By Eileen Sun, GLF Integrated Power

Designers of wearables — including smartwatches, fitness
trackers, heart monitors, and many other devices — are constantly trying to find ways to increase
battery life in order to improve user convenience. Yet for all of the advances
that have been made in their design and capabilities, wearables often require
frequent recharging. This is because most wearables are using the same load
switches designed for bigger devices like tablets, which have much larger
battery capacity than wearables, and where battery-draining leakage current and
ON resistance are less of a concern.

To address this challenge, new ultra-low leakage current load
switches are setting the standard for efficient power management in wearable
devices. By using advanced load switches, efficiency can be dramatically
increased. These savings will translate into either a smaller battery or a
longer battery life and will differentiate wearable products in a very
competitive market.

Battery efficiency in wearables
In wearable products, load switches perform the essential
battery-management tasks with minimum footprint and nearly zero leakage current
and/or ON resistance. However, IC load switches that are meant for laptops and
tablet computers simply can’t meet these demands.

An example of a typical wearable application (Fig. 1) includes several environmental
sensors, GPS, display, memory, wireless connectivity, and a fast, feature-rich
microcontroller (MCU). In order to extend battery life, the circuitry includes
a number of load switches to turn on devices sequentially or place the device
in “sleep” mode and to turn off functions as need­ed.

In this respect, the example shown in the block diagram is not
materially different than that of a typical handheld tablet or mobile phone.
The difference lies in the size and performance of these devices.

Fig. 1: New high-performance load switches can
add hours or even days to wearable battery life.

Many of the functional devices utilized in wearables (including
connectivity, sensing, and MCUs) come with power-saving modes that offer developers
a false sense of lower-power control. Table
1
shows examples of low-power mode current drawn by a typical mix of ICs
and modules.

Table
1: Typical low-end
wearable functions and the power they draw.

GLF-Integrated_Table-1

Table 2 describes the current drain of other
features required in higher-performance applications. Together with the MCU/APU
and other devices, standby current can really add up. All of the required
features can turn a device into a “power monster.”

Table
2: Typical devices
that do not offer the needed low standby current.

GLF-Integrated_Table-2

Criteria for choosing a load switch
Designers of wearables need to consider load switch selection
criteria. The following are examples to compare the performance of available
devices.

Mostly ON vs. mostly OFF
Depending on whether the subsystem is ON or OFF, differ­ent
parameters of the load switch become important. Most switches are optimized for
one but not for both conditions.

In the case of a “Mostly OFF” subsystem, the current standby ISD (standby current leakage) becomes the
most important parameter to consider. The best devices on the market are well
under 1 μA. Table 3 includes devices
that meet this criterion. Marking a significant improvement, the table shows
that an advanced low-leakage current load switch, GLF71301 device, offers
best-in-class ISD at 7 nA.

Table
3: GLF load switch’s
IQ and ISD performance compared to competitive
devices.

GLF-Integrated_Table-3

If the subsystem is “mostly ON,” then the ON resistance of the
switch, RON,
becomes paramount. A designer should look for a RON of less than 100 mΩ at the max
operating range. Table 4 offers a
sampling of some of the devices offering the lowest RON. As shown, the GLF71321 load switch
offers the best RON performance (17 mΩ at 5.5 V and 21 mΩ at 3.3 V).

Table
4: GLF load switch’s
RON
performance compared to competitive devices.

GLF-Integrated_Table-4

Voltage range
Most wearables have several different voltage rails rang­ing
from 1.1 V for the CPU core to 5.0 V for higher-voltage peripherals. A load
switch with a wider range of voltage rail options can help manage inventory and
used throughout. All of the devices listed in Tables 3 and 4 meet this criterion.

It should be clear from the information presented in these
tables that both ISD and RON will have a significant effect on battery life. Another factor
to take into account is the quies­cent current (IQ) of the device. This is the “overhead”
current drawn by the device when in the “ON” state. A quick review of these
same two tables reveal that the GLF devices are up to several hundred times
lower than the others.

The future of wearables
It is estimated that by 2025, there will be over 1 trillion IoT
devices in operation; of these, a significant portion will be of the wearable
variety. Consumers will continue to expect wearables to need less-frequent
charging. This, in turn, will make it necessary for applications engineers to
employ the most battery-efficient design techniques possible.

Selecting the latest
ultra-low leakage current load switches offer designers an effective means for improving
battery life in wearables. Unlike conventional devices more suitable to larger
portable electronics, these wearable-optimized load switches can easily extend
operating time from days to weeks between recharging.