OPA551: Current limit associated with shutdown?

Hi Steve,

The OPA551 current limit is just that, it limits the current to about +/-380 mA and the output produces no more current. The output waveform will be clipped when the output current hits the current limit. There is nothing in the original design review indicating that activating current limiting results in output shutdown. However, the thermal shutdown does indeed shut down the output stage when the output junction temperature hits about 160°C and then resets when the die has cooled to 140°C.

"When driving a short circuit the OPA551 does seem to go into shutdown without getting hot." If the OPA551 is driven to current limit and the full supply voltage is across the output transistor the power dissipation may be high enough for it to go into thermal shutdown. Should the output transistor junction temperature keep hitting 160°C repeatedly as with a shorted output, it will go into thermal shutdown repeatedly (thermal oscillation). It won't dissipate nearly as much power as it does when continuously driving a a load at 200 mA or more and not going into current limit. The datasheet in Section 7.3.2 Current Limit states, "This feature, in combination with the thermal protection circuitry, provides protection from many types of overload conditions, including short-circuit to ground."

Regards, Thomas

Precision Amplifiers Applications Engineering

Thomas-

Thanks; that's what I thought. 

the datasheet also says that repeatedly driving the part into thermal shutdown will degrade reliability.

In the application, the output could get inadvertently shorted during module installation perhaps due to a wiring harness getting crushed (for instance).  In this case, we need the assembly that the OPA551 is within to not be compromised/over-stressed or otherwise made unreliable due to the short. 

Once the short is cleared (it will get figured out sooner or later) the electronics need to resume working as designed.

This is the spirit of the current limit circuit I sent you in email.

-Steve