Getting the most out of battery power

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Almost every device we rely on these days is powered by batteries. That dependence is not slowing down as battery-powered equipment is increasingly being adopted for healthcare, industrial and domestic applications, in addition to our existing consumer devices. These new applications will mainly be employed to implement Internet of Things (IoT) solutions. IoT endpoint applications, such as medical implants, gas sensors, and RFID tags all rely on batteries to function, and the locations and environments that they operate can mean that replacing batteries when they are depleted can be a difficult task. Replacing batteries is expensive if specialists are required, which is pretty normal in industrial usage, and the disposal of the used batteries can be hazardous for the environment. Therefore, it makes sense to ensure that the batteries that we do use last as long as possible. In some circumstances this can be accomplished by designing products to use higher capacity batteries. However, not all applications are suitable for this, especially in IoT endpoints where the size and shape of the device can the most important factor in its design. In uses like these, the designer has to try get every last microamp out of the battery. The best way to accomplish that is to increase the efficiency of the circuit.

It is already likely that the designer will have chosen the most efficient components possible when designing the circuit, so it is difficult to look for gains there. However, there is a way that the LDO can be made to work more efficiently. That is due to how batteries discharge their power. A fully charged battery may initially have a voltage of 3.4 V, and as it discharges, its output voltage will fall over time until it is depleted around 2 V. Normally an LDO is used to provide a low-noise, regulated voltage to the processor and the other components in the circuit. In many applications that will be all that is required. However, for applications where battery life is critical, a very high-efficiency DC/DC converter can be used to bring the battery voltage down to nearer to the LDO’s output voltage. Due to its design, an LDO is less efficient when its input voltage is significantly higher than its output voltage. Using the battery example above, if the LDO is fed a constant voltage of around 2.1 V, it will operate at its highest efficiency throughout the battery's discharge. Therefore, if the DC/DC converter has losses that are less than the losses from the LDO as the battery voltage drops over time, then the battery life will be extended.

Analog Devices has recently launched the MAX38650 nanoPower DC/DC converter for this type of task. The device can lengthen battery life in suitable applications by up to 20%. It consumes only 390 nA during operation and 5 nA of shutdown current, and also offers other features intended to preserve battery life. For example, the device uses a single resistor connected to the device’s RSEL pin to set the reference voltage. The value of the resistor is chosen from a chart. When the circuit is switched on, the device measures the resistor value to set the reference voltage, and then turns that part of the circuit off. This process only consumes around 200μA of current. In normal applications, the reference voltage is set using an external voltage divider, which is constantly on, dissipating energy. If the resistance is increased too much to minimise this wastage, it would make the circuit susceptible to RF coupling - introducing noise into the power supply. The MAX38650 is also designed to operate at 100% duty cycle to allow it to convert an input voltage right down to its output voltage level, further extending the life of the battery. Another feature of the device that helps extend battery life is reverse current blocking to stop energy flow getting back to the battery and protect against active discharge.

The device is intended for applications, such as IoT endpoints that use very little current, with a maximum output current of 100 mA. It can operate from 1.8V to 5.5V input voltage and has peak efficiencies of 95%, with high efficiency throughout the load range. It is available in a 1.58mm x 0.89mm, 6-pin wafer-level package.