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How would you make a linear regulator's output voltage dependant on its input voltage

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si2030

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Hi there,

This is causing me a bit of grief. I have a linear regulator.. in this instance its the LT1083. I have a pre-regulator using a TL494. I can alter the voltage of the regulator by adjusting the pulse width from ~30volts to ~1.5 volts.

The linear regulator is downwind of the switching converter. If I change the voltage on the converter say from 20volts to say 10volts.. how can I force the LT1083 to output a voltage ~1.5volts lower than any voltage it sees at its input??

Effectively making the LT1083 a slave to its input voltage....

There must be a simple way to bias the ADJ pin based on the the input voltage?

I would appreciate any and all comments on this... hoping for someone to help here.


Kind regards

Simon
 
A combination of resistors and a zener diode would probably do the trick. Can you post a schematic of your setup?
 
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Swap the linear regulator for two diodes and you will get an output voltage 1.4V lower than the input. Is that close enough?

Mike.
 
Will this do? The 1083 output is about 1.6V lower than its input.
 
Hi there,

I dont have a schematic as yet as I am building this on a breadboard... I have the TL494 working at 45kHz and this will feed into a transformer. I can adjust the voltage out of this anywhere from the raw output of the transformer to pretty much zero and then add a filter capacitor to smooth the output. I was intending to add a linear regulator after this section turning the TL494 and transformer into a pre-regulator.

My problem is that if I make the linear regulator adjustable then the pre-regulator has to follow the linear regulators voltage level........ or...... I can make the pre-regulator manually adjustable and this then leaves me with the linear regulator tracking the output voltage of the switching transformer.

Why the linear regulator... to reduce the noise (and ripple) the swtiching converter has.... (is this a wrong assumption?) By combining the two I should get the best of both worlds.. higher efficiency and stable low noise output....

Since the LT1083 is a LDO reg I can afford to track the input voltage resonably closely to the output voltage and manage the voltage adjustment at the pre-regulator stage.... wrong... open to ideas here. :)
 
Alec I think thats it... Could you help me by describing how it works in the context of the ADJ of the regulator.... and could you add the asc file... I did try and build it but I cannot find a model for the BZX79C2V7 zener diode... wondering where you got that?
 
With the circuit simulated the 1083 works internally such that its output sits at ~1.2V above the ADJ pin voltage, which in turn is ~2.7V below the input voltage. Hence the output voltage is ~ 2.7-1.2 = 1.5V below the input.
I couldn't find a model for the BZX79C2V7 either, so I cheated and modified the model for another BZX diode in the standard.dio file. If you want to try that (make a backup copy of standard.dio first!) I've attached the .model text and the asc file.
 
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Alec, you should not alter LTSpice core files like that, you'll lose the changes next time it updates and all you have to do to add your own model is put a .model spice line into the schematic, simply press S to add a new spiceline, it also makes the model portable because it's built into the ASC file so no external library is needed.
 
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Keep in mind, doing this puts the linear regulator at the mercy of the zener diode, so you'll have to add the zener diodes variation with supply voltage (which will be small) to the linear regulators line regulation as far as noise goes.
 
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Alec, you should not alter LTSpice core files like that, you'll lose the changes next time it updates and all you have to do to add your own model is put a .model spice line into the schematic, simply press S to add a new spiceline, it also makes the model portable because it's built into the ASC file so no external library is needed.
Thanks for the advice Sceadwian.
 
Hi Sceadwian,

Wondering if you might clarify for me the comment

"..add the zener diodes variation with supply voltage (which wil be small) to the linear regulators line regulation as far as noise goes"

Are you suggesting that the linear regulator will simply follow the ripple of the input voltage thus cancelling out the intention of reducing noise... if so whats the solution to this... make the linear reg adjustable and force the pre - reg to follow at say ~1.5 volts higher?
 
The ripple has to be at high frequency that is easily fixed with a couple of capacitors.

Maybe you need to state your problem better as your solution is confusing.

Mike.
 
No it won't follow the input voltage, it will follow the zener voltage which will vary slightly depending on the input voltage, so you need to calculate what the zener voltage will be at minimum voltage and maximum voltage (the zener current will vary with voltage and the 2k resistor) and add that to your noise figures. Keep in mind as well, as soon as the SMPS voltage drops bellow the zener voltage the regulator will shut down completely as once the zener stops conducting the ref pin will be connected directly to ground via the 2k with no other refrence source
 
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Are you suggesting that the linear regulator will simply follow the ripple of the input voltage thus cancelling out the intention of reducing noise..
No it won't follow the input voltage, it will follow the zener voltage
I've re-run the sim and the output does indeed follow the ripple (which I hadn't previously considered). So it's back to the drawing-board :(.
 
Alec, we might want to find out from si2030 what his end goals are before you scrap the design. Pommie was perfectly correct in asking what he's trying to do rather than how he thinks it should be done.

Suggestions were made based on incomplete information, if we want solutions we need design parameters.
 
Hi,


Yes, the zener diode is a good idea but the linear will also track the input noise to some degree, so a little filtering is in order here too. A resistor in series with the zener (not too high of a value) followed by some capacitance to ground will probably do it, and the zener voltage high enough to surpass any noise on the input. Maybe 2v would be better.
However, this is NOT the correct way to build a switching pre regulator power supply.

Normally the way this is done is the switcher is made to track the linear, not the other way around. This way when the linear is adjusted the switcher tracks it. That's how the circuit i posted back a while works and that's the typical way of doing it.

The problem with making the linear track the switcher (this new circuit) is that the output is no longer regulated with load. As the load increases the switcher output loads more and more which causes a drop in voltage at the output of the switcher. This in turn makes the output of the linear drop too, as much as the switcher plus some more because of its own change in internal impedance. A load that draws a pulsed current for example will cause the output voltage of the linear to bob up and down. It's not as bad as no regulation, but it's not as good as the other way around.

When the switcher tracks the linear (as more typical of this kind of dual regulator) the switcher keeps up with the linear so the output stays constant with load, and the linear is the sole regulator so the output is well regulated.

The way to simulate this new circuit would be to use a DC input voltage with a small series resistance and use a spice model of the linear regulator and use the zener or whatever. Load the regulator WITHOUT changing the input DC voltage and see what happens. The change in internal impedance of the linear regulator will cause a change in output voltage which is more than if it was really allowed to regulate.
There is probably a way to correct this by using the linear regulator as usual but bias it differently, but what's the point if there is a simpler way to do it with a transistor and zener to make the switcher track the linear.
For example, for a fixed output a zener from the output of the switcher to the bottom of the lower resistor of the linear regulator might do it. That way the linear still regulates, but it's still not as good.
 
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Good points MrAl. Can you point the OP at your circuit?
 
Hi alec,

Oh yeah, i guess i posted it in the other thread that was talking about switching pre regulators and linears.

The circuit below shown with the LM2576 switcher chip works with any switcher that requires a voltage feedback from the output normally. The transistor takes control and maintains a near constant voltage across the linear. It doesnt have to be super regulated just always higher than the minimum that the regulator requires. The linear actually does all the regulating just as it normally would.
The set point difference voltage is the sum of the emitter base voltage plus the zener voltage.
Im sure there are alternates for the low voltage zener too such as some series si diodes.
Output current limiting can be added in ways that are typical for the three terminal linear.
 

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  • PowerSupply-w-Sw-PreReg-03.GIF
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I dont have a schematic as yet as I am building this on a breadboard...
You have it back assward. :rolleyes: You draw the schematic, then build the circuit, even if it is just a breadboard.
 
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