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pre-regulator for LDO linear regulator

can anyone explain how the pre-regulator circuit with a pmos threshold setting the differential voltage across a linear regulator works?

the situation is you have a dc/dc switcher going to a linear regulator, and you use a pmos to keep your dropout voltage at a minimum set by the pmos threshold voltage

I saw this thing on the eevblog video, link below

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and it's on the LT3080 datasheet pg 20

here's what I think is going on,

The DC/DC is set to a high output voltage, it keeps going up until it meets the dropout voltage criteria of the linear, at that point the linear will begin outputting, this creates a voltage across Vgs that turns on the pmos, which causes a short that feeds the output of the switching supply straight to its feedback pin... that's about as far as I can go.... after that I think the voltage output of the switcher is dropping.... but it must never drop below the linear's ldo voltage cause the linear never turns off.... i'm lost at this point

much thanks!

Reply to
panfilero
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PMOS?

PMOS? I see a PNP.

As soon as the input to the LDO exceeds a Vbe above the output potential, current flows into the emitter of the PNP and then the resistors connected to VFB, keeping the switcher output from rising.

...Jim Thompson

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| James E.Thompson, CTO                            |    mens     |
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Reply to
Jim Thompson

once the source voltage exceeds the gate voltage by the threshold voltage the PMOS mosfet starts to conduct reducing the set point of the switching regulators.

linears will often output a lower poorly regulated voltage if the input voltage is too low,

The PMOS doesn't turn all the way on, it doesn't becpome a dead short. it only conducts nough on enough that the switcher's output stops increasing.

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Reply to
Jasen Betts

There is a happy medium in VGS somewhere between full on and full off where= the PMOS conducts just enough to produce a voltage at the switcher feedbac= k input equal to the switcher's internal reference ( for practical purposes= ). At that point the feedback network internal to the siwtcher is satisfied= and makes its output lock steady. It stops calling for any further change = in its output, increase or decrease, it locks the output steady.

Reply to
bloggs.fredbloggs.fred

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