Dropping voltage from 8V to 5 V and 5.5 V

I have a source of 8V and I need drop it between 5V and 5.5V. Any suggestion? I don't want use diodes to do it, and I don't want use a 5V regulator ...

leomecma

Re: Voltage drop

If you don't want to use diodes or voltage regulators then the last idea would be a shunt resistor and a parallel regulator ..
I think any switchmode step-down converter, or even unwanted by you 5V regulator, would be much better option ........
Regards,
IanP

Re: Voltage drop

so, you want to create a regulated rail that sits between 5 and 5.5V but you don't want a regulator?

Re: Voltage drop

You dont want to use diodes or you dont want to use a regulator. Why...? Thats what regulators are for. I assume you dont want to use any transistors or are transistors OK.

The following circuit link contains one 1N4001 diode and 4 transistors and a small handfull of components...

h**p://www.rason.org/Projects/discreg/discreg.htm

Alot of regulator circuits "as I am sure you know" use diodes as a voltage reference. I find your question interesting and would like to know what components you are prepared to use IE. resistors, capacitors, inductors, ?Transistors?, IC's, and how many of each. Is it a project, Do you have a specific budget or do you just want to learn about regulators from the ground up.

Re: Voltage drop

I need it because my project as limited resources (costs) then I'm preveting to use a 5V regulator because I need to drop this voltage only because I'll use a 3V3 LDO regulator and it supports only max 6V input ... diode works well, but isn't an elegant solution (3 or 4 diodes).... then I'm searching a component like a diode, with more drop ... zener is dificult to use because I need a current about 750mA, transistor maybe, but isn't exactly that I'm searching .... I think don't exist other options ...


leomecma

Re: Voltage drop

How about you change the regulator you are using to another one, that allows higher input voltage?
After all, you have 4.7V of headroom, so you do not need an LDO to get 3.3V. Any standard regulator would work in this case. An LM317, for example.
And it is likely to be cheaper than the LDO. And since you are set on using a linear solution, the total power loss (huge, I might add) is going to be the same.

Re: Voltage drop

I think that the most efficient solution is a DC-DC converter. You can get free samples from TI (for example). A good DC-DC will supply 3V3 from near any input voltage.

Re: Voltage drop

I have a DC-DC converter, but it's 8V. And I need LDO because it has 1V8, 3V3 and BROWN OUT in same package, for use and LM317 or LM1117 I need use 2 and an external BROWN OUT, for this reason I need drop the voltage for 5 - 5.5V because this IC supports 6V max, and I don't need a accurancy 5V voltage.

leomecma

Re: Voltage drop

Can the 8V be changed to 5V? Or is it used somewhere else?

Re: Voltage drop

It can't change because I need supply a thermal printer with 8V, exactly this..

leomecma

Re: Voltage drop

Well, then if the power loss is not an issue, you can try a transistor plus a Zener, building a very crude regulator. A 4.7V Zener will give you about 5.3V output.

Voltage drop

Well if drop has to be introduced, it could be done in 2 way :
- about 2 Watts has to be disspated somehow (zener ldo resistors ...)
- or voltage must be transformed DC DC convertor with minimum loss .

If you can't change your input and output constraints, there is nothing more than mentioned above 2 methods .

Added after 11 minutes:

Make a switching capasitor DC-DC converter :
- design comparator with hysteresis
Vh = 5,5 Vl= 5 , put its output to FET, so when Voltage is below 5 - FET must be opened and if above 5,5 - closed.
- Connect one side of FET serially with resistor (0,5 Ohm) to 8 V and another to capasitor about 1000 uF.
- Comparators input should control voltage on capasitor above .

Well , i dont know whether this switching capasitor DC-DC converter will satisfy objectives, but at least you can try to test it .

OR

redesign your DC - DC converter to provide 2 voltages

Re: Voltage drop

use 7805 regulator