FIG. 1A shows a prior art output voltage controlled hysteretic voltage regulator comprising a transistor for connecting a load to a supply voltage Vs through an inductor L when the output voltage Vo falls below a first reference voltage Vref 1, and for disconnecting the output voltage Vo from the supply voltage Vs when the output voltage Vo exceeds a second reference voltage Vref2 higher than the first reference voltage Vref1. In the example of circuitry of FIG. 1A, an S/R flip-flop turns the transistor 2 on/off in response to the output voltage Vo. Because the feedback is based on the output voltage Vo, the prior art hysteretic voltage regulator of FIG. 1A may exhibit poor transient response if there is a sudden change in load current.
FIG. 1B shows a prior art inductor-current/output-voltage controlled hysteretic voltage regulator which helps address the poor transient response of the prior art regulator of FIG. 1A. With the prior art design of FIG. 1B, a current sensor 6 senses the inductor current 8. When the inductor current 8 falls below a second reference current Iref2, and the output voltage Vo falls below a reference voltage Vref, the S/R flip-flop 4 turns the transistor 2 on. When the inductor current 8 rises above a first reference current Iref1, the S/R flip-flop 4 turns the transistor 2 off. Although this design may improve the response to transients in the load current, it may also cause higher current ripple leading to inefficient operation. The current ripple increases because the inductor current 8 is forced to ramp from Iref1 to Iref2 and then back to Iref1 (or lower depending on the output voltage Vo).