• State Resolved
  • Locked Locked
  • Replies 5 replies
  • Subscribers 51 subscribers
  • Views 189 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

OPA548: Current limit turn off time

Azfer Hasni
Prodigy 10 points
Part Number: OPA548

The OPA548 has a current limit feature. The datasheet does not specify the turn off time for the part if the current limit is exceeded. Please let me know the turn off time if the current limit is exceeded. 

  • 0
    TI__Mastermind 47716 points

    Hi Azfer,

    Please let me know the turn off time if the current limit is exceeded. 

    The information below is shown that the shut down state when I_E/S pin is Low (output is disable) or current limit is exceeded.  

    Internally, the OPA548 E/S pin is the base of a PNP transistor and a 15kΩ resistor that pull up to an internal bus that is biased at a voltage higher than the 0.8 V, or 2.4 V levels. The PNP transistor is ON when the input level is low, and OFF when the input level is high. Hence, the small increase in the current sourced by the input in the low state. The 15kΩ resistor accounts for the rest of the E/S pin input current. Therefore, as the datasheet indicates the E/S pin only sources current, and it doesn't sink current.

    If you have additional questions, please let me know. 

    Best,

    Raymond

  • 0
    TI__Guru* 93105 points

    Hello Azfer,

    The OPA548 current limit feature is a real time feature. It monitors and tracks the output current continuously. There is an internal op amp in the current sense circuit that senses the output current across a sense resistor. During current limit the sense circuit forward biases a clamp transistor which is connected across the B-E junction of each output transistor. It is a linear circuit and does require some finite time to perform the current limit function and then to stop limiting. TI hasn't characterized the timing, because the limiting is so immediate in application. Based on the circuit design it likely goes into and out of current limits in matter a of tens of microseconds, or less.

    Regards, Thomas

    Precision Amplifiers Applications Engineering

  • 0 in reply to Thomas Kuehl
    Guru 140200 points

    Hi Thomas,

    Thomas Kuehl said:
    Based on the circuit design it likely goes into and out of current limits in matter a of tens of microseconds, or less.

    lasting much longer would result in a destroyed chip, right? Relaxed

    Kai

  • 0 in reply to kai klaas69
    TI__Guru* 93105 points

    Hi Kai,

    When I observe the OPA548 going into current limit with an O-scope I don't see any evidence of a high current spike at the onset, or release of current limit. It appears to occur smoothly and happen instantaneously. However, in such testing the frequency is usually low such as 10 Hz to a few kilohertz and almost always a sine wave. The OPA548 doesn't have high bandwidth and the applications it is used in don't require high bandwidth. Possibly, if the OPA548 where operate towards it higher usable bandwidth and slew rate, and driven with a fast dv/dt square wave we might be able to detect the current limit occurring. However, we would be looking at an limiting reaction that takes place in tens of microseconds, or less.

    It does appear that between the current limit and thermal shutdown features the very high power conditions that might damage the OPA548 silicon die are usually kept from being reached. In general, the OPA54X power op amps have demonstrated a good track record with regard to ruggedness.

    Thanks, Thomas

    Precision Amplifiers Applications Engineering

  • 0 in reply to Thomas Kuehl
    Guru 140200 points

    Thanks, Thomas Relaxed

    Kai