Figure 8: MCP16311 with parallel MCP1703
The Intermittent, Very High Power, Battery and Capacitor Hybrid Desig n
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.
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