Lithium-ion to primary battery power design conversion strategies: Page 5 of 9

August 19, 2014 //By Terry Cleveland, John Haroian & Adam Jakubiak
Lithium-ion to primary battery power design conversion strategies
Terry Cleveland, John Haroian & Adam Jakubiak  consider some lithium-ion to primary battery power design conversion strategies.
battery voltage can range from 3.0V to 4.2V throughout the charge and discharge cycles. Three alkaline batteries connected in series will measure about 4.5V when fresh, but as low as 2.4V when depleted. Similarly, three primary lithium batteries connected in series will measure about 5.1V when fresh, but about 3.6V when depleted. The application, however, may require a regulated 3.3V rail, which suggests that a power supply with buck-boost capability is needed for this configuration.
 
A SEPIC converter is capable of stepping the input voltage up or down, to regulate the 3.3V system voltage. In addition to providing system regulation, the primary-battery SEPIC design doesn’t require protection circuitry or back-to-back P-channel switches to protect the battery. Instead, an isolation capacitor provides input-to-output shorted switch protection and the control IC, such as an MCP1632, can provide short-circuit-output and current-limit protection with minimal external components.
 
For low-power standby applications, a parallel MCP1700 1 µA IQ LDO can be used in parallel with the SEPIC power train, as shown in Figure 7. With the SEPIC power train disabled, the MCP1700 will provide a “keep alive” voltage rail while consuming 1.6 µA of battery current with no load.
   

Figure 7: MCP1632 Application Diagram with 3 series batteries and optional MCP1700 @ 2.0V

The Pure Buck, Four-Cell Design

For applications with high peak-current demands or long run-time requirements, additional series cells can be added to increase the input voltage of the system. Increasing input voltage reduces input current, resulting in smaller size and lower cost for the connectors, wiring and distribution switches. A low-power, integrated synchronous buck converter can step down four primary cells, in series, to a nominal 3.xV system voltage, with an input range of 3.6V to 6.4V. The MCP16311 buck converter integrates the entire switch-mode power system, including compensation. This reduces system cost and size while operating over a wide input-voltage range and reducing input current, using a

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