Analysis and comparison of power system topology options for portable systems - News - Global IC Trade Starts Here 工业

This article will explore various power supply topologies, particularly focusing on converting Li-Ion battery voltages to 3.3V voltage rails, which is the standard supply voltage for most portable devices. Additionally, we'll examine the diverse applications of buck/boost converters and explain why tailored buck/boost converter solutions are often necessary.

As shown in Figure 1, designing a circuit to convert a Li-Ion battery voltage to a 3.3V voltage rail presents unique challenges. When fully charged, the typical lithium-ion battery discharge curve starts at around 4.2V. The X-axis begins at "-5 minutes," representing the open-circuit voltage when the battery is fully charged. At "0 minutes," the battery is connected to the load, causing the voltage to drop due to internal impedance and protection circuits. The voltage gradually falls to approximately 3.4V, then quickly drops as the discharge cycle nears completion. To maximize the battery's stored energy, the 3.3V rail requires a step-down converter for most of the discharge cycle and a boost converter for the remainder of the cycle.

Figure 1: Discharge curve of a 1650mA-hr 18650 lithium-ion battery.

The challenge of efficiently generating 3.3V voltage rails from lithium-ion batteries has existed for some time, with various solutions proposed. This article examines several common approaches, including cascading buck and boost converters, standalone buck converters, buck/boost converters, and low-dropout regulators (LDOs), discussing the pros and cons of each design and comparing their impact on system runtime.

Cascaded Buck and Boost Converter Solutions

The cascaded buck and boost converters consist of two separate converters: a buck converter and a boost converter. The buck converter stabilizes the voltage at a medium level (e.g., 1.8V), while the boost converter steps up the medium voltage to 3.3V. This architecture is highly effective for systems requiring lower voltage rails since it ensures 100% battery power utilization. However, the two-stage conversion process results in reduced efficiency compared to single-stage solutions.

The overall power conversion efficiency is the product of the efficiencies of the buck and boost converters. Under typical conditions, both converters achieve around 90% efficiency, leading to an overall efficiency of 81%. Due to the dual-converter setup, this approach increases the number of components and the overall system size, making it less suitable for compact portable devices and increasing costs.

Independent Buck Converter Solution

Using a standalone buck converter to convert the Li-Ion battery voltage to 3.3V is another option. However, this solution is often overlooked due to the battery discharge curve (as shown in Figure 1). The buck regulator cannot generate 3.3V from the entire discharge curve of the battery. Many buck converters enter a 100% duty cycle mode when the input voltage approaches the output voltage. Under this condition, the converter stops switching and simply outputs the input voltage directly. The output voltage equals the input voltage minus the voltage drop across the converter, which depends on the MOSFET on-resistance, the DC resistance of the output inductor, and the load current. This determines the minimum battery voltage that remains within regulation.

Assuming the system maintains the 3.3V rail at 95% of its nominal value, the minimum battery voltage for system operation can be calculated using the following formula:

Vbattery_min = Vout_nom × 0.95 + (Rdson + RL) × Iout

Where Vout_nom is the nominal 3.3V output, Rdson is the power MOSFET on-resistance, RL is the output inductor’s DC resistance, and Iout is the 3.3V output current.

When the battery voltage drops to Vbattery_min, the system typically shuts down to prevent damage to the 3.3V rail. Even if the battery retains 5-15% of its capacity, the system might turn off prematurely. The exact amount of remaining battery power before shutdown depends on factors like component resistance, load current, battery age, and environmental conditions.

Most engineers opt against using this standalone buck topology due to these limitations. However, closer examination reveals that the conversion efficiency of standard buck/boost or cascaded buck and boost topologies is often lower than that of a single buck converter. While these architectures maximize battery usage, they are less efficient than standalone buck converters. In many cases, a standalone buck converter provides longer runtime than other options. By 2005, the fully integrated standalone buck converter was considered the optimal choice for generating a 3.3V rail.

Low Dropout Regulator Solutions

Another less common solution involves low dropout regulators (LDOs). Similar to the standalone buck solution, LDOs do not fully utilize the battery's stored energy since the input voltage must exceed the output voltage plus the LDO voltage drop. For example, if the LDO voltage drop is 0.15V, the 3.3V output voltage starts dropping once the battery voltage falls below 3.45V. This inefficiency means more battery power remains unused compared to standalone voltage regulators.

Despite these drawbacks, LDOs offer specific advantages. They are typically the smallest in size, making them ideal for systems with strict space constraints. Additionally, LDO solutions tend to be the least expensive, making them suitable for budget-conscious applications. Many engineers dismissed LDOs due to their inefficiency, but further analysis shows that they perform well in certain scenarios.

When the fully charged lithium-ion battery starts at 4.2V, the initial efficiency of the LDO is 78%, improving as the battery voltage decreases.