Designing power architectures: Creative use of buck regulators

Article By : Haifeng Fan

In this article, the buck regulator IC will be used to illustrate various application designs.

Switching regulators are widely used in industrial, infrastructure, communications, consumer, and automotive applications. One of the most popular switching regulator topologies is the buck converter (also known as a step-down converter). Many power management IC manufacturers offer buck regulator ICs with built-in controller and integrated FETs. Such buck regulator ICs are primarily intended to implement buck converters realizing step-down conversion. Nevertheless, they can be used to create many other designs to meet various application needs, such as inverting power supplies, bipolar power supplies, and isolated power supplies with single or multiple isolated voltage rails. This article introduces a variety of designs using buck regulator ICs, explains their operational principles, and discusses the practical considerations to implement these designs.

Step-down converter with a buck regulator IC

The ISL85410 buck regulator IC will be used to illustrate various application designs.

A step-down converter is required when the desired voltage level is lower than the available voltage source in the system. For example, take a system that has a 12V battery as input, but lower voltage rails such as 5V, 3.3V, or 1.2V are desired to power microcontrollers, I/O’s, memory, and FPGAs. By efficiently converting a high voltage to a low voltage, the buck converter extends the system’s battery life, reduces heat dissipation, and improves reliability. Figure 1 shows the simplified schematic of a step-down converter.

Figure 1 Simplified schematic of a step-down converter

The output voltage has the same polarity as the input voltage, and the voltage conversion ratio in continuous-conduction-mode (CCM) can be expressed as:

Where D is duty cycle and ranges from 0 to 1, which indicates the output voltage (VOUT) is always less than or equal to the input voltage (VIN).

Inverting power supply with a buck regulator IC

While positive voltages are commonly used and available in electronic systems, negative voltages are sometimes also required. In such cases, an inverting power supply will be required to generate a negative voltage from a positive input. The inverting buck-boost converter is one of the popular solutions to address these application needs.

Figure 2 compares the power stage of a buck converter with an inverting buck-boost converter, showing that an inverting buck-boost converter can be derived by switching FET Q2 and inductor L1. This topology change results in different voltage conversion ratio and inverting polarity of the output voltage:

In an inverting buck-boost converter, the output voltage amplitude can be either higher or lower than the input voltage, and the output voltage is negative with respect to ground of the input voltage source.

Figure 2 Power stages of buck converter and inverting buck-boost converter

The inverting buck-boost converter can be implemented with a highly integrated buck regulator IC. Figure 3 shows a simplified implementation example using the ISL85410 buck regulator. When configuring a buck regulator as an inverting buck-boost converter, power designers must pay attention to two major differences. The first difference is the connection of the return (RTN) of input voltage source (VIN). In the buck converter shown in Figure 1, the RTN of input voltage source is also the device ground (i.e., the AGND/PGND pins of the buck regulator), while the RTN of the input voltage source and the device ground are no longer the same in an inverting buck-boost converter. Hence, the input voltage source must be applied across VIN pin and RTN instead of AGND/PGND pins when implementing an inverting buck-boost converter.

The second difference is the voltage stress on VIN pin with reference to AGND pin. This voltage in a buck converter is always equal to input voltage (VIN) regardless of the output voltage. By contrast, the VIN pin in an inverting buck-boost converter has to tolerate the sum of input voltage and output voltage (VIN+VOUT). For instance, in a design converting 24V to −5V, the voltage stress on VIN pin is 29V rather than 24V. Keep in mind that the voltage stress on VIN pin should never exceed the absolute maximum voltage rating specified in the IC datasheet.

Figure 3 Simplified inverting buck-boost converter implementation

[Continue reading on EDN US: Bipolar power supply with a buck regulator IC]

Haifeng Fan is principal applications engineer with Renesas Electronics’ Industrial Analog and Power Business Division.

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