INA138: INA138

Bruce Doan

Prodigy 210 points

Part Number: INA138

Tool/software:

How would I extend this design to work with sources as high as 700VDC

1 month ago

0 Javier Contreras4 1 month ago

TI__Mastermind 32340 points

Hello Bruce,

This circuit design: https://www.ti.com/lit/an/sboa295a/sboa295a.pdf goes through the design process. I think the limiting factor for the voltage will be dominated by Q2 in image above which is Q1 in the design I have shared the link for.

The schematic that you attached also has a guide https://www.ti.com/lit/ug/tidu833/tidu833.pdf that mentions this as well in section 3.3 PNP Transistor

Regards,

Javier

0 Bruce Doan 1 month ago

Prodigy 210 points

It doesn’t say specifically how to increase the supply tolerance. I have seen stacked (series) transistor designs.

Or maybe just a higher voltage transistor.

any suggestions?

0 Javier Contreras4 1 month ago in reply to Bruce Doan

TI__Mastermind 32340 points

Bruce,

Basically the supply voltage is provided with the Zener voltage. The two components are Q1 and R3 in the image below that must tolerate your high to low voltage transition. I assume either a higher voltage transistor or stacked transistors could work but please keep in mind what current you lose through leakage and power on the R3 with maintaining a current across the zener to keep it active during your circuits full operation condition. I would suggest simulation to confirm. You can start with this: https://www.ti.com/tool/download/SGLC001

0 Bruce Doan 1 month ago in reply to Javier Contreras4

Prodigy 210 points

I understand the requirement for sufficient current through the zener.

I attempted TINA simulation but could not find the way to add the INA138 to the schematic

0 Javier Contreras4 1 month ago in reply to Bruce Doan

TI__Mastermind 32340 points

Bruce,

Did the link above I sent you not work. It has the INA138 in that tina simulation file. You could simply cut an paste into your existing design.

Regards,

Javier

0 Bruce Doan 1 month ago in reply to Javier Contreras4

Prodigy 210 points

Got the link, thank you.

Next question is , could this be modified to use a MOSFET instead of the PNP bipolar transistor?

0 Javier Contreras4 1 month ago in reply to Bruce Doan

TI__Mastermind 32340 points

Hello Bruce,

Yes it can. See section 3.3 https://www.ti.com/lit/ug/tidu833/tidu833.pdf. Below for convenience.

Regards,

Javier

0 Bruce Doan 1 month ago in reply to Javier Contreras4

Prodigy 210 points

I am not finding PMOS that will work properly.

Back to the PNP bipolar

Trying to stack 2 devices to get higher voltage breakdown

not getting proper biasing configuration

0 Javier Contreras4 1 month ago in reply to Bruce Doan

TI__Mastermind 32340 points

Bruce,

You just have a voltage divider and the current goes through the resistor rather than the transistors. Also please be aware of the power on the resistors.

0 Bruce Doan 1 month ago in reply to Javier Contreras4

Prodigy 210 points

One more thing.
If I want to protect the INA138 against large current overloads (such as a short circuited load) , can I use a series resistor and clamp diode across the input terminals?

0 Javier Contreras4 1 month ago in reply to Bruce Doan

TI__Mastermind 32340 points

Hello Bruce,

This can be done and we have some documentation (https://www.ti.com/lit/ug/tidu473/tidu473.pdf) on this but you need to be aware that the INA138 is floating relative to your main ground so placing the protection needs to be to GND of the device. All absolute max ratings need to be avoided and they are relative to the GND of the device. Keep in mind that resistance into the device will also cause some gain error. Some of our other devices we recommend under 10Ω in series because of this.

Most of our devices are more concerned of the voltage changes during a transient current or short circuit event rather than the current itself so keep that in mind as you design your protection for the INA138.

If you explain the potential event that you are protecting from I can see how that would impact the INA138. I do normally a transient simulation with the conditions and add parasitics expected in the system to get a rough idea of voltages and currents. I have seen this as a good tool to find what I will need to protect from and the speeds required.

Regards,

Javier

0 Bruce Doan 1 month ago in reply to Javier Contreras4

Prodigy 210 points

I was meaning limiting the differential voltage from Vin+ to Vin-

0 Javier Contreras4 1 month ago in reply to Bruce Doan

TI__Mastermind 32340 points

Hello Bruce,

This will be an issue as you are creating a resistive path from the Rshunt to VIN- creating a voltage drop. These devices are not exactly like amplifiers and have an IB in the µA range. This will cause

Also from your schematic are you assuming R1 is damaged and open? Would the R1 accept power of 4watts. 2V*2V*/1Ω? I say 2V as that is our Absolute Maximum Voltage rating differentially at our inputs. Most of the current will flow across R1 and I think that will get damaged first before the INA138.

Regards,

Javier

0 Bruce Doan 1 month ago in reply to Javier Contreras4

Prodigy 210 points

I intend to limit the current flow with a fast acting fuse in the source.
The sense resistor would only see a momentary high current (.25 W rated resistor)
My only concern is exceeding the maximum differential voltage on the INA138

Please advise

0 Javier Contreras4 1 month ago in reply to Bruce Doan

TI__Mastermind 32340 points

Bruce,

Can you just place two diodes in series to do this?

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INA138: INA138