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Converting lithium-ion to primary battery

Posted: 17 Sep 2014 ?? ?Print Version ?Bookmark and Share

Keywords:lithium-ion? power source? DC-DC converter? LED? boost converters?

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 doesnt 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 A 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 A 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 unique auto-detected low-power mode when the system load is inactive. A parallel approach can be used to further reduce system input current, by using the MCP16311 EN input to shut down switching while powering the system with a low IQ LDO, similar to the MCP1703, during sleep or inactive periods, as shown in figure 8.

Figure 8: MCP16311 with parallel MCP1703.

The intermittent, very high power, battery and capacitor hybrid design
There are many situations in embedded designs that require infrequent, but very high, peak currents to operate. Consider, for example, a portable medical device with a display and an internal motor or pump. Assuming the motor or pump runs intermittently, the current consumed could be several orders of magnitude higher than the typical operating drain. Depending on the load or resistance, the current required from the battery could easily exceed one amp. Consider another example of an embedded device with a high-power radio, such as an 802.11n transceiver or a cellular modem. It may generally draw very low amounts of power, but when the device needs to transmit data over the network, the system needs to allow the radio to power up and transmit, which will briefly consume large amounts of current.

Figure 9: Block diagram of microcontroller, capacitor and battery.

When considering how to power this type of application, it is important to understand the internal-resistance levels of various battery systems. Lithium-ion batteries typically have low internal resistance and are capable of handling very high current spikes. Within primary batteries, the internal resistance is somewhat higher. When alkaline and primary lithium batteries are fresh, they have similar and comparatively low internal resistance. However, as alkaline batteries are discharged, their internal resistance will gradually increase. Thus, when large amounts of current are required, the voltage on a stack of alkaline cells will drop precipitously as the required current rises. Conversely, the internal resistance of primary lithium batteries will remain low throughout discharge, and will provide longer runtimes in applications with high-current demands.

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