Tiny Buck-Boost Converter for Low Current Applications

Introduction

One common challenge for many battery powered portable applications is creating a regulated output voltage above or below the input source. Traditional buck-boost approaches, such as a dual inductor SEPIC converter or cascaded regulators, are unacceptable in most portable devices because of their large solution size and low efficiency. Smaller footprint, integrated charge-pump solutions can switch between buck and boost operation, but charge-pumps achieve good efficiency at only a few operating voltages, while efficiency dips below 50% at others. Another compact and simple approach forgoes a portion of the battery capacity and uses a buck (step-down) only solution, but the advantages are hard to justify when much of the battery capacity is not used, as with certain Li-Ion chemistries and a 3.3V output, or with two alkaline cells and a 3.0V or 2.5V output.

The LTC3531 is a single inductor 200mA buck-boost converter that generates a regulated output voltage from a wide input voltage between 1.8V and 5.5V while maintaining high efficiency. It is an excellent fit for low power applications where a tiny total solution size is required. The LTC3531 is available with fixed outputs [3.0V or 3.3V] or with an adjustable output that can be set between 2.0V and 5.0V. The fixed output versions require only two small ceramic capacitors, a miniature inductor, and the ThinSOT IC. All versions of the part are available in a thermally enhanced 3mm × 3mm DFN packages. A photo of the LTC3531 demo-board is shown in Figure 1.

Figure 1. Compact 3.3V buck-boost application.

Generating a Clean 5V from a Noisy USB Cable

Generating a clean 5V output from a USB cable or wall adapter can be a challenge when the combination of source impedance and load transients cause noise and voltage droops. USB cable voltage can vary between 5.25V and 4.35V, while the maximum allowed decoupling capacitance on the input is 10μF. The trace labeled “VIN” in Figure 2 shows what can occur with a 4.7μF input capacitor and a 100mA load step from a powered device. The LTC3531 produces a clean 5V output (VOUT) with less than 100mV of peak-to-peak ripple using a 22μF VOUT capacitor. VIN and VOUT are DC aligned at 500mV per division in Figure 2, showing significant improvement in noise and voltage droop. Inductor current is also shown with operation in both boost (VIN < VOUT) and buck-boost (VIN ≅ VOUT) modes.

Figure 2. Noisy USB cable input to clean 5V output.

A complete schematic of the USB to 5V application is shown in Figure 3 along with efficiency and power loss curves versus load current. The LTC3531 operates in Burst Mode operation, with just 20μA of quiescent current, providing high efficiency over several decades of load current. All four switches have an RDS(ON) of about 0.5Ω when operating at 5V, providing >90% efficiency at higher load currents.

Figure 3. USB to 5V application.

Maximizing Li-Ion Capacity for 3.3V

When compared to a straight buck converter, the LTC3531 allows lower input voltage operation when providing a 3.3V output from a Li-Ion input source. Typical capacity curves for coke and graphite anode Li-Ion batteries are shown in Figure 4. Coke types have a lower cut-off voltage at 2.5V, where graphite types have a flatter discharge curve and a 3.0V cut-off. Solid lithium polymer batteries have discharge curves similar to graphite.

Figure 4. Typical 1C lithium-ion/polymer capacity curves.

The equivalent series resistance (ESR) of the Li-Ion battery causes additional voltage drops at the terminal at higher load currents. To make matters worse, the battery protector circuit adds additional series resistance and the effects of ESR lower system efficiency as the battery is discharged. To guarantee a 3.3V output, a buck only design may need to use a cut-off voltage of 3.5V or 3.6V. This translates to a capacity loss of approximately 45% for the coke cell and 20% for the graphite—both significant reductions in run time. Furthermore, while graphite or polymer cells are more popular because of their flat discharge curve, new chemistries with greater capacity per volume are on the horizon with expected discharge curves resembling the coke anode.

The wide input voltage range of the LTC3531 allows a regulated 3.3V to be produced from all Li-Ion chemistries, two or three alkaline cells, or a 5V source such as USB. The LTC3531 automatically transitions between buck, 4-switch (buck-boost), and boost modes based on the voltage difference between VIN and VOUT. Figure 5 shows a 3.3V application circuit, along with efficiency vs input voltage for a 100mA load. Maximum load current capability vs input voltage (VOUT = 3.3V) is shown in Figure 6. As expected, efficiency and load current capability are reduced with input voltage.

Figure 5. Lithium Ion to 3.3V schematic and 100mA efficiency curve.

Figure 6. Output current capability vs input voltage (VOUT = 3.3V).

3.0V Flash Memory Application from Two Alkaline Cells

Inexpensive MP3 players and other relatively low capacity, low cost portable devices often replace a hard disk drive (HDD) with flash memory and Li-Ion batteries with disposable alkaline—a good fit for the LTC3531. A complete schematic for a two cell alkaline to 3V flash memory supply is shown in Figure 7. Efficiency is better than 80% with fresh alkaline cells and better than 70% with depleted batteries. Note that overall efficiency is lower in this application (relative to the Li-Ion application) because of the lower drive voltage for the switches—switch RDS(ON) increases with decreasing drive voltage. The adjustable version of the part can be used to power lower voltage flash memories (i.e. 2.5V) with similar performance results.

Figure 7. Two AA or two AAA to 3.0V application.

Peak current requirements for flash memory are typically lower than a HDD, since there is no disk spin-up, but load transients still occur when memory is accessed. The LTC3531’s response to a 10mA–100mA load step is shown in Figure 8. Burst Mode® operation provides a rapid transient response since there is no compensation loop to slew. Peak-to-peak voltage ripple plus load step is under 50mV with a 47μF output capacitor. The output voltage ripple stays fairly constant over input voltage. Half the output capacitance (22μF) results in approximately twice the load step plus voltage ripple (100mV).

Figure 8. Transient response of the circuit in Figure 7.

Summary

The LTC3531 provides a simple, compact buck-boost solution for lower current, portable applications. A complete solution, 1mm in height, can fit in a 35mm2 footprint. The part maintains high efficiency over a wide range of input voltages and load currents, extending battery run time, while providing the flexibility to address many designs such as 2-cell alkaline, USB, and present day or emerging Li-Ion chemistries.

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Eddy Wells