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Trying to build a supply with pwm dac, no 0 though

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Moriambar:
Hello,
insipred by the uSupply I am trying to build a 0-10V supply, just for fun.
My idea is to use, in this phase, an LT3080, but to regulate voltage I want to use a pwm dac. Why? Just to experiment. don't need precision.
The schematic I'm using is in [attach=1] (Please excuse the crudity of this model. I didn't have time to build it to scale or paint it)

So I hooked up my arb with a sqw (tried 1 to 100kHz, all worked) 5V pp (with 2.5V dc offset) and tried the output. All is good until the lower bound: once I reach 1.7V the output gets "stuck" meaning that no matter how much I reduce the pwm duty cycle (or even hooking the input to GND) the output does not get lower than 1.7V. The arb is on high impedance (not 50R) and I actually don't know what's wrong…

One other thing I notice is that sometimes when booting up the circuit, if I'm not careful and don't switch the pwm first, the output is 9V and no matter the pwm that does not change.
So I know I'm doing many things wrong, can you help me plz?

EDIT: prettied a bit the attachment

drdm:
Check the voltage on the SET pin.
I don't think your amp is rail-to-rail, so even if you apply 0% PWM at the input, the output of the amp may not reach 0V.

Moriambar:

--- Quote from: drdm on December 03, 2021, 01:28:45 pm ---Check the voltage on the SET pin.
I don't think your amp is rail-to-rail, so even if you apply 0% PWM at the input, the output of the amp may not reach 0V.

--- End quote ---
Ok I'll try later, but I used a 358 just as Dave uses on his uSupply schematic (although he uses a real dac for the set pin).
it should go down to 0 iirc, perhaps not going to the upper rail but I think it should go down to 0.
I'll check for good measure though, maybe there's something there
thanks

Kim Christensen:
Like drdm said, the LM358's output won't go all the way to ground. It'll get close, but not all the way to zero, depending on how much current it is trying to sink.
Your other problem will be that the LM3080 needs a minimum 1ma load. Quote from datasheet:

--- Quote ---A minimum load current of 1mA is required to maintain
regulation regardless of output voltage. For true zero
voltage output operation, this 1mA load current must be
returned to a negative supply voltage.
--- End quote ---

The LM334Z in your circuit is set for apx 0.64ma with a 100 \$\Omega\$ resistor. The resistor should be less than 64 \$\Omega\$ so it sinks 1ma or more. But the other problem with the LM334Z is that it won't maintain current regulation at 1ma with less than apx 1V across it. So that'll be another problem that will crop up with that circuit.

Moriambar:
Thanks. I'm actually curious on how Dave's design could work then down to 0V…
What is the magic sauce that makes that possible?
ps: btw the lm358 only goes down to about 1V with "pwm" input hooked straight to gnd

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