The inverting regulator is similar to the step-up regulator. During the charging cycle of the inductor the load is isolated from the input. The only difference is in the potential across the inductor during its discharge. This can best be demonstrated by a review of the basic inverting regulator circuit (Figure 6-32).
waveform demonstrated in the step-up configuration [see Figure 6-30(b)].
Figure 6-33 shows the inverting applications both with and without an external pass transistor. Design equations are also included.
Note that in the inverting configuration the internal diode is not used. An external diode must be used because pin 4 (substrate) must be the most negative point on the chip. The cathode of the internal diode is also the cathode of a diode connected to the substrate. When the cathodes are at the most negative voltage in the circuit, there will be conduction to the substrate resulting in unstable operation.