Several fictitious examples can illustrate dc/dc converter selection for a subsystem in a portable product. For simplicity, the examples focus on selection for core logic such as a digital signal processor or CPU. These examples are devices A (a toy), B (an instrument) and C (a cell phone). Together, they show situations where use of LDOs, charge pumps or switching regulators is optimal.
All three examples assume operation from a 1,500-mA-hr, Model CGR18650H Li-ion battery from Panasonic. The nominal battery voltage is assumed to be 3.6 V, at an ambient 25 degrees C. For simplicity, dc/dc converter options are simplified to an LP2982 LDO, an LM2663 charge pump and a hypothetical multimode, step-down, switching dc/dc converter.
The LP2982 has a quiescent current of 65 microamps, 50-mA load capability and 120-mV drop-out. The LM2663 charge pump has a 1.3-mA quiescent current, 90 percent efficiency and a full load of 200 mA. The mystery switching dc/dc converter has a pulse frequency modulation (PFM) with 160 microamps quiescent current, and 77 percent full load efficiency at an output of 1.5 V.
Device A, a toy, has the input-output differential associated with the parallel Li-ion cells and is around 3.6 V - 3.3 V = 0.3 V. If an LDO is used, the resulting full-load efficiency is 91.5 percent. Total battery life, due to current consumed from the Panasonic Model CGR18650H Li-ion battery is 404 hours.
Device B is a medical diagnostic device, or a portable industrial device. Maximum load current is on the order of 100 mA, and an LDO is not optimal, since the device spends most of its operational time in full power, and the efficiency from a Li-ion cell is only 50 percent. The total battery life, due to current consumed from the Panasonic Model CGR18650H Li-ion battery is 36.6 hours.
Device C is a complex one, such as a CDMA cellular phone. This device uses a state-of-the-art 1.8-V baseband processor, dynamically clocked with a 200 mA full-power operating mode, 2 mA low-power mode and 10-microamps halt mode. Battery life for comparative LDO, charge-pump and switching power-supply solutions is 98 hours, 172 hours and 205 hours, respectively. The total battery life, due to current consumed from the Panasonic Model CGR18650H Li-ion battery, is 205 hours.
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