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Switching pre regulator

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OM222O:
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?

Vovk_Z:
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.

Vovk_Z:
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.

hugo:
TP0610 is a p channel mosfet and 2N7002 a n channel one ... you could be wrong  ;)

Vovk_Z:
Oh, yes. It was BSS92.

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