# Energy Management Integrated Circuit for Multi-Source Energy Harvesters in WBAN Applications
## 4 Circuit Block Designs
### Analog Blocks: Path Switches, Comparators, and Oscillator  
One analog blocks is composed of path switches, comparators, and an oscillator. The path switches are managed using digital control logic to determine the output paths used to supply the input energy. The two output paths (low-power path and high-power path) are shown in Figure 6a. In this system, the input ports are constrained to three choices. The number of input ports can be expanded according to the needs of an application. When designing path switches, the resistance of a switch is a major factor to be considered. Switches with high on-resistance can lose relatively large amounts of power during energy transmission. The equation for dissipated power in a path switch is  
\[\mathrm{P}=\mathrm{I}\cdot R^{2} \tag{3}\]  
In Equation (3), \(\mathrm{P}\) is dissipated power in a path switch, \(\mathrm{I}\) is the current that flows through the path switch, and \(R\) is the on-resistance of the path switch. When current flows through a switch, the power dissipation in the switch is determined by the magnitude of current and the resistance of the switch. This is the reason why the on-resistance of the path switch is very important. If the width of the switch is too small, on-resistance may increase. This may cause too much power dissipation when current flows through the path switch. If the width of a switch is too large, on-resistance may decrease. However, it occupies a large area, and in the off-state, leakage current may also increase. In this case, by using the body bias technique, off-resistance may be controlled. If a current of 10 \(\mathrm{\SIUnitSymbolMicro A}\) flows through a switch with 1 \(\mathrm{\SIUnitSymbolOhm}\) resistance, 10 \(\mathrm{\SIUnitSymbolMicro W}\) of power is wasted through the path switch. For low-power applications, 10 \(\mathrm{\SIUnitSymbolMicro W}\) is not negligible. Ideally, a path switches with no on-resistance, no distortion, and zero time delay is desired. However, it is hard to implement these in real process technology. To minimize the limitations of real process technology, the path switches were designed with bilateral CMOS devices. The on-resistance of both NMOS and PMOS devices changes with the channel voltage. This nonlinear resistance can cause errors in accuracy as well as distortion. By using the bilateral CMOS switch, on-resistance is minimized, and linearity is also improved. The target application of this paper is for WBANs, and the harvest power is limited to micro watt scale. Because an excessively low on-resistance requires a large implementation area, the switch size should be optimized for micro watt scale to achieve efficiency better than 90%. A comparator detects the magnitude of the harvested energy stored in the temporary storage, Cap_n. The comparator design is shown in Figure 6b. The inputs to comparators are placed on  
Figure 5: State classification for the harvest energy input. the gates of transistors to show high impedance on the sensed nodes. The role of the comparators is to inform the state of the stored energy in input capacitors to a digital control logic. When the stored energy is higher than the energy threshold, the output of the comparator reaches a high state, and the digital control logic turns on the path switch. After providing the stored energy to the output, the voltage of the input capacitor starts to decrease. When designing the comparators, operating speed is not an important issue because it detects the voltage of a capacitor. The capacitor stores energy temporarily, and the change of voltage in the capacitor is not rapid. Rather than operating speed, power consumption is a more important issue. To reduce the power consumption in comparators, digital control logic guides the operation of comparators. If no operation is required, the bias currents in the comparators are blocked by an enabling signal from the digital control logic. By adapting enabling techniques, the power can be reduced by as much as 80%.