8.2.3 Slow Turnon 15-V Regulator

An application of LM317A includes a PNP transistor with a capacitor to implement slow turnon functionality (see Figure 21). As V_IN rises, the PNP sinks current from the ADJ rail. The output voltage at start up is the addition of the 1.25-V reference plus the drop across the base to emitter. While this is happening, the capacitor begins to charge and eventually opens the PNP. At this point, the device functions normally, regulating the output at 15 V. A diode is placed between C1 and V_OUT to provide a path for the capacitor to discharge. Such controlled turnon is useful for limiting the in-rush current.

Figure 21. Slow Turnon 15-V Regulator