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Voltage mode buck regulators have become commonplace in today's broad market. Many of these regulators require external compensation. External compensation allows the regulator to be used with a wide range of components and gives the user control over the systems' performance. While the user has considerable control over the system, many find themselves lacking the proper resources to take advantage of this feature.
This paper will discuss the technical motivation behind compensation, derivation of analytical and design oriented transfer functions, establish criteria for the selection of a particular type of network, and provide an illustrative example. In Part 2 you will find a series of 'quick sheets' and appendices that are a useful reference in the design of compensation networks.
Motivation
Due to the abundance of materials related to elementary feedback theory, this paper will assume the reader has prior knowledge of properties including the gain-bandwidth product and the reduction of noise (or other disturbances) due to feedback. We will simply say that the purpose of compensation (feedback) is to alter the closed loop response of a system to meet a given set of requirements.
The requirements of a closed loop system may be classified as rise time, overshoot, and settling time as shown in Figure 1.
Figure 1: Time domain characteristics
These time domain characteristics are related to each other and the frequency domain characteristics of the system. Design usually takes place in the frequency domain since the specifications are more convenient and it gives the simplest criteria with which to work.
Design Criteria
Since the motivation for compensation is to alter the system performance, we will need a model for our particular system. A simplified switcher model is shown in Figure 2a.
Figure 2a: Switcher Model
One should note that the components inside the dashed box are integrated within a switcher. In some cases the FETs will be used externally. This is, however, of no consequence in the treatment of loop response.
While the model shown in Figure 2a is useful for understanding the function of the control loop, this model may be simplified to aid in our analysis.
The comparator and FETs may be lumped into one circuit element, a voltage controlled voltage source (VCVS) of gain A V/V. This circuit element is typically referred to as the modulator. The modulator gain is either PVIN/VRAMP or a given (constant) value for feedforward topologies.
Figure 2b: Linear, small signal switcher model
While there are a number of tradeoffs and interrelationships relating the loop gain to the closed loop performance, a phase margin of 45°-90° and crossover frequency of 1/5th-1/10th the switching frequency should provide good performance, it is typically advisable to have a phase margin of 52° and a crossover frequency of 1/5th the switching frequency.
These requirements may be met by analyzing each component in the system as shown in the following section.
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