Is there a general tool for implementing DC/DC voltage conversion?

Most applications or subcircuits require a constant voltage supply within a certain voltage tolerance range for proper operation. Battery-powered applications, such as wireless sensors and personal handheld devices, require voltage conversion to generate the desired output voltage as the battery discharges and the voltage drops with it. Applications powered by fixed power rails (such as optical modules, wired sensors, active cables, or dongles) may also require voltage conversion.

This article explores whether a buck-boost converter can be an ideal solution for voltage conversion and whether it can be a universal tool for any type of DC/DC voltage conversion.

When to use a buck-boost converter

Generally speaking, a boost converter can effectively convert a DC voltage to a higher voltage level if the available supply voltage for a circuit or subcircuit is lower than the desired voltage. The buck converter performs voltage conversion if the available supply voltage is higher than the required voltage.

A buck-boost converter, on the other hand, is suitable for applications where the supply voltage range is above and below the desired output voltage. The buck-boost converter consists of a buck converter and a boost converter, and the block diagram shown in Figure 1 more clearly reflects its structure.

Is there a general tool for implementing DC/DC voltage conversion?

Figure 1: Buck-Boost Converter Block Diagram

A buck-boost converter combines a buck converter (green in Figure 1) with a boost converter (orange in Figure 1) in the same structure, allowing the output voltage to be efficiently stepped up and down. The control loop can decide whether the device needs to operate in buck or boost mode based on the actual input voltage and the programmed output voltage.

For example, suppose a Li-Ion battery with a typical voltage range of 4.2V to 2.8V is to provide an output voltage of 3.3V. If a buck converter is used, the cutoff voltage of the battery must be greater than 3.3V. The disadvantage is that the battery cannot be effectively utilized. stored electrical energy. The buck-boost converter helps to fully utilize all the power of the battery because the buck-boost converter can also consume stored power when the input voltage is at or below 3.3V, as shown in Figure 2.

Is there a general tool for implementing DC/DC voltage conversion?

Figure 2: A buck-boost converter can completely drain the battery

Using a Buck-Boost Converter as a Voltage Regulator

A second common use for a buck-boost converter is as a voltage regulator. If there is a change in the power rail (eg 3.3V ± 10% variation) and the load requires a more precise regulation voltage (eg 3.3V ± 5% tolerance), then a buck-boost converter that can regulate the voltage is required . If components are sensitive to supply voltages (such as transimpedance amplifiers in optical modules); if other DC/DC pre-regulators are not tightly regulated in industrial applications; or if other components in the power path (such as electrical fuses, load switches, or long cables) increasing the voltage change as the current increases, the voltage may need to be regulated more tightly. This problem cannot be solved with a boost converter or a buck converter alone. However, buck-boost converters are able to regulate changing input voltages to the tighter limits required. Figure 3 shows the transient response of the TPS63802 to a fast ±0.5 V/10 µs line with significantly less than ±0.1V output voltage undershoot/overshoot.

Is there a general tool for implementing DC/DC voltage conversion?

Figure 3: TPS63802 Line Transient Response at VI = VO = 3.3V, ΔVI = ±0.5V

Other Applications for Buck-Boost Converters

There are other reasons to choose a buck-boost converter rather than simply a buck or boost converter. One of the reasons is power ORing. Imagine a device like a baby monitor powered by a 5V USB wall adapter or two AA main batteries, ranging from 3V (when the batteries are new) to 1.6 V (when the batteries are dead). Only a buck-boost converter can handle a wide input voltage range from 5V (wall adapter) to 1.6V (no wall adapter connected and the battery is dead) and still produce a 3.3V rail for the system. In addition to the buck-boost converter, you only need two external diodes to avoid cross currents from the wall adapter to the battery and to switch seamlessly to the battery when the wall adapter is unplugged.

Buck-Boost Converter Limitations

The internal control loop of a buck-boost converter is usually designed to continuously switch between buck and boost modes when the input voltage is close to the output voltage. This operation is acceptable, but still has some drawbacks: mode switching may result in different switching frequencies, higher output voltage ripple, and more electromagnetic interference (EMI). The second negative effect is that the efficiency may drop slightly at this time.

To avoid the negative effects of mode switching, look for devices with dedicated buck-boost modes that keep the output voltage ripple low. For example, TI’s new TPS638xx family of buck-boost converters features buck-boost mode and hysteresis to avoid switching between easily filtered noise spectrum and lower EMI.

Can a buck-boost device handle all types of voltage transitions?

Since a buck-boost converter “contains” a buck converter and a boost converter, it can be used for DC/DC voltage conversion. So from this perspective, the answer is yes. But more details still need to be considered. Table 1 illustrates whether a given input voltage range versus desired output voltage can be a good solution for a buck, boost, or buck-boost converter.


Table 1: Overview of DC/DC Conversion Topologies

Let’s go back to the original question: Is there a universal tool for implementing DC/DC voltage conversion? Actually not sure. When a buck-boost converter is not required, analog designers working on high-volume product development will prefer the performance optimization of a dedicated boost or buck converter. However, designers working on low-volume product development may decide that certain trade-offs are worth considering.

Using buck-boost conversion (to buck-boost, buck, and boost) provides the following benefits:

Scale across projects to save time and reduce design risk.

The number of different DC/DC converters can be reduced to a list of easy-to-use buck-boost converters.

Purchasing procedures can be simplified, inventory complexity reduced, and price leverage and supply stability improved.

Buck-boost can disconnect the load from the power supply during shutdown, while other topologies may require additional load switches.

Other resources

· Learn how to choose a DC/DC converter that maximizes battery life in pulsed-load applications.

· Learn how to use a non-inverting buck-boost converter for voltage regulation.

· Learn how precise thresholds enable pins to prevent over-discharge of the battery.

The Links:   LQ150X1LG92 SKM200GB128D

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