Designing a fully integrated buck-boost converter to extend Li-ion battery life

For portable power applications, to fully utilize the advantages of advanced battery technology such as small size and high energy density, it is necessary to achieve efficient operation over the entire battery discharge voltage range. This poses a severe challenge to lithium-ion battery-powered systems that require a 3.3V bus voltage. Although standard buck converters have high conversion efficiency when converting 4.2-3.0V lithium-ion battery voltages to lower output voltages such as 1.8V, and standard boost converters have high conversion efficiency when converting lithium-ion battery voltages to higher output voltages such as 5V, neither of the above converters is the best solution for generating the commonly used 3.3V bus voltage. Although SEPIC and traditional buck-boost can fully utilize the battery power, they have disadvantages such as low efficiency, high cost, large circuit board area and high number of components. The TPS6300x with three configuration structures can solve these problems. The TPS63000 has an adjustable output range from 1.2V to 5.5V. The fixed output voltages of the TPS63001 and TPS63002 are 3.3V and 5.0V respectively. All of the above products are available in a space-saving 10-pin QFN (DRC) package.

TPS63001

TI's TPS63001 features minimal component count, small board area, and low cost, and can efficiently convert lithium-ion battery input voltage to a 3.3V bus voltage. In addition to the buck and boost functions, it also integrates switching FETs, compensation and protection functions in a 3×3 mm QFN package. Only three external components are required to ensure operation: input and output capacitors and an inductor. The converter has a peak efficiency of 96% (see Figure 1) and a peak output current of 800mA, which is sufficient to power most portable loads. The wide input voltage range of 1.8 to 5.5V can work with many common power supplies, such as two or three alkaline batteries, NiMH batteries, and 3.3V and 5V buses.



Figure 1 TPS63001 efficiency graph at 1.8-5.5V [320mA load (VOUT=3.3v)]

Figure 2 shows a typical 3.3V power supply that can be powered by a single lithium-ion battery. The 1.5MHz switching frequency allows the use of a small 2.2μH inductor and small 0603-size ceramic input and output capacitors. High efficiency combined with a low number of external components reduces the total solution size to only 6×6mm (see Figure 3).



Figure 2 Typical application circuit


Figure 3 Typical layout in a 6×6mm board space

Advanced control topology maximizes efficiency

The TPS6300x is based on a standard H-bridge buck-boost power stage as shown in Figure 4, and includes both buck and boost switching FET configurations connected to a single inductor. Unlike the standard buck-boost mode that continuously switches four FETs simultaneously, the TPS6300x uses a proprietary modulator design that switches only two of the FETs at a time. This control mechanism significantly reduces unnecessary switching losses. The TPS6300x can operate in either buck or boost mode at higher efficiencies than conventional buck-boost mode, further reducing power losses.



Figure 4 Block diagram of the power section

When the Li-ion battery discharges to and below 3.3V, the buck-boost converter must switch from buck mode to boost mode. At this transition point, many buck-boost control schemes suffer from reduced efficiency, power supply jitter, or output voltage instability. The TPS6300x seamlessly transitions between buck and boost modes on a pulse-by-pulse basis as needed, providing constant PWM switching across the buck and boost ranges without overlap or dead time between the two modes.

More Features

The TPS6300x also includes other integrated features that enhance the experience in portable applications, such as extremely low quiescent current (less than 50μA), a user-selectable power save (PS) mode that maintains high efficiency at light loads, and external synchronization to help minimize system noise.

The average current mode control topology provides fast transient response and low output ripple in both buck and boost modes. The output voltage regulation tolerance is ±1% over the input and load range. The internal compensation is optimized for 2.2 to 4.7μH external inductors with 10 to 22μF output capacitance.

The short-circuit protection acts as a foldback current limit, and the maximum output current limit is reduced from 1.7A to 800mA when the output voltage drops 3%.

This reduces the device power consumption during output overload conditions. Normal operation can be resumed after the overload is removed. The advantage of this method is that large output capacitors such as supercapacitors can be charged.

The PS mode feature maintains very high efficiency even at light loads below 300mA. In PS mode, the switching time is only guaranteed to increase the output voltage to slightly above the output voltage set point, and then the switching is stopped until the output voltage drops below the set point again. This "on-then-off" switching mode provides extremely high efficiency at light loads.

Other Applications

The TPS6300x can also drive white LEDs (WLEDs) in current regulation mode, that is, the output voltage divider network is replaced by a resistor in the WLED loop. Since the typical forward voltage drop of WLED is 4.2-3.5V, it is problematic to power it with a lithium-ion battery in most power topologies because the power supply needs to both buck and boost its output voltage. The buck-boost function of the TPS6300x solves this problem well and can easily provide 500mA of current for lighting or flash applications.

Conclusion

The TPS6300x is an ideal solution for converting lithium-ion battery voltage to a 3.3V bus voltage. It has the characteristics of high efficiency, small circuit board area, low cost, and seamless transition from buck mode to boost mode. It is an ideal choice to help design engineers complete high-performance and fast designs.
Published by: Xiaoyu