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Design >> Analog Design >> how to design this protect circuit
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Message started by circuit_cook on Nov 3^rd, 2009, 12:00pm

Title: how to design this protect circuit
Post by circuit_cook on Nov 3^rd, 2009, 12:00pm

Hi,
I need to design a protect circuit for a node, which has 2kOhm resistor connected to output of a -2v to 5.5v opamp output. I only have 3v devices to design this -2v to 5.5v protection, and I can't pull this node down.
Anyone has some good idea.

Judy

Title: Re: how to design this protect circuit
Post by loose-electron on Nov 4^th, 2009, 1:18pm

The question is a little confusing - perhaps posting a picture to better define the problem better would be a good idea.

There are methods of stacking devices in series to get higher voltages, but I am not yet sure what you are trying to do.

Title: Re: how to design this protect circuit
Post by circuit_cook on Nov 5^th, 2009, 9:32am

Hi,

Here is what I want to achieve, a classAB driving a 16Ohm load with variable gain setting from -30 db to 4dB by feedback resistance. there is a charge pump on chip, when it is enabled, the opamp power supply is (vdd=5.5 and vss=-2, ouput common mode is 0v), when it is disabled, the opamp power supply is (vdd=5.5 and vss=0, output common mode 1.5. All I have is 3.3v device, the output switch SW1 and SW2 is realized with cascode transistor, but how to protecte the switch S1, S2 etc. I can't pull A, B, C, D to 0, when it is open loop.
Another question is there any limiation for this architecture, with pragrammable gain comined classAB, driving a 16Ohm load, the minimum feedback resistance is 2K in the lowest setting.

Judy

Title: Re: how to design this protect circuit
Post by loose-electron on Nov 5^th, 2009, 9:48am

some basics -

fitrst, driving into 16 ohms is going to make for some very big transistors.

You want a gain that for the most part, is an attenuator.

Have you consider just using a resistor divider to do most of this?

Still a little unclear on the "protect" circuit you are trying to achieve.

Title: Re: how to design this protect circuit
Post by circuit_cook on Nov 5^th, 2009, 12:14pm

Hi,

Thanks for the reply, the gain is from -30db to 4dB, so most part it is attenuator.
What do you mean with resistor divider, for which part, right now the gain is set by selecting different tap in a resistor ladder.
the protection is for switch S1, S2 in the tap, I need to put some fancy voltage on the gate, so it won't exceed the 3.6v stress. I wonder what is the best way to protect them from the -2 to 5.5v.

Judy

Title: Re: how to design this protect circuit
Post by loose-electron on Nov 6^th, 2009, 10:29am

is s string of diodes suitable?

Title: Re: how to design this protect circuit
Post by Mayank on Nov 6^th, 2009, 11:26am

Hello,
Will some CDM structures help ?? as in the ones used for standard ESD Protection.

--Mayank

Title: Re: how to design this protect circuit
Post by circuit_cook on Nov 6^th, 2009, 3:00pm

Hi,

Thank you for the above two replies. I will give it some more thoughts. I thought about using diode too, the tricky thing is the negative supply could be 0 or -2.

-Julie