Some battery-powered devices require large amounts of current in a short period of time but spend most of the time in sleep (power-down) mode. The momentary large-load current demands large batteries to meet the time requirement, even though the average current consumption is low. For instance, a system operates for 1.5 sec every 10 hours and needs 500 mA at 3.3V during the operation. Although the average current is only 21 µA, small, "coin" batteries cannot drive such a heavy load.

To eliminate the need for larger batteries, the circuit in Figure 1 solves the problem by gradually building up energy in a supercapacitor. The device releases the energy when it is needed. Because the supercapacitor has low internal impedance, the momentary current can easily exceed several amperes.

Because a coin-type lithium battery delivers 3V and the supercapacitor's rated voltage is 2.5V, the circuit uses a voltage-controlled switch to cut off the battery once the voltage on the super-capacitor reaches 2.2V. This design uses a 1.5F, 2.5V supercapacitor from PowerStor (www.powerstor.com), model A1030-2R5155. IC₁, a micropower voltage comparator with built-in 1.245V reference, senses the voltage on the supercapacitor. R₃ provides 0.5V hysteresis to the comparator. When the voltage is lower than 1.7V, the comparator's output is low and thus turns on the p-channel MOSFET, Q₁. The battery charges the supercapacitor. Once the voltage on the supercapacitor reaches 2.2V, the comparator switches high to shut off Q₁. You could use this low-to-high transition at Point A as a battery-charge-complete indicator or to trigger another device, such as a microcontroller's interrupt line.

Q₂, another p-channel MOSFET, controls the discharge of the supercapacitor. When Point B is floating, the switch is off. When an open-drain or open-collector device pulls down Point B, the switch is on. Because the voltage on the supercapacitor continuously drops when the switch is on, you can use a boost dc/dc converter to generate a constant output voltage. Select a boost converter with the lowest possible working input voltage to obtain the maximum energy from the supercapacitor. For example, you can use an LTC3402 to generate a stable 3.3V output. Once it starts, the LTC3402 can work with input voltages as low as 0.5V. The energy from the supercapacitor is 1/2V²C, or 1/2[(2.2V)²×1.5F–(0.5V)²×1.5F]=3.4J.

Check out our Best of Design Ideas section!