Author Topic: Switching pre regulator  (Read 1315 times)

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Offline OM222OTopic starter

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Switching pre regulator
« on: July 23, 2020, 09:30:39 am »
I have always used just a battery and a LDO in applications where noise is inportant but obviously battery and LDO means really awful battery life and low effeciency overall.

I would like to implement a switching pre regulator , followed by a filter (ferrites and capacitors) and then the LDO. I have searched quite a bit on google but didn't find anything other than control the output of the regulator with something like a dac or digital pot but that's not ideal.

Is there a way to have the output of the LDO in the feedback loop of the pre regulator so adjusting the output of the LDO, automatically adjusts the switching regulator?
 

Offline Vovk_Z

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Re: Switching pre regulator
« Reply #1 on: July 23, 2020, 12:24:31 pm »
Yes, look at  LT3080 datasheet, p.21, 'Adjustable High Efficiency Regulator'.
You need only a Mosfet, and that's it. You use resistive feedback divider circuit of a DC/DC 'prereg' as it is, and just add a Mosfet Drain-Sourse in parallel to this feedback divider of a DC/DC regulator, and gate with series resistor to the linear (LDO) regulator output.
I used this way to make a XL4016 as pre-regulator for LM350/LM338. Works fine.
« Last Edit: July 23, 2020, 12:31:50 pm by Vovk_Z »
 

Offline Vovk_Z

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Re: Switching pre regulator
« Reply #2 on: July 23, 2020, 12:36:11 pm »
If I'm not wrong I used usual 2N7002 Mosfet, it gives about 2.5-4 V voltage drop. So for low-drop LDO regulator you may need some logic-level (low-level) Mosfet.
 
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Offline hugo

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Re: Switching pre regulator
« Reply #3 on: July 23, 2020, 03:58:58 pm »
TP0610 is a p channel mosfet and 2N7002 a n channel one ... you could be wrong  ;)
 

Offline Vovk_Z

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Re: Switching pre regulator
« Reply #4 on: July 23, 2020, 04:17:52 pm »
Oh, yes. It was BSS92.
 

Offline David Hess

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Re: Switching pre regulator
« Reply #5 on: July 23, 2020, 11:31:56 pm »
Is there a way to have the output of the LDO in the feedback loop of the pre regulator so adjusting the output of the LDO, automatically adjusts the switching regulator?

Yes, that can be easily done.  However first I suggest reading the Linear Technology application notes written by Jim Williams about low noise switching regulators:

https://www.analog.com/media/en/technical-documentation/application-notes/an70.pdf
https://www.analog.com/media/en/technical-documentation/application-notes/an101f.pdf
https://www.analog.com/media/en/technical-documentation/application-notes/AN118fb.pdf
 

Offline Pawelr98

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Re: Switching pre regulator
« Reply #6 on: July 24, 2020, 01:20:27 am »
Classy TL494 can do that because the error amplifiers can be tied to anything you want.
With a bootstrap circuit (just like the LT part has) and a proper driver it should be able to drive mosfet switch and provide decent efficiency.
 

Offline Marco

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Re: Switching pre regulator
« Reply #7 on: July 24, 2020, 05:49:18 pm »
Why not a PNP to have a little easier to predict drop?
 


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