SBOA583 December   2023 OPA205 , OPA206 , OPA210 , OPA2206 , OPA2210 , OPA2392 , OPA2828 , OPA320 , OPA328 , OPA365 , OPA392 , OPA397 , OPA828

 

  1.   1
  2.   Abstract
  3. 1 Introduction
  4. 2 Circuit Configuration Impact on Common-Mode Range
  5. 3 Practical Input Limitations
  6. 4 Input Phase Reversal (Inversion)
  7. 5 Common-Mode Limitations Inside Bipolar Amplifiers
  8. 6 Common-Mode Limitations Inside CMOS Amplifiers
  9. 7 Rail-to-Rail CMOS Amplifiers
  10. 8 Output Swing Limitations Inside a Bipolar Op Amp
  11. 9 Linearity of Output Swing Specifications
  12. 10Output Voltage Swing vs Output Current
  13. 11Classic Bipolar vs Rail-to-Rail Output Stage for CMOS and Bipolar
  14. 12Rail-to-Rail Output and Open-Loop Gain Dependence
  15. 13Output Short-Circuit Protection
  16. 14Overload Recovery
  17. 15Supply Current During Input and Output Swing Limitations
  18. 16Summary
  19. 17References

13 Output Short-Circuit Protection

The vast majority of op amps have an output short-circuit protection circuit. This circuit limits the output current to protect the device when the output of the op amp is shorted to ground. In fact, the output can be shorted to any voltage between the power supply and the circuit protects the device from damage. Shorting the op-amp output to a voltage outside the supply limits turns on the output ESD structures and if unprotected, damages the amplifier. The short circuit limit can be seen on the output voltage versus output current curve as an abrupt drop in output voltage when the limit is exceeded. Note that the short-circuit current is generally designed to have a negative temperature coefficient so that the device does not go into thermal runaway(1). For example, if the short-circuit limit is tripped at room temperature, the device self-heats, but the current limit decreases as the temperature increases to reach equilibrium. Figure 13-1 shows how short-circuit limit decreases with increasing temperature. For example, the positive short-circuit limit is 67 mA at 25°C and 60 mA at 85°C. The figure also illustrates that the short-circuit limit is not necessarily symmetrical depending on the output voltage of the amplifier because different types of transistors drive the positive and negative output swing. For example, the positive short circuit limit at 25°C is approximately 67 mA, but the negative is 55 mA.

GUID-20231005-SS0I-XVHN-NN7H-MMFS3NKJXX6Q-low.svg Figure 13-1 Short-Circuit Limit vs Temperature for OPA392

Figure 13-2 illustrates a simplified op-amp output short-circuit protection. Q1 and Q4 are the classic bipolar output stage. Transistors Q2 and Q3 and the output current sense resistors R1 and R2 form the short-circuit protection. For example, assume the output is driving a positive voltage, and is shorted to ground. Doing this causes a large current to flow through Q1. This current develops a drop on R1 that turns on Q2. Turning on Q2 steals base current from Q1 which decreases the output of Q1. This effectively limits the output current by turning down the output drive transistor. The same phenomena occurs on Q3 and Q4 if the amplifier were grounded when driving a negative output.

GUID-20230929-SS0I-GW5T-SGLM-H1FZ9GVGWNB3-low.svg Figure 13-2 Short-Circuit Protection Inside Bipolar Amplifier
1.

Thermal runaway occurs when an increase in device temperature causes an internal change in device operation which further increases the device temperature. This condition leads to device damage. This problem is not a concern on modern amplifiers. Thermal runaway is mentioned here to explain the temperature coefficient for short-circuit current.