The OPA564-Q1 provides overcurrent protection to the load through an accurate, user-adjustable current limit (I_SET pin). The current limit value, I_LIM, can be set from 0.4A to 1.6A by controlling the current through the I_SET pin. Setting the current limit does not require special power resistors. The output current does not flow through the I_SET pin.

A simple resistor to the negative rail is sufficient for a general, coarse limit of the output current. Figure 6-30 exhibits the percent of error in the transfer function between I_SET and I_OUT versus the current limit set resistor, R_SET. The ±3 sigma distribution is derived from one lot of material characterized at room temperature. As significant variation can occur from unit to unit and across operating conditions, do not use the adjustable current limit to set an exact output current, but rather, to protect the device. Figure 6-31 and Figure 6-32 show how this error translates to variation in I_OUT versus R_SET. The dotted line represents the ideal output current setting that is determined by the following equation:

The mismatch errors between the current-limit set mirror and the output stage are primarily a result of variations in the approximately 1.2V band-gap reference, an internal 5kΩ resistor, the mismatch between the current limit and the output stage mirror, and the tolerance and temperature coefficient of the R_SET resistor referenced to the negative rail. Additionally, an increase in junction temperature can induce added mismatch in accuracy between the I_SET and I_OUT mirror. See Figure 8-7 for a method that can be used to dynamically change the current limit setting using a simple, zero-drift current source. This approach simplifies the current-limit equation to the following:

The current into the I_SET pin is determined by the NPN current source. Therefore, the errors contributed by the internal 1.2V band-gap reference and the 5kΩ resistor mismatch are eliminated, thus improving the overall accuracy of the transfer function. In this case, the primary source of error in I_SET is the R_SET resistor tolerance and the beta of the NPN transistor.

The primary intent of the current limit on the OPA564-Q1 is coarse protection of the output stage; therefore, exercise caution when attempting to control the output current by dynamically toggling the current-limit setting. Predictable performance is better achieved by controlling the output voltage through the feedback loop of the OPA564-Q1.