““Smaller size, lower cost, and higher efficiency” are the market requirements for a new generation of portable devices, but it is difficult for design engineers to optimize these three requirements independently. An optimized solution must be based on overall system requirements, taking into account factors such as size, cost, and work efficiency. For design engineers, there are many choices for power topology, including buck converters, low dropout regulators (LDOs), buck/boost converters, etc., but each has its own advantages and disadvantages, and the choice should be weighed .
“Smaller size, lower cost, and higher efficiency” are the market requirements for a new generation of portable devices, but it is difficult for design engineers to optimize these three requirements independently. An optimized solution must be based on overall system requirements, taking into account factors such as size, cost, and work efficiency. For design engineers, there are many choices for power topology, including buck converters, low dropout regulators (LDOs), buck/boost converters, etc., but each has its own advantages and disadvantages, and the choice should be weighed .
This article will discuss various power topologies, especially the pros and cons of converting Li-Ion battery voltages to the 3.3V voltage rail (the supply voltage for most portable devices). This article will also explain the different applications of buck/boost converters and explain why buck/boost converter solutions need to be “tailored”.
As can be seen from Figure 1, the design of converting the Li-Ion battery voltage to a 3.3V rail is challenging. A typical Li-Ion battery discharge curve starts at 4.2V when fully charged. The starting point of the X axis is “-5 minutes”, and the corresponding voltage is the open circuit voltage when the battery is fully charged. At “0 minutes”, the battery is connected to the load, and the voltage begins to drop due to the internal impedance and the protection circuit. The battery voltage slowly drops to about 3.4V, and then the voltage begins to drop rapidly as the discharge cycle is nearing the end. To fully utilize the battery’s stored power, the 3.3V rail requires a step-down converter for most of the discharge cycle and a boost converter for the remainder of the discharge cycle.
Figure 1: 1650mA-hr 18650 Li-Ion battery discharge curve.
The question of how to efficiently generate a 3.3V rail from Li-Ion battery voltage has been around for a long time, and its solutions have been varied. This article discusses several common solutions, including cascaded buck-boost, buck/boost, buck, and LDO power topologies, and discusses the pros and cons of each design, as well as system runtime measurements and comparisons.
Cascaded Buck and Boost Converter Solutions
A cascaded buck and boost converter consists of two separate and separate converters, a buck converter and a boost converter. A buck converter regulates the voltage at a medium voltage (like 1.8V), while a boost converter steps up the medium voltage to 3.3V. Being able to utilize 100% of the battery power makes this architecture ideal for systems requiring lower voltage rails. However, due to the two-stage conversion mechanism, this is not the best solution from an efficiency standpoint.
Effective power conversion efficiency is the product of the buck regulator efficiency and the boost regulator efficiency. Operating under the above voltage conditions, the typical efficiency value of both buck and boost converters is 90%, so the effective power conversion efficiency of the 3.3V converter is 90% × 90% = 81%. Since the architecture includes two independent converters, the number of components and the size of the system are increased, which is not only difficult to implement in small portable products, but also increases the cost.
Standalone Buck Converter Solutions
Using a step-down converter can also convert the lithium-ion battery voltage to 3.3V, but this solution is often overlooked and not widely used. Design engineers typically abandon this solution after looking at the battery discharge curve (shown in Figure 1) because the buck regulator cannot generate 3.3V as seen from the battery full discharge curve (shown in Figure 1). voltage rail. Many buck converters go into 100% duty cycle mode when the input voltage of the buck converter drops close to the output voltage. Under this condition, the converter stops converting and outputs the input voltage directly. In 100% duty cycle mode, the output voltage is equal to the input voltage minus the voltage drop of the converter. This voltage drop is determined by the (MOSFET on-resistance, the DC resistance of the output Inductor, and the load current), which sets the minimum battery voltage that is still in regulation. Let’s say the system thinks the 3.3V rail is down 5% and is still in regulation , the minimum battery voltage at which the system works can be calculated using the following equation.
Among them: Vout_nom is the rated value of 3.3V, Rdson is the on-resistance of the power MOSFET, RL is the dc resistance of the output inductor, and Iout is the output current of the converter at 3.3V.
When the battery voltage drops to Vbattery_min, the system must shut down below the minimum tolerance to avoid corrupting data by running on the 3.3V rail. The system may shut down even if the battery still has 5-15% charge remaining. How much battery power is left before the system shuts down depends on a variety of factors, including component resistance, load current, battery age, and ambient temperature.
Most design engineers would avoid using a separate buck topology for this reason, but a closer look at actual system runtime shows that standard buck/boost, cascaded buck, and boost topologies have lower conversion efficiencies than separate buck topologies. The efficiency of the voltage converter is much lower. Although these topologies make good use of battery power, they are far less efficient than buck converters. In many cases, the stand-alone buck converter runs longer than the other two topologies. Until 2005, fully integrated buck converters were considered the best option for generating 3.3V rails.
Low Dropout Regulator Solutions
Another less commonly used solution is the LDO. Similar to the “buck alone” solution, the LDO cannot fully utilize the full battery power because it can only stabilize when the input voltage is greater than the sum of the output voltage and the LDO voltage drop. pressure. If the voltage drop of the LDO is 0.15V, when the battery voltage is lower than 3.3V+0.15V=3.45V, the 3.3V output voltage starts to drop. The battery power that cannot be fully utilized due to this solution is likely to be much more than the voltage drop solution alone. Despite these drawbacks, LDOs also have advantages in certain circumstances.
LDO solutions are typically the smallest in size, making them ideal when space is critical in the main system. LDO solutions are also typically the lowest cost, making them ideal for low-cost applications. Many design engineers gave up using this solution due to the inefficiency of LDO, but after careful study, it can be found that the efficiency in this application is still good:
When the starting voltage of a fully charged Li-ion battery is 4.2V, the initial efficiency of the LDO is 78%, and its efficiency increases with decreasing battery voltage.
Buck/Boost Converter Solutions
Buck/boost topologies are widely used. This topology combines all the advantages of the other solutions mentioned above. As the name suggests, this topology has both buck and boost functions, so it can utilize 100% of the battery power.
The way a buck/boost converter is deployed determines its extremely high conversion efficiency. For example, Texas Instruments (TI) fully integrated buck/boost converter TPS63000 achieves a conversion efficiency of about 95% from 3.6V to 3.3V. High conversion rates mean that battery power is fully utilized for maximum runtime. A fully integrated buck-boost converter with integrated power switches, compensation components, and feedback circuits is not at a disadvantage compared to the component count and size of a buck solution, and requires only input capacitors, output capacitors, and inductors for external components . Highly integrated single-chip IC solutions help reduce overall system cost.
The buck/boost power stage is shown in Figure 2. The topology consists of a buck power stage with 2 power switches and a boost power stage with 2 power switches, which are connected by a power inductor. These switches can operate in three different modes: buck/boost mode, buck mode, and boost mode. Specific IC operating modes have specific input-to-output voltage ratios and IC control topologies.
Figure 2: A buck/boost power stage consists of a buck power stage with 2 power switches and a boost power stage with 2 power switches.
Buck/Boost Converters Are Not All The Same
The need for buck/boost converters in portable applications has been around for a long time, but their size and efficiency requirements are often very stringent. Only recently has semiconductor packaging technology developed to integrate four MOSFET switches and corresponding control loops into a small package.
Although different buck/boost solutions have the same power stage topology, the control circuits vary widely. Three standard buck/boost converters are currently available, the first with four MOSFET switches active during each switching cycle, which produces standard buck/boost waveforms. A careful analysis of these waveforms shows that the effective current (RMS) through the inductor and MOSFET is much higher than that of a standard buck or boost converter, which results in increased conduction and switching losses in a standard buck/boost converter. Running 4 switches synchronously also increases gate drive losses, resulting in a sharp drop in efficiency at low output current states.
The second new buck/boost control method operates only 2 MOSFETs per switching cycle, reducing losses. As can be seen from Figure 2, this control scheme can operate in three different modes. When Vin is greater than Vout, the converter turns on Q4 and turns off Q3, and then uses Q1 and Q2 as a standard buck converter; when Vin is less than Vout, the control circuit turns on Q2 and turns off Q1, and then uses Q3 and Q4 as a standard boost converter is used. However, this control mode presents some operational and control issues in the transition region between buck and boost modes. To address these issues, a standard buck/boost mode can be used during the conversion process. These control issues can be resolved because all four switches are active in standard buck/boost operation. But the increase in switching losses and RMS current makes the efficiency dip in the conversion region, and this efficiency dip region is close to the battery voltage (where most of the battery charge is provided at this time), so in most regions of the battery discharge curve, the converter works in the inefficient buck/boost mode.
The third buck/boost control mode eliminates the transition region between buck and boost modes, resulting in significant performance and efficiency improvements. TI’s TPS63000 buck/boost converter includes an advanced control topology that addresses various issues faced by standard buck/boost converters. Regardless of the mode of operation, the TPS63000 has only two switches active per switching cycle, which not only reduces power consumption, but also maintains high efficiency during the full battery discharge curve. Unlike some solutions, the TPS63000 integrates all compensation circuits and operates with only 3 external components, minimizing product size.
Figure 3 shows the corresponding relationship between the discharge curve and the running time when the lithium-ion battery voltage drops to 3.3V in the four solutions.
These solutions include cascaded buck and boost converters, standalone buck converters, LDO converters, and the TPS63000 buck/boost converter. The picture uses a fully charged 18650 lithium-ion battery with a capacity of 1650mAHr. The load current is 500mA, and the system shuts down when the 3.3V rail voltage is 5% below the initial setting. It is required to use the same battery here to avoid data deviation due to differences in battery capacity. As we expected, the LDO had a shorter runtime of 190 minutes, while the buck/boost converter had the longest runtime at 203 minutes and the cascaded buck/boost solution had the shortest runtime , only 175 minutes.
Other factors to consider
The data in Figure 3 was measured under constant DC load conditions, which is a common practice for performance testing, but differs from actual applications. To keep portable applications running for long periods of time, connect the load only when needed and disconnect the load when not needed. Displays, processors, and power amplifiers are the primary sources of significant transient currents on the system battery, and their load fluctuations will cause the voltage on the battery bus to drop due to the battery’s internal source resistance, protection circuits, and distribution bus impedance. If these load fluctuations occur at the end of the discharge cycle, the battery voltage can be reduced to less than 3.3V. Using a buck or LDO solution may result in premature system shutdown, while a buck/boost solution continues to operate through transients, extending system runtime.
The load transient current that is not obvious during the laboratory test is very obvious in the practical application, because the internal impedance of the lithium-ion battery doubles after 150 charge/discharge cycles; when the operating temperature is 0?C Between ~25?C, its internal impedance will also double.
Summary of this article
There are many design options for converting Li-ion battery voltage to 3.3V, and the design engineer can choose the best solution based on the specific requirements of the system. A buck/boost converter is suitable for most systems because it has the longest runtime, smallest size, and relatively low cost, making it the best overall solution for most portable applications.
When choosing a buck/boost converter, it is important to understand that the characteristics of various buck/boost converters are not the same, and it is important to pay attention to factors such as the operating mode, the efficiency of the entire battery operation phase, and the overall size of the solution.
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