An improved architecture for an implantable medical device using a primary battery is disclosed which reduces the circumstances in which the voltage of the primary battery is boosted, and hence reduces the power draw in the implant. The architecture includes a boost converter for selectively boosting the voltage of the primary battery and for supplying that boosted voltage to certain of the circuit blocks, including digital circuitry, analog circuitry, and memory. However, the boost converter is only used to boost the battery voltage when its magnitude is below a threshold; if above the threshold, the battery voltage is passed to the circuit blocks without boosting. Additionally, some circuitry capable of operation even at low battery voltages—including the telemetry tank circuitry and the compliance voltage generator—receives the battery voltage directly without boosting, and without regard to the current magnitude of the battery voltage.