SBVS311A November 2019 – March 2020 TPS7A54
Some applications have transients that place the device into dropout, especially with a device such as a high-current linear regulator. A typical application with these transient conditions may require setting VIN ≤ (VOUT + VDO) in order to keep the device junction temperature within the specified operating range. A load transient or line transient with these conditions can place the device into dropout; for example, a load transient from 1 A to 4 A at 1 A/µs when operating with a VIN of 5.4 V and a VOUT of 5.0 V.
The load transient saturates the error amplifier output stage when the gate of the pass element is driven as high as possible by the error amplifier, thus making the pass element function like a resistor from VIN to VOUT. The error amplifier response time to this load transient (IOUT = 4 A to 1 A at 1 A/µs) is limited because the error amplifier must first recover from saturation, and then place the pass element back into active mode. During the recovery from the load transient, VOUT overshoots because the pass element is functioning as a resistor from VIN to VOUT. If operating under these conditions, apply a higher dc load or increase the output capacitance in order to reduce the overshoot.