COVER STORY FEATURE


rectification to optimise efficiency over a wide range of load currents. It can offer over 90% efficiency for loads ranging from 15µA to 50mA and only requires 720nA of no load quiescent current in regulation, thereby extending battery life. The combination of a 3mm x 3mm DFN package (or MSOP-10) and only five external components offers a very simple and compact solution footprint for a wide array of low power applications. Figure 1 shows a typical LTC3388 application circuit.


actuators may also benefit. An LDO works great for such rails since they have low output ripple.


 Bluetooth/RF connectivity system rails also require low noise. A low dropout regulator or, since output currents can be high, an LDO post-regulated switching regulator is a good choice.


 Processor power (the “brains” of the wearable). From TI OMAP, ARM Cortex MCUs, DSPs, GPS chips or FPGAs for example, have a variety of low-voltage rails, spanning low level to high currents. These can be powered by LDOs or switching regulators.


 Batteries need care and feeding so as to avoid overcharging which thus reduces battery cycle life. Accurate battery chargers with onboard termination algorithms ensure longer life for the cell(s).


 Compact size and low weight make the wearable device more comfortable for the user. ICs in compact packages provide small solution footprints, thus enabling the device to be offered in a small form factor.


 A feature-rich wearable product means many system rails. Multiple output regulators or power management integrated circuits (PMICs) may well fit the bill. Finally, compact ICs with battery chargers integrated onboard provide a higher level of integration and flexibility.


ULTRALOW IQ IC SOLUTIONS


It is clear that an IC solution that solves the application needs, as well as the associated issues already discussed should have many of the following attributes:  Ultra-low quiescent current, both in operating mode and shutdown


 Wide input voltage range to accommodate a variety of power sources


 The ability to efficiently power multiple system rails


 Accurate battery charge voltage to prevent overcharging


/ MICROMATTERS


 The ability to charge popular battery chemistries such as Lithium


 Simple and autonomous charging operation with onboard charge termination algorithms (no µC needed)


 Small and low profile solution footprints  Advanced packaging for improved thermal performance and space efficiency Fortunately, Linear’s recent ultralow IQ


LTC3388/-x buck regulator family, and its LTC3553 combination buck regulator & single-cell Lithium battery charger PMIC have most of these attributes already. The LTC3388 is an ultralow quiescent


current synchronous buck converter that can deliver up to 50mA of continuous output current from a 2.7V to 20V input supply. This device has a ‘no-load’ operating current of 720nA making it ideal for a wide range of battery- powered and low quiescent power applications, including “keep-alive” supplies and wearables. The LTC3388 utilises hysteretic synchronous


Figure 2: The LTC3553 simplified application schematic


FOR HIGHER SCALE OF INTEGRATION – USE A PMIC The LTC3553 is a micropower multi- function PMIC for portable Li-Ion/ Polymer battery applications. This device integrates a USB-compatible linear PowerPath control manager, a standalone battery charger, a 200mA high efficiency synchronous buck regulator, a 150mA LDO and pushbutton control, all in a compact 3mm x 3mm QFN package, (see Figure 2 for its typical application circuit). For applications operating with low currents in standby mode, the IC’s pin-selectable Standby Mode reduces battery drain current to only ~12µA while maintaining regulation of all outputs, see Figure 3’s graph for details. The LTC3553 is highly suited for personal navigation devices (PNDs), media players, portable medical and industrial devices and other small- battery portable device applications with small capacity batteries that consume low amounts of power. The smart wearables market is


Figure 3: LTC3553 battery drain current operation


exploding and includes a wide variety of products for the health & fitness, medical, infotainment, military and industrial application spaces. The core architecture for a smart wearable device depends on the product type, but essentially boils down to a micro- controller, MEMS sensor(s), wireless connectivity, battery and support electronics. Powering a low-current wearable device can prove very challenging. However, Linear Technology offers a large portfolio of products capable of very high performance at such low power levels. Devices such as the ultralow IQ


LTC3388 energy harvesting


buck regulator, and for a higher level of integration, the LTC3553 Lithium battery charger and buck regulator/LDO combination PMIC can enhance the operating life and overall performance of many future wearable devices.


Linear Technology (UK) Ltd. Now part of Analog Devices www.linear.com 01628 477 066


MICROMATTERS | AUTUMN 2017 13


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