The market growth of smart watches continues unabated, fueled by a plethora of new features and applications. Personal health and fitness products include new features like tracking, alerts, and connectivity. The rise in smart watch-controlled automotive is fueled by features like emergency assistance and car status updates. Waterproof and shockproof features as well as brighter displays are driving the growth of this market globally. High-definition cameras, GPS receivers, tiny powerful speakers, ample storage, and other new features are on the horizon. However, the requirement for smaller form factors escalates the challenge of miniaturization in electronics and presents additional ones for longer battery life. This design solution presents an innovative power management system that delivers power with higher efficiency in a very small space, while enabling longer battery life for miniaturized wearable devices (Figure 1).
Figure 1. Smart watch in action.
Ideally, an efficient solution occupies minimal space and integrates the battery charger and required regulation to power the smart watch blocks all in one chip. A single-inductor multiple-output (SIMO) buck-boost converter that implements three switching regulators with a single inductor further reduces the occupied space. Each regulator’s topology is programmable, supporting buck-boost operation for voltages within and above the battery voltage range as well as providing buck operation—inherently more efficient than buck-boost—for voltages below the battery voltage. Finally, high-frequency operation allows the use of a small inductor, further minimizing the space required. Two on-chip LDOs are used for noise-sensitive loads or as load switches. Figure 2 illustrates a highly integrated SIMO PMIC. For simplicity, the external passives are not shown here.
Figure 2. Highly integrated SIMO PMIC block diagram.
Figure 3 is a system power tree, reporting each regulator’s output voltage, load current, efficiency and power dissipation (PD). Three out of five loads are supplied directly by the high-efficiency SIMO switching regulator. The LDOs for the fourth and fifth loads are also supplied by the SIMO, achieving 90% efficiency because of the low dropout (2V to 1.8V). The overall system efficiency is an outstanding 86.2%.
Figure 3. Power tree of a high-efficiency SIMO.
Figure 4 shows the block diagram of the SIMO converter (all the components, except the inductor, are integrated). The switching regulators deliver power with minimum losses, and the clever architecture eliminates the need to have one inductor for each switching regulator.
Figure 4. SIMO PMIC power block diagram.
In this hysteretic discontinuous current mode (DCM) regulator, the inductor is shareable since the inductor current always goes to 0.
In buck-boost mode, the inductor builds up current with M1 and M4 ‘on’ at the rate of VIN/L. When it reaches a programmed limit, the current is delivered to the selected SBBx output, via the M2 and M3_x transistors, as shown in Figure 5.
In buck mode, M1 and M3_ turn on, delivering current to the output while building current in the inductor at the rate of (VIN - VSBBx)/L. When the inductor current reaches the programmed limit, energy from the inductor is delivered to the output by turning on M2 and turning off M1.
Notice how in buck mode the current is directed to the output during the entire cycle, while in buck-boost the current is delivered to the output only during the phase in which M2 and M3_x transistors are on. The higher delivery of current to the output, per cycle, makes the buck converter the most efficient architecture.
Outputs are serviced in the order in which they are requested by their output error comparators, also known as first-in-first-out (FIFO).
Figure 5. SIMO current waveforms illustration.
As shown in Figure 5, the three switching regulators are serviced one at a time and the inductor current goes to 0A, avoiding cross-regulation issues.
Thanks to its SIMO switching regulators and its integrated LDOs, the small MAX77654 PMIC (WLP, 2.79mm x 2.34mm x 0.5mm), delivers power with minimum losses in a PCB space that is 41% less than the typical implementation (see next section). In Figure 6, all the PCB active and passive components are accounted for.
Figure 6. SIMO PMIC solution with minimal board space (19.2mm2).
The total board space occupied is merely 19.2mm2.
The ability to program buck mode whenever needed brings a greater efficiency advantage. In Figure 7, the SIMO buck operation shows a 10% efficiency advantage vs. the buck-boost operation. The programmable inductor peak current limit, IP_SBBX, is set to 0.5A.
Figure 7. Buck vs. buck-boost efficiency.
A typical smart watch power management system is shown in Figure 8. A PMIC implements the battery charger, a buck converter which powers the micro, and an LDO which powers the display. A second IC, a dual LDO, powers the sensors and Bluetooth®. For simplicity, the external passives are not shown here.
Figure 8. Typical hearable power flow diagram.
The complete power tree for a typical implementation is shown in Figure 9. In this typical space-constrained solution, a heavy use of LDOs results in an overall efficiency of only 73.8%.
Figure 9. Power tree of a typical solution with lower efficiency.
A comparison between the two solutions’ power performance is shown in Table 1.Table 1. SIMO Advantage vs. Traditional Solution
|Parameter||Traditional Solution||SIMO||SIMO Advantage|
|Li+ Battery Current||176.6mA||152.6mA||SIMO saves 24mA|
|System Efficiency||73.8%||86.2%||SIMO is 12.4% more efficient|
|Minimum Li+ Battery Voltage||3.3V due to 3.1V LDO||2.7V||SIMO allows more discharge|
The SIMO solution’s superior efficiency results in smaller battery drain, while its wider operating range, down to 2.7V, prolongs the smart watch’s untethered operation.
All active and passive components for the typical power flow diagram shown in Figure 9 are accounted for in the solution drawing of Figure 10.
Figure 10. Board space of a typical hearable solution (32.4mm2).
This typical hearable solution occupies a board area of about 32.4mm2, or 69% more than that of the SIMO-based solution (19.2 mm2). Here, the relatively low level of integration, the use of multiple LDOs, and bigger passives results in a solution that is inefficient in both space and power.
We discussed the advantages of powering a smart watch with a highly integrated PMIC based on the SIMO architecture. We compared it to a typical solution with a low level of integration that suffers from inefficiencies in terms of both PCB space and power consumption. Thanks to a unique SIMO architecture, the MAX77654 PMIC efficiently delivers more power in a smaller space, enabling longer battery life and smaller form factor for smart watches and other miniature portable applications.
Boost: Step-up switching regulator
Buck: Step-down switching regulator
LDO: Low dropout regulator
Li+: Lithium-ion battery
PCB: Printed circuit board
PMIC: Power management integrated circuit
SBBx: SIMO buck-boost voltage regulator output x
SIMO: Single-inductor multiple output voltage regulator