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ysc New Member Offline Posts: 2	over voltage protection Aug 14^th, 2006, 7:48pm Can some body give me ideas to design a circuit that takes an input voltage from 2.8v to 5.25 volts but limits the output to less than or equal to 4.0v. The current drawn is of concern too. any help is appreciated.
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ACWWong Community Fellow Offline Posts: 539 Oxford, UK	Re: over voltage protection Reply #1 - Aug 15^th, 2006, 2:28am what is the power supply voltage ?
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ysc New Member Offline Posts: 2	Re: over voltage protection Reply #2 - Aug 15^th, 2006, 6:04am The power supply voltage can vary from 2.8v to 5.25v. The output of this circuit should be less than or equal to 4v. In other words this circuit does not do anything for input voltages less than 4.0v. when the input voltage crosses 4.0v this limits the output voltage to 4.0v .
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ACWWong

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Posts: 539
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Re: over voltage protection
Reply #3 - Aug 15^th, 2006, 6:52am

ysc wrote on Aug 15^th, 2006, 6:04am:

The power supply voltage can vary from 2.8v to 5.25v. The output of this circuit should be less than or equal to 4v. In other words this circuit does not do anything for input voltages less than 4.0v. when the input voltage crosses 4.0v �this limits the output voltage to 4.0v .

I'm confused, as your last post contradicts your previous post...

anyway I think what you need is a regulated output supply of 4V. This means for supply voltages 4V to 5.5V you get a limited output at 4V max. For supply voltages 2.8V to 4V, the output is limited at the supply voltage.
You can design a low drop-out regulator using a banggap refence voltage and multplying it up with an oaamp and an output PMOs and resistive divider. Google voltage regulator or LDO for info.

I hope this answers what you want.

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loose-electron Senior Fellow Offline Best Design Tool = Capable Designers Posts: 1638 San Diego California	Re: over voltage protection Reply #4 - Aug 15^th, 2006, 9:20am Agreed with the prior post - you want an LDO regulator. A conventional voltage regulator architecture won't get it done. http://www.national.com/pf/LP/LP3997.html The above is one example, there are many more out there. Take a look at the internal block diagram of the device to get the idea. hope that helps, Jerry
Back to top	Jerry Twomey www.effectiveelectrons.com Read My Electronic Design Column Here Contract IC-PCB-System Design - Analog, Mixed Signal, RF & Medical IP Logged