OPA547: How much ESD current can pass into the output of this amplifier?

Hi Rick,

Most parts have a 10mA limit on their inputs and outputs due to limits on internal ESD protection diodes.  How much momentary current can the output of this amp take with a strke like that?

Yes, the maximum current via ESD protection diodes at input and output pins shall be limited to +/-10mA, when ESD diodes are turned on. The maximum current is tailored for ESD ratings, which ESD strike duration is typically within a couple hundred nsec range. However, these EDS protection diodes are unable to withstand longer duration of ESD current >10mA, which these ESD diodes are unable to absorb the amount of energy to clamp an overvoltage and limiting current in the worst case events described above.  

I prefer just a single TVS diode to ground instead of putting that ESD current into the rail, but that could go as high as 24V, or 900mA. 

Understood. The OPA547's output pin has to be protected against overvoltage and overcurrent and/or output pin that is short to a higher output voltage as mentioned above. 

If you are able to share the schematic with us, I will see if there is a simpler way to protect the worst case scenario with the minimum amount of external components. 

Enclosed is an application note about power amplifier's overvoltage protection scheme used in a resolver application, which it may be used as a reference in a similar scheme in OPA547. 

https://www.ti.com/lit/an/sboa447/sboa447.pdf?ts=1628195400076&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FALM2403-Q1

Best,

Raymond

This is the basic idea of what I have.  I've stripped out a bunch of supporting circuitry.

Hi Rich,

What application is this? Is this a 10V voltage regulator?

If Vp node under the worst case scenario is able to clamp an overvoltage at approx. 15V range and limited the output current (< +/-500mA), the Safety of Operating Area indicates that the part's Tcase will be ok.

Enclosed is Tina simulation above.  

OPA547 Overvoltage Protection 08052021.TSC

Best,

Raymond

Hi Raymond,

It is a 10V regulator of sorts.  It's a precision excitation supply for a sensor.  I have this amplifier buried inside the feedback loop of a precision amplifier that takes its feedback from the connection point of the sensor at the end of a cable.  This design is essentially already working in an internal device what we use, but I want to make it more bulletproof for another application.  After looking at the existing ESD protection a little closer, I think I can do a better job protecting the output.  Anyway, to your point on the data sheet, I assumed that was what the output limits would be with the amplifier output stage driving the output under normal conditions.  With an ESD event, it happens so quickly that I doubt the amplifier output would be able to respond and sink (or source) the current required.  Therefore, I didn't think the output current rating would apply all that much.  One other thing.  Maybe it is best that I put the current into the supply rail.  In the case where this unit is powered down, an ESD event would produce currents that are even higher.

Hi Rick,

With an ESD event, it happens so quickly that I doubt the amplifier output would be able to respond and sink (or source) the current required. 

ESD protection has two functions. 

1. To protect a part from ESD damages from human or machine handlings when a part is out of a circuit. 

2. When the part is in a circuit, it will redirect excessive current to power rails when there is an overvoltage occurred at an output or input pins, but the amount of current that can go through ESD diode is limited to +/-10mA. 

When an ESD strikes occurred near the vicinity of in circuit components, the ESD pulse energy is typically finding a closest ground path (like a device ground or chassis) and discharge it immediately. The ESD energy should not find a path through the inner components of a device. This is typically tested and verified via ESD test for a device. 

(There is another class of test which is called pin injection tests or lightning inducted transient susceptibility test. The pin injection is typically performed directly onto a pin of a device or cable bundle tests, which can be found via DO-160 test procedures for Airborne Equipment. I do not think your system is applicable in this case). 

Your overvoltage protection on an output pin is fault condition test with time duration in sec to minute range. If OPA549's output voltage is clamped to Vcc of the OPA549's power rail and ESD diode current is limited within +/-10mA, in case that ESD current steering diode is turned on momentarily, the part will be safe without damage. Of course, it is good to have adequate design margins to cover the entire operating temperature range. 

If you have additional questions, please let me know. 

Best,

Raymond