The key to extending the battery life of wearable devices - voltage regulators

Wearable devices have become the next wave of opportunities in the electronics industry. As shown in Figure 1, smart watches are today's most popular wearable devices. The healthcare market, including healthcare, fitness and rehabilitation, presents greater opportunities. Most wearable devices have a lot in common.

Wearable device must:

· ready to go

· Small size, light weight, easy to wear

· Rechargeable or disposable battery provides long enough time

· Support short-term work, most of the time is idle or dormant

· Ability to stay in idle or sleep mode for a very long time

These requirements put forward higher requirements for product technology. The size of the battery must be smaller and longer and longer. For example, a smart phone's battery capacity is about 2000 mAh; while a smart watch's battery is much smaller and about 10 times lower, and requires a charging interval that works the same as a smart phone that works with it. This requires a huge breakthrough in the performance of electronic components of smart watches. In other words, these devices must be very small and have the lowest power consumption in both active and inactive modes (standby and shutdown modes).

1 everything is low power

Traditional voltage regulator designs focus on power efficiency in the operating mode, from light load to heavy load to full load. A small percentage of efficiency advantages can determine the success or failure of the market. With the deep understanding of the efficiency curve and the bottleneck of technology development, the focus is shifting to optimizing energy efficiency in non-working mode. The non-operation mode corresponds to the system's idle mode (on but in standby mode) or sleep mode (system off).

Figure 1 wearable lifestyle

With the popularity of wearable devices, this shift is inevitable because wearable devices spend most of their time in standby mode with little time to work. Obviously, if idle and sleep modes take up most of the time, then energy savings during this period are very important. Therefore, every nanoampere of current is important because it extends standby time and saves valuable battery power.

For example, a 40mAh, 1.55V coin cell battery is ideal for powering wearable devices. If the wearable device consumes 4μA, the battery will have a shelf life of about 1 year before it runs out. If the current consumption is reduced by 1μA, the storage time of the wearable device can be extended by about 3 months! Considering the total current consumption of the voltage regulator in the ultra-portable device, the quiescent current is 10μA and the shutdown current is a few μA. It is not difficult to judge that any improvement in parasitic current is very beneficial.

2 regulator low shutdown current

In order to guarantee the battery life of wearable devices, we must consider the new voltage regulator technology with ultra-low current in non-operating mode. One such option is the MAX1722, a boost converter optimized for wearable products that uses state-of-the-art energy-saving techniques to minimize system sleep mode or current drain when the regulator is turned off. In shutdown mode, all regulator control circuits are turned off, with only inevitable parasitic leakage, minimizing the discharge of the battery and output capacitors. The BATT pin (ILB in Figure 2) has a few nanoamps of leakage current, and the OUT pin (ILO) has tens of nanoamps of leakage current. The leakage current on this pin is high due to the power supply chain on the OUT pin.

Figure 2 boost converter static / shutdown current

3 regulator low quiescent current

When the system is in idle mode, the voltage regulator is normally in standby and can be activated at any time when a command is received. In standby mode, the voltage regulator drives a very light load, so its quiescent current is very important and has a significant impact on regulator efficiency. In this state, the MAX1722 operates with minimal overhead. The quiescent current on the input (IQB in Figure 2) is only a few nanoamps, and the quiescent current on the output (IQO) is a few microamps, which is the control circuit for all Benchmark. Figure 3 shows the typical quiescent current of the MAX1722 with only 1.5μA at 3.3V output. The curve is measured at ambient temperature. This parameter is guaranteed to be less than 3.6 μA over the temperature range of 0 to +85 °C. Therefore, with the 150mA current in the operating mode and the ultra-low current in the non-operating mode, the MAX1722 in a small 2mm x 2mm μDFN package is ideal for powering solutions in wearable devices.

Figure 3 quiescent current of the MAX1722 boost converter OUT pin

English translation:

Quiescentcurrentintooutvs.Outputvoltage: static output current and output voltage

Quiescentcurrent: quiescent current

Outputvoltage: output voltage

4. Summary

We discussed the importance of minimizing the static and shutdown currents of voltage regulators in system idle or sleep mode. Let's take the MAX1722 as an example to show that when these two parameters are reduced to one or more orders of magnitude lower than conventional products, the battery life can be greatly extended. Maxim's future offerings will further reduce the size of the regulator and further isolate its circuitry from the battery and output capacitors in non-operating mode, challenging the limits. Further reduce parasitic leakage, avoid consumption of energy storage element current, and extend system operating time.

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