This work presents a single-input radio frequency energy-harvesting (RFEH) interface with 3-D hill-climbing (HC) maximum power point tracking (MPPT). We use constant energy packet extraction (CEPE) control that facilitates power detection with low quiescent current. To reduce the error of the CEPE scheme at a low-voltage conversion ratio (VCR), we modify it with turn-on time compensation. We obtain the 3-D MPPT by searching the optimal: 1) the number of rectifier stages; 2) dc-dc input impedance; and 3) rectifier shunt input capacitance. Furthermore, we replace the RF circuit with an artificial neural network (ANN) model to speed up the simulation. The prototype chip for the 2.4-GHz RFEH interface fabricated in a 65-nm CMOS exhibits > 97% MPPT accuracy. The low-power design facilitates the highest sensitivity (- 28.5 dBm) and efficiency (10.4% at - 20-dBm PIN) among the compared works with single-input rectifiers. In addition, it shows near-consistent efficiency across varying temperatures.