Battery-operated systems: A typical challenge for power-management systems that use these types of devices is the battery-powered application. For example, a large number of battery applications use one lithium-ion (Li-ion), rechargeable battery cell and require an output of 1.8 V at a current from 0 to 300 mA. The nominal voltage of a Li-ion cell is 3.6 V, with an output voltage range between 2.8 and 4.2 V. For this application, the feasibility of SPC, charge-pump, and LDO solutions can be evaluated in terms of efficiency.
The charge pump is probably the most unlikely device to fit in this application, primarily because it's an inverting device. Consequently, it couldn't easily provide a 1.8-V output. Although this is obviously a very critical specification, the charge-pump circuit has other problems in this application. Its efficiency-versus-current performance is only optimum for output currents of 1 to 10 mA (Fig. 6, again). Although the charge pump is an inexpensive solution (because the external components are capacitors only), the device's efficiency doesn't suit this application well.
The LDO is a possible solution to this application problem. Implementing the TC1185 in a circuit is very simple due to the low external device count. Though the device can be designed into this application, the efficiency-versus-source voltage isn't as good as the other solutions. Because the battery has a large output voltage range, the frequency of battery charging is higher with the LDO, compared to the SPC solution. In fact, the total LDO power dissipation is 240 mW when the input voltage is 4.2 V and the output is loaded to 100 mA.
The SPC device is the best choice for this type of application. The TC105 can easily power a 1.8-V output with good regulation. Its efficiency-versus-source voltage performance is comparatively very high—approximately 90% over the entire source-voltage range (Fig. 5, again). Although the efficiency versus output current isn't superior at every current, it remains the best choice across the entire output-current range (Fig. 6, again).
The bottom line is that in battery-powered applications, efficiency considerations are at the top of the list when considering a new design's power management. Available choices include SPCs, charge pumps, and LDOs. For systems that have wide source-voltage variations, the SPC is by far the best choice. If the system operates over a small range of low output currents, the charge pump can yield the best efficiency in a system. Finally, if the system requires a good low-noise regulated output, and the power dissipation is manageable, the LDO will provide satisfactory results.