Buck-Boost Enables High Power Density and Higher System Efficiency for Systems with Back-Up Battery

要約

Systems equipped with critical communication functions may come with a backup battery in case of any power outages or glitches to ensure continuous operation. Many of these systems are catching on to USB C trend to receive and provide power. Wide system input voltage could vary from 12V down to 2.7V depending on the backup battery chemistry and configuration. Given such a wide input voltage range, the power architecture can be quite complex. In this design solution we show how a small, monolithic buck-boost converter can simplify the system power design by providing higher power, optimizing solution size and system efficiency.

Introduction

Systems equipped with critical communication features utilize backup batteries to maintain communication during any power outages or glitches. This approach is widely used in security and smart home monitor panels (as shown in Figure 1), specialty tablets, mobile payment terminals, etc. Some systems powered by AC adaptors are moving to USB C power as new designs shift toward the universal and interchangeable USB C adaptor for flexibility. Utilizing USB C as an input power source, 12V is the most common implementation to provide power to the entire system. Backup batteries can vary quite a bit but single- or dual-cell lithium-ion or three to four cell alkaline/NiMH batteries are most widely used. This means when the system is getting power from the backup battery, the input voltage can be as low as 2.7V. Overall, the input voltage for the systems mentioned above has a wide range from 2.7V to 12V.

Critical communication modules built into these systems also can vary quite a bit, including, but not limited to, Bluetooth, Wi-Fi, and long-term evolution (LTE). While communication modules are different, they generally require power between 3.3V to 3.6V and ranges from tens of milliamps to hundreds of milliamps of current. Some systems come with speakers which require 5V of power supply. There may also be a downstream USB port demanding 5V and depending on the regulation, require up to 2.4A of power. If a system implements USB C port and adopts USB C power delivery (PD) standards, the USB C port may demand higher voltage such as 9V and current up to 3A.

With such a wide input voltage range from 2.7V to 12V, generating consistent 5V and 3.3V output may seem trivial with multiple buck and boost converters. However, when considering the need to optimize system efficiency to extend battery life and when form factor becomes a limitation, the power design complexity increases, especially when USB C PD is involved.

The USB Type-C PD specification allows to increase the charge voltage up to 20V and current up to 5A for a maximum power of 100W. For portable devices other than laptops, 18W (9V/2A) provides quick charge to keep users on the go. The USB C PD specification adds another tier of complexity to the power design because an additional 9V power rail up to 2A needs to be considered.

This design solution discusses how a buck-boost converter simplifies the power design for such systems described earlier and also how it supports adding 9V, 2A, 18W capability to the USB C port and introduces a novel highly integrated and high-efficiency power management implementation.

Figure 1. Smart security monitor panel.

Typical Power System

Figure 2 illustrates a typical power design with a system fully equipped with a USB port, speaker, LTE, and Wi-Fi.

Due to the wide input voltage range, an intermedia bus voltage for the downstream buck converters is needed. This can be achieved with a bypass boost converter. When the adaptor voltage is present, the boost converter enters bypass mode where the intermedia bus is the adaptor voltage and the downstream buck converters generates 5V and 3.6V. When the adaptor power is missing and backup battery is in use, the boost converter boosts the battery voltage to a calculated voltage so the buck converters can operate within its duty cycle limitation.

Although this architecture supports all the power requirements, it is far from efficient, not to mention a high cost BOM and large PCB size. The two-stage power conversion, first boost then buck provides low system efficiency. In this case, assuming 90% efficiency from the boost converter followed by 93% efficiency from the buck converter results in an 83.7% overall system efficiency. Low system efficiency drains the backup battery when in use and to maintain operation during power outages, a larger battery capacity is needed which in turn adds to the system size and cost. On top of low system efficiency, four sets of DC-DC converters with quite a count of external components are needed and thus, additional PCB space and cost. As the system's power requirements increase, the power generated by the boost converter also increases, creating the need of an even larger inductor.

Figure 2. Typical power solution with a USB port, speaker, LTE, and Wi-Fi.

Discrete High-Power Delivery Solution

As USB C and USB PD trends, some designs may implement a downstream USB C port and provide USB PD capability for fast and convenient charging of peripherals illustrated in Figure 3.

Figure 3. Typical power solution with a USB C port supporting USB PD, speaker, LTE, and Wi-Fi.

In the design (Figure 3), in order to accommodate the USB PD dynamic voltage change, a buck-boost converter with DVS is required. This will separate the USB port power generation away from the previous intermedia bus to be generated independently. Since the output of the buck-boost is not fixed, a discrete or low integration buck-boost solution is used with a dedicated buck-boost controller. This approach provides and meets the requirement of USB C PD, however due to the low level of integration, it will take up a significant amount of PCB space. Figure 4 shows low-integration buck-boost setup requiring an active PCB area of 63mm², not considering other external components which makes the solution size even larger.

Figure 4. Low-integration buck-boost power solution PCB size (63mm<sup>2</sup> active area).

Figure 4. Low-integration buck-boost power solution PCB size (63mm² active area)

Integrated High-Power Delivery Solution

Figure 5 shows a highly integrated solution in which controller, driver, and MOSFETs are integrated on board of a single IC, the MAX77831. The MAX77831 transforms the typical two-stage (boost then buck) design into a single-stage conversion to provide high efficiency to support 5V speaker and 3.6V LTE and Wi-Fi. For 5V output, the MAX77831 can provide 92.8% efficiency, 9% higher compared to the 83.7% two-stage efficiency. Through I²C communication, the MAX77831 can provide buck-boost output dynamically up to 18W to support the USB PD capability for the downstream USB C port. As described earlier, 18W is sufficient to quickly charge a vast majority of peripherals including smartphones.

Figure 5. Highly integrated power solution with a USB port supporting 18W USB PD, speaker, LTE, and Wi-Fi.

Figure 6 shows the new PCB size. In this case, the active area is reduced to 21.9mm², a 61% saving! This solution also reduces the external component count by 50% to further reduce the solution size and cost.

Figure 6. Highly integrated power solution PCB size (21.9mm<sup>2</sup> active area).

Figure 6. Highly integrated power solution PCB size (21.9mm² active area).

Integrated Buck-Boost Converter

The MAX77831 monolithic buck-boost converter in Figure 7 is a high-efficiency, high-performance regulator targeted for systems requiring wide input voltage range (2.5V to 16V). It allows systems to change the output voltage and load current capacity dynamically through I²C interface. It supports the standard 5V USB VBUS requirement as well as 9V all the way up to 15V with total 18W of continuous power.

Figure 7. Highly integrated power solution application diagram.

The integration of low RDSON N-Channel MOSFET transistors on board assures superior efficiency. In Figure 8, the IC shows an efficiency advantage across most of the operating range, versus a competitor solution, with a full 3-point advantage at full load. Also, the IC can outperform competition with higher output power at higher efficiency, truly optimizing power density in any given design.

Figure 8. Power and efficiency competitive advantage.

The buck-boost operation is smooth, with near-seamless transition from one mode to another. Figure 9 shows both the small boost-to-buck transition overshoot (+110mV) and the buck-to-boost undershoot (-100mV).

Figure 9. Smooth buck-boost transition.

Conclusion

Connected devices continue to proliferate to various market segments and many are powered from adaptors with backup batteries to keep the system operational during power outages. This design solution reviewed the shortcomings of typical two-stage power conversion, large PCB space, and high cost due to low integration. Subsequently, it showed that the MAX77831 monolithic buck-boost converter is the best power solution to cope with such a wide input voltage range of 12V down to 2.7V from backup batteries, optimizing system efficiency for longer battery life, small PCB size, and lower BOM cost.