OPA2202: Input current

Note (3) talks about these diodes; note (4) (which you have quoted) talks about the diodes to the supply rails.

This calculation is correct for differential input voltages.

HI Nishie,

The internal ESD structures of the OPA2202 may start turning on around 0.55V, where the forward voltage of the diodes varies with temperature.

As Clemens has mentioned, the input current calculation you show above divided by 100Ω + 100Ω = 200Ω is correct to protect for a differential overvoltage condition, where the input differential voltage must not exceed ±0.5V during a fault, OR the current to the inputs must be limited to less than ±10mA.

Also, as mention in the Absolute Maximum Ratings table, the input terminals are diode-clamped to the power-supply rails. Therefore, input signals that swing more than 0.5 V beyond the supply rails (common-mode voltage) must be also current-limited to 10 mA or less.  In this case, the input current calculation is divided by a single 100Ω resistor.

If in the application, the device will be submitted to conditions that exceed the absolute maximum ratings, a common practice is to use external clamping circuit.  This is typically accomplished using series resistors to limit current or a combination of series resistors, external Schottky diodes on the op-amp input to stir the current, and TVS diodes on the op-amp supplies.

In the example below, a clamp circuit is designed to provide input protection for a common-mode  ±40-V continuous overvoltage fault. The Schottky diodes, used here on the op amp inputs, have a metal semiconductor junction that offers a lower forward voltage drop than the silicon junction diodes, such as those internal diodes used for ESD protection in the op amp. The Figure below details how this attribute of the external protection clamp circuit works in conjunction with those internal ESD diodes. The current during the ±40-V fault is limited to 20 mA using the 1.24-kΩ, 1/2-W R_LIMIT resistor, as shown in Figure below.

In this example, the BAS40 is a small-signal Schottky diode with a forward voltage close to ~380 mV at 1 mA. In comparison, the internal ESD structure has a forward voltage somewhere around ~550 mV at the same forward current. Therefore, the Schottky diodes turn on before the amplifier’s internal ESD diodes, and most of the in-rush current flows through the external clamp. The internal ESD structure can only withstand 10 mA, while the external Schottky diode can handle forward continuous currents up to 200 mA, providing strong protection.

The transient voltage suppression (TVS) diodes are used to clamp the power rails, sinking the clamp circuit current to keep the supplies below the op amp’s ±20-V absolute supply rating. TVS diodes are like Zener diodes, but designed for fast, large transient power dissipation. The SMF12A shown is a unidirectional TVS with a reverse standoff voltage of 12 V, a breakdown voltage of 14.7 V and a maximum clamping voltage of 19.9 V. 

Please review the TI Precision Lab Session explaining overstress or EOS protection:

https://www.ti.com/video/series/precision-labs/ti-precision-labs-op-amps.html

To find the session on EOS protection, look into "Topics" and browse the bottom of the page, you will find 4-sessions on op-amp:

Here is also a short article that discusses the op-amp input protection using Schottky diodes protecting an op-amp.

https://www.ednasia.com/provide-robust-input-overvoltage-protection-for-amplifier-analog-input-modules/

Let us know if you have any questions,

Thank you and Regards,

Luis Chioye

HI Nishie-san,

Yuta Nishie said:

1) If the current is 10 mA or less, how much Voltage can the potential difference between the + and negative terminals be input? I would like to know the information that it can withstand 0.5V to xV.

2) Also, when a potential difference of 0.5V or more is input  (less than 10mA) , is there a regulation on the input time?

If the device is exposed to differential input voltages above ±0.5V rating, or common-mode voltages that swing more than 0.5V above or below the supply rails, the designer must limit the current to less than 10mA at all times.

When limiting the current with either an external clamp, or external series resistors, by default, the voltage seen by the internal ESD structures is also indirectly limited to a safe level, since the voltage drop occurs across the external protection resistors. Also, when the input current flowing through the op-amp inputs is kept low, well below the ±10mA, there is not enough power to cause damage.  If the device is expected to be exposed to an overvoltage condition in the application, I recommend keeping the current through the inputs below the ±10mA rating with some margin.  As shown on the TI Precision Lab tutorials, limit the current using external series resistors OR by stirring the current outside the device, implementing a clamp with a combination of series resistors and Schottky diodes. 

Thank you and Regards,

Luis