I need a circuit, when higher voltage is occured, it should normally conducts the voltage. But when the lower voltage is occured, it must boost or double the voltage. for example, if my input voltage is 15V, output voltage must be around 14V. if my input voltage is 3V, it should boost the voltage say about 6V. Is is possible? is there any component to do that? Please help

 

Yes you need to use a buck boost ic like the Ti range types, but you have to set the required out put voltage with a set of resistors. Here is one type..

http://www.linear.com/product/LTC1871

 

Thanks for the reply. I will look into the datasheet.

In my design, output is fixed and say it is about +5V. and the input voltage is varying, for some times it will be +20V and some times it will +3V. For all input voltage range, output must be fixed +5V. This ic can do this?

Well i have another query,
If my input varies from 200V to 3V, is there any ic or circuit which gives fixed +5V? Please help !!!

 

At this point in time, the chip that converts 3 volts to 5 volts and converts 200 volts to 5 volts is still in magic fairy land.

 

It is possible. But i don't know still how to make it possible. I have seen a power supply in my office lab. It does 230V to +14V and +3V to +10V...

 

The power supply in the office lab is not an "ic" and it does not make 5 volts out of 3 volts. That is what you asked about. If you wish to ask about a laboratory power supply, that is a different matter altogether.

 

Because of curiosity, i opened the power supply, it is just an ic and nothing apart from that. It looks like a mosfet (4 pin smd package). I surfed in the net, but still couldn't find that ic. I have applied 230V to one terminal, at the output im getting 15V. And applied 3V at the same terminal, at the output i got nearly 10V. How can a single component will act like buck boost without adding any other external components.

 

A four pin ic is connected to 230 volts, or 3 volts, and provides the adjustable, regulated output of a laboratory supply? You must post photos!

 

for example, if my input voltage is 15V, output voltage must be around 14V. if my input voltage is 3V, it should boost the voltage say about 6V

For all input voltage range, output must be fixed +5V.

So.... which is it? Should the output be fixed, or should it depend in some strange relationship on the input as described in your first post?

 

I don't want the output to get fixed. do u have a circuit which i mentioned in 1st post? so that i can add a regulator to make the output to get fixed 5V.

 

I am totally confused. You state that you don't want the output to get fixed and then say that you want to add a regulator to make it fixed. Which is it? What are the specifications for the circuit you want to build? Don't include anything that comes before it or after it, what specs does the circuit need to meet?

 

sorry about the confused post.. my circuit need to meet this specs, when input voltage varies from 3V to 230V, output voltage should be 5V.

 

Divide and conquer. No single circuit will do two orders of magnitude like this, you may need 3 or more separate circuits and your challenge is to figure out how many and where to draw the lines between them.