Voltage Input-to-Output Control for Linear Regulators—Part 2: Operation and Reference Designs

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Abstract

This is Part 2 of a two-part article series on voltage input-to-output control (VIOC) in low dropout regulators (LDOs). Building on the foundational concepts introduced in Part 1, this article expands on VIOC system design, demonstrating how the latest generation of LDOs can maintain a constant input-to-output voltage differential to achieve key performance advantages such as improved power supply rejection ratio (PSRR), optimized power dissipation, and robust fault protection. It highlights the simplicity of implementing VIOC through reference designs and convenient evaluation methods, including LTspice® simulations and demonstration hardware. The article also explores VIOC integration in negative voltage topologies and reviews earlier VIOC implementations, including those using discrete components and legacy LDO architectures. By streamlining coordination between switching regulators and LDOs, VIOC enhances circuit performance and offers versatile solutions for modern power management systems.

Introduction

Part 1 of this series laid the foundation for the use of the voltage input-to-output control (VIOC) feature in the latest generation of low dropout regulators (LDOs), and now Part 2 dives deeper into the capabilities. This article also makes VIOC system design more accessible by offering ready-to-use reference designs and straightforward evaluation methods. Additionally, it explores how VIOC can be seamlessly implemented in negative voltage topologies—broadening its applicability—and provides historical context by tracing the evolution of VIOC in earlier implementations.

VIOC Explained

The VIOC feature in the latest generation of LDOs is implemented by a difference amplifier with a gain of one that is internal to an LDO, as shown in Figure 1. The difference amplifier is drawn to show it is made from an operational amplifier and discrete resistors. In its most basic configuration, the output of the difference amplifier is connected directly to the FB pin of the switching regulator that provides power to the LDO. That connection between the VIOC and FB pins closes a feedback loop that servos the switching regulator output voltage to a voltage that is higher than the LDO output by the voltage at the difference amplifier output that is connected to the switching regulator FB pin. The FB pin is the external connection to the switching regulator IC error amplifier inverting input.

An error amplifier is an electronic component, typically implemented using an operational amplifier, that amplifies the difference between two input signals—usually a reference voltage and a feedback signal from a system output. This difference, known as the error signal, is used to adjust the system to maintain a desired output, making the error amplifier a critical part of feedback control systems such as voltage regulators, power supplies, and servomechanisms.¹

The difference amplifier that implements VIOC also handles housekeeping tasks, such as preventing the LDO input from dropping too low and turning off the LDO. For full details on VIOC behavior, refer to the VIOC section of the LDO data sheet.

Switching regulator FB pin voltages can vary from approximately 0.4 V to 1.2 V depending on the switching regulator IC. If the voltage at the switching regulator’s feedback pin is lower than the desired LDO input-to-output voltage, an additional resistor can be added between the switching regulator’s FB pin and the LDO’s VIOC pin. This resistor allows you to connect the VIOC signal at the difference amplifier output to a higher voltage point as shown in Figure 1. VIOC of the LT3041 LDO shown in Figure 1 and similar LDOs cannot easily achieve an input-to-output voltage for the LDO that is lower than the switching regulator FB pin voltage.

To understand how the difference amplifier implements VIOC, consider the output of the difference amplifier as one of its inputs. If the output of the difference amplifier is a certain voltage, then the voltage at its positive input must be higher than the voltage at its negative input by that same difference amplifier output voltage. Since the negative input of the difference amplifier is connected to the LDO’s output, the difference amplifier can adjust the switching regulator’s output voltage to be higher than the LDO’s output voltage by an amount that is the same as the difference amplifier’s output voltage, which is determined by the switching regulator’s FB pin voltage.

Figure 2 shows the LT3073 LDO with a VIOC circuit design that allows the LDO to have an input-to-output voltage that is lower than the switching regulator’s feedback pin voltage. This VIOC method includes an offset voltage that makes the LDO input-to-output voltage 800 mV less than the VIOC pin voltage. This type of VIOC is common on LDOs that operate at lower input-to-output voltages and have higher current ratings. The Figure 2 circuit also limits the switching regulator’s output voltage. The feedback resistor divider for the switching regulator is designed to prevent the switcher’s output voltage from becoming too high in situations where the LDO is not enabled or unable to close the VIOC feedback loop. Figure 3 shows the LT3041 from Figure 1 in a VIOC circuit with a limit on the LT8608 switching regulator output voltage.

The LDOs discussed so far are positive LDOs that operate with positive voltages, but VIOC operation is also possible for negative LDOs that operate with negative voltages. VIOC circuits for negative LDOs may use switching regulators designed to produce negative voltages that can have a feedback pin named FBX instead of FB. Figure 4 shows a switching regulator in a Ćuk configuration that is designed to produce a negative voltage. VIOC of the negative LT3099 LDO controls the Ćuk output voltage.

The circuit in Figure 5 is a scenario that is becoming more common with a buck regulator that normally produces positive voltages, used to produce a negative voltage by being configured as an inverting buck-boost. Since standard buck regulators don’t have an FBX pin or an FB pin designed to work with negative voltages, this VIOC circuit requires a level shifter. This level shifter is the portion of the circuit that includes the LT1636 op amp and the network connected to the op amp’s positive input.

VIOC is easy to make work because it only requires one wire in addition to the switching regulator-to-LDO connections that would be made anyway. VIOC works with any switching regulator with an FB pin. Note that there is no FB pin on Silent Switcher® 3 (SS3) regulator ICs, so normally VIOC is not used with those ICs. VIOC has some limitations if the switching regulator FB pin is connected to an internal divider resistor that also connects to the output, like on some µModule® regulators.

A µModule regulator is a highly integrated system-in-package (SiP) solution that combines multiple electronic components, such as DC-to-DC controllers, power transistors, input and output capacitors, compensation components, and inductors, into a single compact package.²

Figure 6 shows a VIOC circuit that pairs one channel of the LTM4616 µModule regulator with the LT3078 LDO. The µModule has an internal 10 kΩ resistor between the FB pin and the output of the switching regulator. The VIOC pin current and, consequently, the VIOC pin voltage are related to the current of the 10 kΩ resistor, which changes proportionally when the LDO and switching regulator output voltages increase or decrease. This dependance of the VIOC pin voltage on the LDO output voltage limits the LDO output voltage adjustment range for the circuit in Figure 6.

Convenient Ways to Evaluate the Operation of VIOC Systems

LTspice® can be used to quickly simulate and evaluate VIOC operation in a controlled environment, allowing for the testing of different scenarios and parameters without the need for physical hardware. This makes it an efficient tool for verifying circuit behavior and optimizing a design before implementation.

The most efficient way to evaluate VIOC functionality using hardware is by connecting a switching regulator demonstration circuit to an LDO demonstration circuit with minimal modifications. Figure 7 shows two such circuits configured for quick VIOC testing. For the final design of VIOC circuits, it is usually necessary to check circuit stability by doing a load transient test on actual hardware. It is common to add capacitance at the switching regulator output to improve stability—see the data sheet VIOC reference designs for guidance.

A Previous VIOC Version and VIOC that is Not Built-In

Most of the discussion so far has been about LDOs with the latest generation of VIOC. There is also a VIOC method that uses discrete components and an earlier generation of LDOs with VIOC. Figure 8 shows an LDO circuit with one of the earlier generations. The VIOC pin of the LDO is connected to the ITH or VC pin of the switching regulator, and the LDO’s VIOC circuitry automatically controls the LDO input-to-output voltage to around 300 mV. The I_TH or V_C pin is the external connection to the switching regulator IC error amplifier output. Early 5 V, 5 A LDOs like the LT3070-1, LT3071, and LT3072 feature this VIOC method.

Finally, the last type of VIOC to discuss is the VIOC method that uses discrete components. VIOC capability can be added to almost any LDO by using discrete components. This method was especially popular with early current source reference LDOs starting with the LT3080, since it is relatively easy to vary the output voltage of LDOs with current source references by using an external voltage signal or a digital potentiometer. A current source reference LDO is a regulator that uses a current source driving a resistor to determine the output voltage, instead of a voltage reference. Figure 9 shows a circuit that uses discrete components for VIOC. The source-to-gate voltage of the IRF7342 PMOS determines the input-to-output voltage of the LDO. See the LT308x data sheets for more reference designs with this type of VIOC.

Conclusion

Integrating LDOs and switching regulators using VIOC technology unlocks powerful advantages for modern power management design. VIOC maintains a stable input-to-output voltage, boosting PSRR to reduce output noise, improving power dissipation, and enhancing fault protection and startup reliability. It also delivers consistently strong load transient response—essential for high performance systems.

VIOC simplifies voltage coordination between switching regulators and LDOs, streamlining the design process and elevating overall circuit efficiency. The latest generation of LDOs with VIOC support offers flexible, high performance solutions for today’s complex electronic applications.

To accelerate your design journey, explore detailed application notes, reference designs, and LTspice simulation models available at analog.com. Take advantage of simulation tools to test and optimize your circuits before hardware implementation— saving time, reducing risk, and enabling smarter innovation.

References

¹ Error Amplifier (Electronics). Wikipedia.

² μModule Regulators. Analog Devices, Inc.

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