Lithium-ion to primary battery power design conversion strategies: Page 4 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.
making them well suited for boost-converter applications that require constant power.
 
A single-cell solution requires a boost converter to step the single-cell voltage range up to a usable, regulated 3.0V or 3.3V system input-voltage range. Readily available boost converters, such as the example from Figure 3, integrate the entire switch-mode power solution, including compensation components, and provide the system with a low-current shutdown (< 1 µA) and low-current standby (<14 µA), along with the ability to deliver up to 100 mA of continuous current during operation. Additional integrated features block the typical boost-converter input to output DC connection and provide recoverable output short-circuit protection, as shown in Figure 4.
 

Figure 3: Single-cell MCP16251 boost converter

 


Figure 4: Showing blocking path and current-limit function


The High-Power, Two-Cell Design

For higher-power applications, two cells in series can be used, providing longer run time and higher current. As an example, Microchip’s MCP1643 high-current boost converter can regulate up to 500 mA from two cells in series for high-power LED applications, while providing true disconnect shutdown, current limit and short-circuit protection (see Figure 5). A high (1 MHz) switching frequency enables small inductors and capacitors, reducing size and cost. The MCP1643 can also be turned off, minimizing battery drain to less than 1 µA. A regulated current source can be developed with a single integrated device, from two primary lithium batteries (see Figure 6). A low, 120 mV Vref is used to maximize system efficiency. Optimum performance is enabled by their high and consistent voltage profile across a wide range of discharge rates.

Figure 5: Two series cells-powered MCP1643 application, shown driving high-power LED
     

Figure 6: Output current capability of two-cell + MCP1643 solution
 
The Buck-Boost, Three-Cell Design

For some applications, the input-voltage range can be greater than or less than the desired output voltage. This can be true for either primary or rechargeable batteries. For example, lithium-ion

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