OPA2170: clamp diode

SIDDHARTH MOHAN

Part Number: OPA2170

Hi All , can anyone please let me know when does the internal clamp diode start conducting 10ma? as per datasheet the inputs should not exceed vcc by more than 0.5v so does this apply o the input pins after the internal 2.5K resistor on both internal input pins ?

thanks

over 5 years ago

0 Paul Goedeke over 5 years ago

TI__Mastermind 30600 points

Hi Siddharth,

The datasheet values for the maximum input voltage and current apply for the device as designed, which includes the 2.5k input resistors found in Figure 38. It is difficult to provide a consistent point when 10mA conducted current will occur - once the diodes are turned on, the amount of current flowing can escalate quickly.

You can use additional series resistance to further limit the current into the input pins if you are at risk of exceeding VCC by 0.5V or more in your application. But of course, any conditions outside the absolute maximum table isn't guaranteed and the device may perform in unexpected ways or be damaged.

-Paul

0 Ron Michallick over 5 years ago

TI__Guru*** 140026 points

Siddharth

Those input resistors might be 250 ohms each. I found data that said 'input to input' that data showed one diode plus 400 ohms.
The absolute maximum is 10mA, I would recommend limiting to 1mA. Any current flow to VCC has to be consumed by something.

0 SIDDHARTH MOHAN over 5 years ago in reply to Ron Michallick

Genius 3205 points

thanks for the reply Ron, the block diagram that i see shows 2.5K,may i know where is 250ohms coming from?
Also can you please elaborate upon "I found data that said 'input to input' that data showed one diode plus 400 ohms."

0 kai klaas69 over 5 years ago in reply to SIDDHARTH MOHAN

Guru 140200 points

Hi Siddharth,

why so complicated? Datasheet clearly says: No more than 0.5V above the rails. This is meant directly between the input pins and the supply pins and not from any point within the internal circuitry. And, of course, it's not meant behind the 2k5 resistor.

The usual maximum voltage drop across such a protection diode would be 0.3V, by the way. So, very probably, the internal 2k5 protection resistors make that you are allowed to apply an input voltage which is even 0.5V above the rails instead of 0.3V.

Kai

0 SIDDHARTH MOHAN over 5 years ago in reply to kai klaas69

Genius 3205 points

thanks for the reply Kai, but if we apply any input 0.5v above rail then these diodes won't conduct 10ma due to the large 2.5K resistors in series.. The 10ma or even 1ma current rating will require input to be much higher than the rail so how is this 10ma rating being justified ?

0 kai klaas69 over 5 years ago in reply to SIDDHARTH MOHAN

Guru 140200 points

Hi Siddharth,

the justification is just to give a very simple rule of thumb of how to deal with overvoltages at the inputs. A rule of thumb which covers the worst case and offers reserves. It's not the intention to explain in detail what current flows at what input overvoltage.

By the way, if you look carefully at figure 38 of datasheet you will notice that there are also other protection diodes which do not profit from the internal protection resistors.

Again: if your input voltage isn't more than 0.5V above the rails everything is fine. But if your input voltage can exceed the rails by more than 0.5V, then install an external current limiting resistor and limit the current to less than 10mA to prevent damage.

Kai

0 Ron Michallick over 5 years ago in reply to kai klaas69

TI__Guru*** 140026 points

Siddharth,

The input resistors are close to 2.5 k ohms. The data sheet goes into great detail including why 10mA was chosen. See section 8.3.3 Electrical Overstress on pages 19 to 21.

There is an additional reason for 10mA maximum. If VCC was 36V and one input was 0V and the other 36V, then input current would be just under 36V/(2*2.5k ohm) = 7.2mA , at abs max 40V it would just under 8mA.

0 SIDDHARTH MOHAN over 5 years ago in reply to Ron Michallick

Genius 3205 points

Hi All,

Do you provide any thermal models for the IC’s?

Also Can I get some idea on which tool is the thermal simulation of these IC’s done at the package and pcb level ?

regards

0 Paul Goedeke over 5 years ago in reply to SIDDHARTH MOHAN

TI__Mastermind 30600 points

Hi Siddharth,

We do not offer any thermal models for the devices at this point. The necessary Theta JC and other values are provided in the datasheet, table 7.5. If there is additional information that you need that is not provided, please let us know, we can request additional information from our thermal modeling team.

If you are concerned about the way device parameters shift with temperature, some typical plots are usually provided for reference (offset vs temp, Ib vs temp, CMRR vs temp) and give a good sense of how much change to expect. For other parameters, we usually just give guidelines of 30-50% change over process, variation, and temp.

For some more information on thermal caluclations, I recommend taking a look at our TI precision labs video on this topic:
training.ti.com/ti-precision-labs-op-amps-power-and-temperature

Regarding the tool we use, I'll have to check with the modeling team and see.

Best,
Paul

0 SIDDHARTH MOHAN over 5 years ago in reply to Paul Goedeke

Genius 3205 points

thanks of the reply Paul, its fine if the thermal models are not available. But would be helpful to know which tool is used at TI for thermal simulation of the IC at the device,package and pcb level. I use cadence sigrity PowerDC, Sigirty thermal. Has any of these been tried?

regards

0 kai klaas69 over 5 years ago in reply to SIDDHARTH MOHAN

Guru 140200 points

Hi Siddharth,

what temperatures are we talking about?

Kai

0 Paul Goedeke over 5 years ago in reply to SIDDHARTH MOHAN

TI__Mastermind 30600 points

HI Siddharth,

Any thermal modeling tool should work well, I can't comment specifically on what TI uses. I do recommend that you take a look at our Semiconductor and IC package Modeling app note - it covers relevant processes and standards for determining the parameters.

www.ti.com/.../spra953c.pdf

Best,
Paul

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OPA2170: clamp diode