Current limit in an LDO is defined by establishing an upper boundary for the current supplied. Unlike a constant current source, LDOs supply current on demand but can also control the total power regulated. Current limiting is achieved through internal circuitry controlling the output stage transistors inside the LDO.

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Source: Texas Instruments

 

This is a classic current-limit circuit for an LDO and is commonly referred to as a “brick-wall” current limit due to its abrupt current stop once the limit is reached. In this internal circuit, the LDO measures the output voltage for feedback but also measures a scaled mirror of the output current against the internal reference (IREF).

 

In a brick-wall current limit, the upper boundary is defined and the LDO supplies current incrementally until the limit current limit is reached. Once the current limit is exceeded, the output voltage is not regulated and is determined by the resistance for the load circuitry (RLOAD) and the output current limit (ILIMIT).

 

The pass transistor will continue this operation and dissipate power, as long as the thermal resistance (θJA) allows for healthy power dissipation where the junction temperature is within acceptable limits (TJ < 125°C). Once VOUT goes too low and the thermal limit is reached, thermal shutdown will turn off the device in order to protect it from permanent damage. Once the device has cooled, it will turn back on and regulation can proceed. This is particularly important in cases where a short circuit may present itself, as the LDO will proceed to regulate VOUT to 0V.

 

To learn more about LDO current limits, foldback current limiting, click here.