suitable
>
> IF I'm right assuming that ADJ can't be directly voltage-controlled
> (correct me if I'm wrong) then adding modulation will require either
> grounding the pot through a FET, so that it can be switched out, or
> paralleling it with some resistive element
>
> Such as -
>
> Digital pot
> PWM-ed 4066 switch (effective R =3D Ton/Toff)
> Linearly-driven FET
>
> The first two imply some additional logic, perhaps a 12F. The
> third seems simpler, if the FET could be is used as a resistor
> with R (simply) controlled between Ron and 10k ohms. If the
> pot is left in circuit, then R would be from Ron to Roff
>
> Any other ideas ?
>
> Joe
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