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Part Number: OPA454
I've been trying to utilise the Flag pin in order to turn on one of 2 LED's:
essentially I want the red LED to come on when the current through the output of the op-amp is exceeded.
In the datasheet in section 6 the description of the flag status pin is: open drain....active low and goes active for an overcurrent or over temperature event. This implies that the active event is an over current or over temp event and this will go low.
I've tested this in TINA and on the bench and it is LOW normally and goes HIGH during such an event!
Also once the device overheats it no longer holds the flag status, so does this mean I have to latch the signal myself?
The OPA454 Flag function as described in the datasheet Pin Functions table states, "The Status Flag is an open-drain active-low output referenced to E/D Com. This pin goes active for either an overcurrent or over-temperature condition." This, as discussed in a very recent E2E post, was found to contain incorrect information. In actuality, the Status Flag is active-high when either an overcurrent or over-temperature condition exists. Thus, the open-drain flag transistor is turned off and the pull-up resistor pulls the drain up close to the V+ voltage level. The OPA454 simulation models this behavior correctly.
Your circuit will have to be changed to accommodate this difference in the flag on/off states. Do note that the flag pin does not have a lot of current sinking capability when in the normal operating, low-state mode. With a 100 kilohm pull-up resistor and a +50 V supply the flag transistor will only sink about 500 uA (or about 100 uA with a 10 V pull-up voltage - see Fig. 24). If you are using a +50 V for the V+ supply, your circuit will develop about 7.5 V across a 15 kilohm gate resistor. That should be plenty high to turn a 2N7002 MOSFET on.
Additionally, even though the OPA454 is rated for somewhat more than 50 mA output current while in normal operating mode the current limit doesn't activate until a much higher level, typically +/-125 mA. Keep in mind that the over current LED won't activate until the current is at the higher levels. The LED should activate at about 150 °C as the datasheet indicates when thermal shutdown occurs.
If the OPA454 goes into thermal shutdown around 150 °C the output will turn off and cease delivering current to a load. When that occurs the power in the output stage is much reduced and the OPA454 cools causing the die temperature to drop. Once the die temperature drops below 130 °C the output stage should resume operation. During the time the output is in thermal shutdown the flag should remain high.
There is something about your circuit I don't quite understand. The OPA454 circuit is similar to a unity-gain buffer configuration. However, U2 is connected directly to the summing node of the op amp. It is not clear the function U2 provides and its connection to the summing node.
Precision Amplifiers Applications Engineering
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