OPA189: Vf value of internal diode

Hi Liu,

an additional diode clamping is possible but you would have to be very careful not to ruin the low input bias currents of OPA189.

Can you show a schematic of your planned protection circuit?

Kai

Hi Tom,

Is there a specific application that you'd like to add external input protection circuit? 

You mentioned that "an external protection should be work faster than internal circuit", which it may not be true.  Typically, the internal protection diodes start to conduct earlier than normal external PN type of diodes due to the higher die temperature. Also, you may ruin the  low input bias current features in OPA189, as Kai indicated. 

Based on the datasheet, I'd say that internal protection diodes are more than adequate, see the attached image of the datasheet. If I am able to locate  additional performance data on the internal protection diodes, I will post it. 

Best,

Raymond

Hi Tom,Kai

Thank you for your reply.

I have attached our application.

15V or higher voltage(~50V) may be input when unintended switch error. (In normal case, INA will be unconnected  until measure system is stable.)

But, to keep low noise spec, Rin should be under 1kohm in our experiments. So, input current will be exceeded absolute max rate of input current.

This is why that we need external protection.

Best Regards,

Tahara

Hi Tahara,

I would do it this way:

Kai

Dear Kai-san,

Thank you for your reply.

I think the way you provided is good.

But I'd like to estimate the current(Vf(of internal diode) / 150ohm)  which flows input.

So could you please provide us detailed information of ESD protect circuit in OPA189.

Best Regards,

Tahara

Hi Hiroto,

in the worst case 50V drops across the 470R resistor. -> 50V / 470R = 106mA.

The voltage drop across BAV199 at a current of 106mA is less than 1.2V according to the datasheet of BAV199.

So, in the worst case 1.2V / 150R = 8mA is flowing into the OPA189, which is within the absolute maximum ratings of OPA189. But in reality this current is much smaller because also across the internal ESD cell a voltage is dropping.

You can increase the 150R resistors of protection scheme up to the allowed noise level.

Kai

Dear Kai-san,

I think, If internal diode of OPA189 is ON faster than external one, 106mA current will flow into the OPA input in a moment.

Is this my misunderstanding?

Best Regards,

Hiroto Tahara

Hello Kai,

Thank you for your prompt reply.

I am working together with Hiroto-san as distributor FAE.

Initially Hiroto-san wanted to know internal ESD diode characteristics to verify reliability of external Diode protection circuit workaround.

Instead of that, should we simulate input current is below absolute maximum rating?

Is this better way for system circuit design review rather than verifying ESD diode IV characteristics itself?

Regards,

Mochizuki

Hi Hiroto, hi Mochi,

the OPA189 can withstand a 4kV ESD (human body model). This means that the ESD cells can handle a current of more than 1A for a period of 30ns with a peak current of 2.7A. So, even if the BAV199 does not turn-on immediately during your 50V overvoltage event but needs some nanoseconds, the OPA189 should not be in danger.

What is the maximum leakage current across the shunt (left of the 470R resistors) during normal operation you can allow? Maybe you can use a BAV99 instead of BAV199? Or even a Schottky diode?

Kai