To overcome the voltage drop, especially if alkaline batteries are used, an energy reservoir needs to provide additional current when needed. A capacitor is the natural element to use, and a wide range of sizes and capacitances is available. With values up to and above 1F available in the lithium-ion voltage range, there remain two additional challenges in implementing a solution. First, the capacitor will have very low internal resistance - particularly when empty - so a current limit needs to be placed between the primary cells and the capacitor. Second, introducing a current limit assumes an intentional resistance that needs to be controlled and managed to keep the efficiency high. A microcontroller with integrated analog that controls an external MOSFET solves both of these issues. The remaining design challenge centers on sizing the capacitor and MOSFET.
Figure 9 illustrates the blocks needed to control and protect the capacitor, and which items are integrated into a microcontroller. A suitable microcontroller will contain an internal oscillator, A/D, comparators and op amps, so that the external components needed are limited to just the MOSFET.
Figure 9: Block diagram of microcontroller, capacitor and battery
Figure 10: Flow chart of software algorithm
To design the solution, two elements need sizing:
● Capacitor value
● Current limit
For a design example, the assumed current peak will be 1A for a maximum of 500 mS, with a minimum repeat rate of 5 seconds between peaks.
The capacitor needs to keep its voltage value between 3.0V and 4.2V, during the 1A discharge of 500 mS, and recharge in 5 seconds.
Based on the capacitor discharge equation and the voltage assumptions above, the following equation sizes the capacitor:
C = t*Ipeak
A 0.5F capacitor provides the necessary reserve to deliver 1A for 500 mS.
To recharge the capacitor for