In the step-up regulator, the formulas change slightly. During the charging cycle (S1 closed) the inductor (L) is charged directly by the input potential. The peak current is not related to the load current as it was in the step-down regulator because during the inductor charge cycle the blocking diode D1 is reverse-biased and no charge is delivered to the load.
The circuit in Figure 6-30(a) delivers power to the load only during the discharge cycle of the inductor (S1 open). The diode (D1) is forward biased and the inductor discharges into the load capacitor. Figure 6-31 shows a positive, step-down configuration both with and without an external pass transistor. Design equations are included.