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IC Design Advice needed on current sense amps

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electronrancher

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All the current sense amps I've seen compare top rail to a sense voltage, created by a small series resistor. I would like to ask if anyone has or has seen a new circuit in a power supply book or something that is worth trying. Can you scan me the page, or scan a quick handwritten schematic?

I could probably adapt a circuit made from 2n3904 or something, so if you guys have a nice sense amp in descretes, please let me take a look at those also!

Other than that, I am not looking for datasheets of IC's (ha - unless you are the designer, or have the internal schematics!) or website addresses except maybe IEEE papers or something..

Moderators: Too bad there's not an IC Design forum!! hint hint..
 

goodboy_pl

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Please be more precise!
What is the range of the current?
How much it must be accurate?
DC or AC?
the application?

May be I can help u!!!

BEST!
 

electronrancher

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the circuit will be DC, as far as this amp is concerned. i will probably build it on a 20v process, but that does not matter as far as the circuit is concerned - i could build it on 80v process and just change mosfet models...

Setting the range of the current will change depending on user selection, but i have already designed that part.

Basically, I am sensing current limit across an NMOS. I can use sense transitors if I want to get current output instead of voltage output, but layout is a little trickier..

The smallest IC will have about 100mA ILIM, biggest part will be up to 3-5A.. This is no big deal, we are just sensing a few mV in either case. Accuracy should be about 0.1% of the range. (like 1mA on 1A Ilimit, 100uA on 100mA Ilimit).

What approach do you think?
 

goodboy_pl

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I think u want to measure the current of a switching mosfet when it is ON to limit it by turning it OFF.
if so there are 3 solution:
1. use external presiion resistor to sense the current.
it requires extra pin also a 1-2 watt presision resistor. also some glitches will be introduced to the current due to PCB and resistor parasitic LC.
2.sense MOSFETs with kelvin points to mirror a small fraction of current for sensing. normaly this method is used for very high current switches like 40A and upper but maybe useful for Integrated design specially to use very good device matching in IC.
3.sensing Vds of MOS : this method requires external ultrafst diode to prevent IC failure. also it is not a presision method but sufficient for ILIM application. International Rectifier has used this method in several MOSGATE Drivers. for changing its current/voltage range an external reference is necessary!
Hope I inswered your question.

BEST!
 

electronrancher

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hmm, all those techniques are possibilities but i was asking for circuit implementation. like for example, using a 100mOhm sense resistor gives 100mV sense voltage at 1 amp. This is ok, except that top of resistor is 5v Vcc rail, and bottom of resistor is 4.9v. How to measure 100mV off top rail when Vdsat of a PMOS is 80mV? Vcc powers the amp and acts as noninverting input - now we are horribly trioded!

It is kind of tricky situation, actually.. Which is why I submit for review by you guys!
 

Btrend

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I had made a current sense circuit without sensing resistor in IC.
This method is to add a sensing MOS that have the same voltages at
D,G,S,B with the swithching mos but it's size is 1/N of the latter one.
If matching layout is done, this current sense will give error only from
the matching issue.
 

bastos4321

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Some time ago I have used a sence resistor and a switch capacitor amplifier to cancel the offset of the amplifier and get the signal from the resistor. The switchs need to work in the high side. I use clock boost to make it working.

Bastos
 

sherazi

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hi to all;

i want to design a current overload sensor for a 250Vac load its a motor, thats driven by micro controller via triac (optocoupled via moc) ...

now i want to use a shunt current overload sensor ic to be used (if it can be) or another solution ... so that i can sense motor stuck that makes overload and hence should send motor a signal that makes the mc to stop the motor

Added after 53 minutes:

well i use ASC712 hall effect sensor instead? or should iuse any other hall effect sensor? can any one give me some sugestions?
 

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