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KrisBlueNZ

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Because of those corrections, the proper value for R1 is about 0.54Ω. I suggest you use four 2.2Ω 0.5W 1% metal film resistors in parallel, which will give you 0.55Ω, and connect another resistance in parallel to drop that down to 0.54Ω. The calculated value is 29.7Ω and 30Ω would be close enough, or use a 50Ω trimpot and a very accurate low ohms meter (using a four wire measuring method) to adjust the trimpot to give the right value.

I don't think there's any point being that accurate, because the figures I used in my calculations are all just typical parameters, not exact values.

Did you read and understand the section of post #13 that I quoted in post #19?
 

Sravanthi

Feb 4, 2015
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Because of those corrections, the proper value for R1 is about 0.54Ω. I suggest you use four 2.2Ω 0.5W 1% metal film resistors in parallel, which will give you 0.55Ω, and connect another resistance in parallel to drop that down to 0.54Ω. The calculated value is 29.7Ω and 30Ω would be close enough, or use a 50Ω trimpot and a very accurate low ohms meter (using a four wire measuring method) to adjust the trimpot to give the right value.

I don't think there's any point being that accurate, because the figures I used in my calculations are all just typical parameters, not exact values.

Did you read and understand the section of post #13 that I quoted in post #19?


Yes, I understood.Thank you for your reply and your suggestion.

Can you give any suggestion for this question.
Is there any diode with 0.2 to 0.3V as forward drop.(This is not related anything with above discussion regarding clamping the voltage).
 

KrisBlueNZ

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Yes, Schottky diodes can have forward voltages that low, at lowish currents. Germanium diodes have low forward voltages as well. Both Schottky and germanium diodes have quite high leakage currents, and the "knee" in their VF vs. IF graphs may not be as sharp as the knee in the graph for a standard silicon diode.
 

Sravanthi

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I am using LMP90100. Here VIO pin voltage range is -0.3V to 6V. So to protect from higher than 6V I connected TVS diode in reverse bias.
So now to protect from less than -0.3V what can I do?

Can you tell particular number(either schottky or germanium diode )with these requirements.
 

KrisBlueNZ

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Show us a schematic.

Can you connect the diode in series with the power source, instead of reverse parallel?
 

Sravanthi

Feb 4, 2015
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LMP90100 VIO is the input pin. To this pin I have connected hall sensor output reading (Which gives current output So I connected a resistor and voltage across it is given to LMP90100).LMP90100 has internal ADC which gives digital reading.
VIO input range -0.3V to 5.3V.So I have connected a TVS diode to clamp higher than 5.2V voltages.
Now to protect less than -0.3V what can I do?
 

KrisBlueNZ

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It's not ideal to pull any input pin of any IC beyond the supply rails, but the IC will have internal clamping diodes, and provided you don't feed too much current into the pin, it is normally acceptable.

There's a bit of a clue to this in note 4 below the absolute maximum ratings table. It says "When the input voltage (VIN) exceeds the power supply (VIN < GND or VIN > VA), the current at that pin must be limited to 5mA and VIN has to be within the Absolute Maximum Rating for that pin." So if you have an external diode to clamp the voltage to (say) -0.8V~+5.2V then add a series resistor between that clamp and the input pin. The maximum voltage drop across the resistor will be about 0.8V so for 5 mA current the resistor should be about 160Ω. Use 180Ω.
 

Sravanthi

Feb 4, 2015
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It's not ideal to pull any input pin of any IC beyond the supply rails, but the IC will have internal clamping diodes, and provided you don't feed too much current into the pin, it is normally acceptable.

There's a bit of a clue to this in note 4 below the absolute maximum ratings table. It says "When the input voltage (VIN) exceeds the power supply (VIN < GND or VIN > VA), the current at that pin must be limited to 5mA and VIN has to be within the Absolute Maximum Rating for that pin." So if you have an external diode to clamp the voltage to (say) -0.8V~+5.2V then add a series resistor between that clamp and the input pin. The maximum voltage drop across the resistor will be about 0.8V so for 5 mA current the resistor should be about 160Ω. Use 180Ω.

Hi...

Didn't get what you are saying...
I have added TVS diode for >5.2V protection(VA is 5V in my design).So now can I add a another diode to protect <-0.3V.
Please send circuit what you are saying...
 

KrisBlueNZ

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Tvs.gif
Just connect a resistor between your TVS and the IC's input pin. I suggest 180Ω. This will not clamp the voltage but it will limit the current that can flow into the IC to a level that will not damage it.
 

Sravanthi

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Did you read what I wrote?
That's fine what you are saving I understood.
To limit the current we have to provide the resistor.
If VIN < GND or VIN > VA we have to provide some provision to bring into absolute ratings.
For Vin>VA protection I have provided TVS diode.Similarly to protect VIN < GND what should be connected I am asking.
 

KrisBlueNZ

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You need to use a TVS diode that conducts when forward-biased. These are called unidirectional TVS diodes. In that case it will clamp the signal to about -0.8V before the resistor. If you're using an SMP3V3 as you showed in post #26 (but without the 38 ohm resistor, which you didn't explain) then that should be OK.

I strongly suspect that you will have problems with your design because you don't seem to understand what you're doing, and you're not describing your project fully here so that I can pick up problems for you. I don't know whether you think it's some kind of secret, or what. But you are really wasting a lot of time by asking narrow questions and giving minimal information. I'm about to give up trying to help you.

Normally I have a lot of patience but you are really wasting a lot of time. I think it might be someone else's turn to bang their head against a wall for you.
 

Sravanthi

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You need to use a TVS diode that conducts when forward-biased. These are called unidirectional TVS diodes. In that case it will clamp the signal to about -0.8V before the resistor. If you're using an SMP3V3 as you showed in post #26 (but without the 38 ohm resistor, which you didn't explain) then that should be OK.

I strongly suspect that you will have problems with your design because you don't seem to understand what you're doing, and you're not describing your project fully here so that I can pick up problems for you. I don't know whether you think it's some kind of secret, or what. But you are really wasting a lot of time by asking narrow questions and giving minimal information. I'm about to give up trying to help you.
Normally I have a lot of patience but you are really wasting a lot of time. I think it might be someone else's turn to bang their head against a wall for you.

Sorry for in convenience. Because of some constraints I am not able to give the full schematic.
Thank you for your suggestions.
 
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