Author Topic: What adjustable linear regulator to use?  (Read 2627 times)

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Offline LukeWTopic starter

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What adjustable linear regulator to use?
« on: March 17, 2019, 07:30:45 am »

Are there any good candidates for a "jellybean" stock part as a more modern replacement for the 317?

In particular, I'd like an adjustable linear regulator to meet the following requirements - any recommendations? Thanks!

500 mA max load

0-5V out

"Rail-to-rail" output voltage, all the way from 0 to 5V.

Adjustment with a manual 10k pot, ideally output voltage should be linear with respect to pot position.
 

Offline awallin

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Re: What adjustable linear regulator to use?
« Reply #1 on: March 17, 2019, 07:55:47 am »
you'll get as many part-suggestions as replies to this one  :-//

my go-to LDO lately has been LT1963.. but I'm sure someone will quickly inform us about a cheaper and better part!?
https://www.analog.com/media/en/technical-documentation/data-sheets/1963aff.pdf
 

Offline coromonadalix

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Re: What adjustable linear regulator to use?
« Reply #2 on: March 17, 2019, 11:42:22 am »
The lm317 has 2.5v dropout

LD1086  LT1083  LT1086  they have 1.5v dropout

LT1085  has 1v

LM1117  has 1.2v

But many of them (3 legged to220)  wont go at zero untill you null out the adjustment pot with a negative voltage,  like the lm723  ....


Ultra Low Dropout LDO regulators 
https://www.st.com/en/power-management/ultra-low-dropout-ldo-regulators.html?querycriteria=productId=SS1733

 

Offline bd139

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Re: What adjustable linear regulator to use?
« Reply #3 on: March 17, 2019, 01:36:49 pm »
LT3080. The thing is the bee's knees!

https://www.analog.com/media/en/technical-documentation/data-sheets/3080fc.pdf

1. Max 1A out
2. Current referenced so goes down to 0V
3. 350mV dropout
4. Single resistor programming
5. Stable with small output cap

Look at the application notes under "lab power supply". That sounds like what you need.

They cost 10x what an LM317 does however in low quantities.
 

Offline Zero999

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Re: What adjustable linear regulator to use?
« Reply #4 on: March 17, 2019, 05:09:01 pm »
Most regulators don't work down to 0V, without a negative power supply. The LT3080 has a minimum load current requirement which means a current sink is required for it to work down to 0V with light loads. Unfortunately such a current sink would require a negative supply to get a true 0V out, although a current mirror does a reasonably good job of getting close without a negative supply.
 

Offline bd139

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Re: What adjustable linear regulator to use?
« Reply #5 on: March 17, 2019, 05:18:21 pm »
Good point. I've never run my LT3080 board without a load.
 

Offline LukeWTopic starter

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Re: What adjustable linear regulator to use?
« Reply #6 on: March 18, 2019, 02:04:56 am »
The LT3080 does seem a little expensive (well duh it's LT) but it seems really simple and easy, quality regulation - better than setting up extra parts and a negative rail converter on an LM317 to make it go down to zero, which will be nearly as expensive and not as good.

Here's a challenge.

The program current on the LT3080 is 10 uA.
Therefore, to go from 0-5V you would have a 500k \$\Omega\$ variable resistor.

But what if the pot was already committed, already glued to the devices, absolutely can't be changed and it's a 10k \$\Omega\$ pot?

Is there any nice way that we could set 0-5V using a pot which must be 10k?
 

Offline soldar

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Re: What adjustable linear regulator to use?
« Reply #7 on: March 18, 2019, 07:34:35 am »
The program current on the LT3080 is 10 uA.
Therefore, to go from 0-5V you would have a 500k \$\Omega\$ variable resistor.

But what if the pot was already committed, already glued to the devices, absolutely can't be changed and it's a 10k \$\Omega\$ pot?

Is there any nice way that we could set 0-5V using a pot which must be 10k?

The answer to your question is right there in the application notes.
All my posts are made with 100% recycled electrons and bare traces of grey matter.
 

Offline Bassman59

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Re: What adjustable linear regulator to use?
« Reply #8 on: March 18, 2019, 07:19:41 pm »
LT3080 is pretty great. Just mind the Vc control voltage, which has to be something like 1.5 V above the output. So unless you have a separate higher-voltage supply it can’t really be used as an LDO, even though Vin to Vout min is 300 mV or thereabouts. (Numbers from memory, I don’t have the data sheet right here.)
 

Offline LukeWTopic starter

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Re: What adjustable linear regulator to use?
« Reply #9 on: March 19, 2019, 02:25:44 pm »
Thanks for pointing out that datasheet schematic I overlooked.

R1 and R2 need to be quite small in order to get as close to 0 as possible when Rset is 0.
Let's say R1 is 3.3k and R2 is 68 ohms. RSet is 0-10k.

That should give us Vout = (3.3k*10uA) + (10k*10uA)*(1+(3300/68)) = 4.99 volts at the 10k pot maximum.
If I've calculated that correctly. Can you confirm that is right?
 


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