Typically, efficiency considerations drive a designer's final decision when selecting an application's battery-power-management strategy. The choices for this strategy are switched-power converters (SPCs), charge pumps, and low-dropout regulators (LDOs). As far as efficiency is concerned, the SPC is the overall best choice for almost all applications. If the application can tolerate the switching electromagnetic-interference (EMI) noise, an SPC's efficiency is relatively independent of line voltage and output current.
In terms of line voltage variations, the charge pump's power efficiency is nearly comparable to the SPC. It's only more efficient for a small range of load currents. Also, charge pumps generally have unregulated outputs. If a charge pump's output must be regulated, it needs to be connected to a linear LDO. So the efficiencies of the charge pump and LDO are multiplied together, resulting in a system with lower efficiency than either the LDO or charge pump alone.
The linear regulator's efficiency can be approximated by the VOUT/VIN ratio. Given this scenario, the efficiency is dynamic, decreasing linearly with increasing line voltage. If the source voltage varies widely, the linear regulator is the poorer choice for a power-supply design. But the good news is that a linear regulator's efficiency is relatively independent of output current load—if the application can tolerate the power dissipation.
With all of these options, it might seem that choosing the right power system for a given application would be difficult. But taking time to carefully select the power-supply strategy can give one a competitive edge by providing the most efficient, compact, low-cost solution for the intended application.
The power-supply management circuit discussed here can be implemented with discrete components, or as a combination of ICs and discrete components (Fig. 1). Its purpose is to match source-voltage changes to the desired output voltage, while complementing the output drive requirements. In the best cases, this task is achieved with optimum efficiency. As mentioned earlier, the ICs typically used in this type of application are SPCs, charge pumps, or LDOs. In all cases, the IC conditions the source voltage to a different output voltage.
Buck-SPC circuit efficiency: Figure 2 shows a simplified example of a buck SPC circuit. This style of converter uses a simple chopping network in combination with a low-pass LC filter. As described here, the buck converter is operating in a continuous inductor-current mode. The input is "chopped" using a pulse-width modulator (PWM) signal to the switch. Then, the resulting pulses are averaged to create a dc output voltage. This converter can only step down the input voltage (from a high value to a lower value).
In evaluating the circuit of Figure 2, the following assumptions are made: (1) the input voltage is always greater than the output voltage, as required for a buck SPC, (2) the output voltage is essentially dc, which implies that the output filter is large enough to completely average this voltage (typically less than 1% variation on VOUT), and (3) there's always a current through the inductor, which is required when keeping the converter in a continuous-conduction mode.