Easy Definition of Fixed Off Time Variable Frequency

In the fixed-off-time switching voltage regulator, the average de voltage is varied by changing the on time (t) of the switch while maintaining a fixed off-time (tOFF). The fixed-off-time switching voltage regulator behaves in a manner opposite that of the fixed-on-time regulator.

As the load current increases, the on time is made to Increase, thus decreasing the operating frequency; this is illustrated in Figure 5-25. This approach provides the capability to design a switching voltage regulator that will operate at a well defined minimum frequency under full load conditions.

Figure 5-25. Frequency vs Load Current for Fixed Off-Time SVR

The fixed-off-time approach also allows a de current to be established in the inductor under increased load conditions, thus reducing the ripple current while maintaining the same average current. The maximum current experienced in the inductor under transient load conditions is not as well defined as that found in the fixed-on-time regulator. Thus additional precautions should be taken to ensure that the inductor does not saturate.