COVER STORY ELECTRONICA 2016 SUPPLEMENT
POWER EFFICIENT DESIGN FOR WEARABLE ELECTRONICS
With the next big growth area in the electronics industry expected to be in wearables, some predictions claim that this market could increase to $10bn by 2020. But this up-take is largely dependant on the user experience, and one of the main attributes that users judge portable products on is battery life. Here Mark Patrick from Mouser Electronics looks at three approaches to save every joule of energy possible
factor in maximising efficiency. This is particularly true for synchronous regulators where efficiencies of over 95% are possible. However, it is not just headline efficiency, or even standby efficiency, that is necessarily the most critical factor. It is necessary to look at the current in different modes for the device and determine the contribution to overall power consumption from each mode after taking into account the switching regulator efficiency at each current level. There are some quite impressive regulators around though, such as the new Analog Devices ADP5301 Step-down Regulators. The quiescent current of these devices
T
o meet the exacting targets of wearable devices, requires designers
to look into every aspect of the design, from the start up time of clocks, to the MOSFET switching times. Sleep mode is a natural first step for
any designer looking for a low power usage design. This will be especially important for wearable designs as they usually won’t be powered totally down. Most wearables will take periodic sensor readings and either store the reading until it can be sent, or send the reading immediately. It makes sense to put the design into
sleep mode between active periods. The device can be brought out of sleep mode by an interrupt, or by a physical input, such as a button push. How often the device is awake will depend on the application. Even within the application, sleep times can vary quite considerably. Dynamic sleep intervals are often used
to allow the device to judge how often it needs to make measurements. In the case of a fitness device, it can wake and check for movement. If there is movement, it will intelligently narrow the time between measurements. In contrast, if there is a lack of movement, it can
S10 OCTOBER 2016 | ELECTRONICS
extend the time until the next measurement, prolonging battery life. The communications protocol between
devices can be important for energy saving. I2C uses pull-up resistors, which dissipate energy. SPI doesn’t have pull- up resistors, so may prove a better choice. Another way that energy can be lost in communications is pin capacitance. To minimise this figure, reduce the data to be transferred as much as possible. To demonstrate how much energy can be lost, if four pins have a capacitance of 5pF in a system running at 20MHz from a 3.3V supply, 660μA will be drawn from just pin capacitance. This figure can be determined from the equation I = 0.5CVf. The current drawn will be the total of
both the data sent and received, which can mount up. This figure can be cut by using a highly integrated chip. Internal communications don’t suffer from pin capacitance, therefore having more peripherals on-board is better from a power consumption point-of-view. On- chip RAM and flash memory offer the same power savings. Choosing a switching regulator for a switched mode power supply is a key
Figure 1:
The diverse market range for wearbale devices is only just at the beginning of its capabilities and market adoption
is down as low as 180nA when not- switching, but still operating in hysteresis mode. It will switch for a short burst to add charge to the output capacitor using the inductor at very light loads, then return to just the quiescent current. The low quiescent current can give efficiencies as high as 80% at 1μA depending on the input and output voltages. It is more likely that there will be lower
figures in practice than this optimum value, but they should still be above 40%. The devices also deliver up to 0.5A and have a single pin programmable output with a fixed resistor. These figures are very impressive compared to older regulators, which would take a few milliamps with no load. If you are using a switching regulator
with an external MOSFET, bear in mind that the MOSFET switching time can result in significant losses. The transition from non-conducting to conducting is the time when a switching MOSFET dissipates the most power. When it is turned fully on, the voltage drop will usually be very small and hence power dissipation will be low. However, partly turned on there will be a significant voltage drop across the MOSFET accompanied by significant current. You therefore want to minimise the
time that the transistor spends in that state by choosing a fast switching device and low gate capacitance. Low ON resistance is a must.
/ ELECTRONICS
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