When the MODE/S-CONF pin is configured for AEE mode, the TPSM82901 provides the highest efficiency over the entire input voltage and output voltage range by automatically adjusting the switching frequency of the converter. This is achieved by setting the predictive off time of the converter. The efficiency of a switched mode converter is determined by the power losses during the conversion. The efficiency decreases if V_OUT decreases, V_IN increases as shown in Equation 4, or both. In order to keep the efficiency high over the entire duty cycle range (V_OUT/V_IN ratio), the switching frequency is adjusted while maintaining the ripple current.

Equation 4.

The AEE function in the TPSM82901 adjusts the on time (TON) in power save mode, depending on the input voltage and the output voltage to maintain highest efficiency. The on time in steady-state operation can be estimated as using Equation 5:

Equation 5.

Equation 6 shows the relationship among the inductor ripple current, switching frequency, and duty cycle.

Equation 6.

Efficiency increases by decreasing switching losses and preserving high efficiency for varying duty cycles, while the ripple current amplitude remains low enough to deliver the full output current without reaching current limit. The AEE feature provides an efficiency enhancement for various duty cycles, especially for lower V_OUT values where fixed frequency converters suffer from a significant efficiency drop. Furthermore, this feature compensates for the very small duty cycles of high V_IN to low V_OUT conversion, which limits the control range in other topologies.