A brief introduction to the role of feedback resistor in Buck converter

Buck converters are widely used in mobile multimedia devices such as mobile phones, GPS, MP3, etc. due to their high efficiency. Currently, many power management chip manufacturers have launched Buck converters with different current capabilities. Although these converters have some differences in current capabilities and protection functions, their main framework structures of circuits are basically the same, which can be divided into two parts: one is the main power part for realizing power conversion, and the other is the control circuit for realizing negative feedback control, as shown in Figure 1.


For Buck converter chips designed by different manufacturers , the devices required for the peripheral circuits will be different. This is because the integration of the chips is different. For example, some manufacturers will integrate the power tube inside the chip; some manufacturers will integrate the compensation network of the control part inside the chip. The more integrated the chip is, the fewer devices are required for the peripheral circuits. Therefore, for customers, the choice of peripheral devices needs to be determined according to the specific chip. However, for any Buck converter chip with adjustable output, it is essential to choose a suitable feedback resistor . Figure 2 is a typical application diagram of the Buck converter AP3406 of BCD Semiconductor . Due to the high integration of the chip, the peripheral only needs input capacitors , output inductors, output capacitors and feedback resistors. This article takes this as an example to briefly analyze the role of feedback resistors and provide a reference for how to choose feedback resistors.



Setting the output voltage

The first function of the feedback resistors Rf1 and Rf2 is to set the output voltage value of the Buck circuit. As shown in Figure 2, in steady state, the voltages of the inverting input and the non-inverting input of the operational amplifier are equal, so the output voltage calculation formula can be obtained: Originator"/> ： VREF is the internal reference voltage of the chip (0.6V in this example). Influence on system stability and dynamic response In order to achieve the anti-interference ability of the system, the Buck converter will have a corresponding negative feedback control circuit in addition to the main power part, and the compensation network is part of the feedback control circuit. The addition of the compensation network can increase the low-frequency gain of the loop, thereby improving the anti-interference ability; at the same time, the compensation network enables the system to have sufficient phase margin, thereby ensuring that the system is in a stable working state and will not oscillate. The part in the yellow box in Figure 2 is the compensation network part, which includes R1, C1, C2 and Rf1 (Note: Rf2 does not work in loop analysis). The transfer function of the compensation network can be expressed as: From the above formula, it can be seen that the compensation network produces two poles, one of which is at 0 and the other is . At the same time, a zero point is also produced . The amplitude-frequency characteristic and phase-frequency characteristic of Gc(s) are made in mathcad, as shown in Figures 3 and 4. The role of Rf1 in the compensation network is to change the gain in the mid-frequency band and will not affect the zero poles in the compensation network. The performance in Figures 3 and 4 is that with the change of Rf1, the amplitude-frequency characteristic of the compensation network Gc(s) shifts up and down, and the phase-frequency characteristic remains unchanged.











Therefore, when the compensation network enters the system loop, the role of Rf1 is to shift the amplitude-frequency characteristic of the loop gain up and down, while the phase-frequency characteristic of the loop gain remains unchanged.


Figures 5 and 6 are the results of the AP3406 loop gain test using a network analyzer. As can be seen from the figures, when Rf1 changes from 300k to 470k and then to 750k, the amplitude-frequency characteristic of the system's loop gain continues to move downward, while the phase-frequency characteristic remains basically unchanged. As a result, the bandwidth and phase margin of the system have changed significantly, and the test results are shown in Table 1.


The bandwidth of the system is an important factor affecting the dynamic response of the system, and the phase margin is an important factor affecting the stability of the system. If you choose a different Rf1, the bandwidth and phase margin of the system will change, that is, the dynamic response and stability will change. Specifically, according to Table 1, when Rf1 changes from 300k to 750k, the system bandwidth changes from 51.1kHz to 26.1kHz, so the dynamic response of the system will deteriorate accordingly, and because the phase margin is sufficient, the system is in a stable working state. Figures 7 and 8 are the dynamic response test results of load switching. From the test results, it can be seen that the loop response speed slows down, resulting in a larger output voltage overshoot and a worse dynamic response effect.



From the above analysis, it can be seen that choosing the right feedback resistors Rf1 and Rf2 plays an important role in AP3406 and similar Buck converters with compensation structures. When choosing feedback resistors, we should not only consider whether the output voltage meets the requirements in steady state, but also the impact of feedback resistors on system stability and dynamic response.