A detailed analysis of the design issues of the sense resistor in the buck converter

A buck converter is used to demonstrate the impact of an incorrectly sized sense resistor and what happens when the _RSENSE filter element is removed. The sense resistor is often incorrectly sized because designers are trying to optimize efficiency and operation, and incorrect sizing can result in degraded performance. Additionally, the sense resistor filter element is critical in providing accurate information to the feedback architecture and if removed it can degrade SMPS performance. The current coming out of the inductor is converted to a voltage through a small sense resistor . This can be seen in Figure 1.

This voltage acts as a signal to the feedback logic, which is used to adjust the output. Selecting the correct value for this sense resistor is critical to ensure that the feedback logic receives an accurate description of the inductor current. It also ensures that the R _SENSE signal complies with the data sheet absolute maximum differential voltage across the sense resistor.

Designers may choose to reduce the value of the sense resistor for efficiency. The sense resistor is placed in series with the inductor and the output so that the device can sense the triangular inductor current waveform and use it in the feedback loop. The power loss in this resistor is measured as P _loss =I ² _L ×R _SENSE . Designers can slightly improve the efficiency of the sense resistor by reducing its value. However, this approach is costly. If the value of the resistor is too small, the signal from the sense resistor will also be very weak. This will cause the signal-to-noise ratio (SNR) to deteriorate because the amplitude of the noise becomes close to the converted inductor current signal. Due to the deterioration of the signal-to-noise ratio, the sense resistor can no longer isolate the main signal, resulting in noise on the output signal. This usually manifests as jitter on the output signal, as shown in Figure 2.

To solve this problem, designers should choose an appropriate R _SENSE value based on the following formula:

Where V _sense(max) is determined in the device data sheet and I _max is the maximum load current that can be drawn. The maximum current (I _max ) that each SMPS can handle is defined as the sum of half of the inductor current ripple and the average load current, as shown in Figure 3.

Selecting a value based on this formula ensures that the R _SENSE value is large enough to adequately capture the inductor current ripple. If designers have difficulty selecting an appropriate value, they can utilize Analog Devices’ LTpowerCAD® ^to calculate the R _SENSE value and obtain recommended values ​​to ensure proper operation. If designers are concerned about the efficiency of their design, they can also utilize the Power Loss and Efficiency tabs in LTpowerCAD to identify sources of power losses in the circuit (such as MOSFET switching losses and inductor DCR losses) and correct these losses by selecting a more efficient device. In addition, the sense resistor can be omitted if the device has an inductor DCR sensing function, which senses the voltage across the inductor, thereby improving efficiency at the expense of reliability and noise performance. The preferred method is to use a sense resistor, but if peak efficiency is required, inductor DCR sensing can be performed on the current waveform.

Designers do not typically select oversized sense resistors when designing an SMPS. However, layout issues can cause resistance in the PCB traces, which when added to the value of the sense resistor increases the total sense resistance. Typically, SMPS chips have a current limit function that is determined by the maximum voltage that can be developed across the sense resistor. When this value is exceeded, the device enters current limiting mode, and the output voltage begins to drop as the load current increases. The device no longer regulates the output voltage. This can be seen in Figure 4.

This phenomenon usually occurs when the trace between the inductor and the sense resistor is longer than necessary, or when the current-carrying trace is connected to one of the sense pins on the chip. Since the selected sense resistor is in the milliohm range, it is sensitive to any resistance added. This problem can be avoided by using a Kelvin connection, as shown in Figure 5.

The sense traces come from the sense resistor and are separated from the PCB pad and current carrying traces. The Kelvin traces are much thinner and are run as close to the sense resistor as possible to avoid adding parasitic resistance. This allows V _SENSE to accurately represent the voltage across the sense resistor. Therefore, when the sense resistor is increased due to lack of proper Kelvin connection, too long traces, or simply due to the wrong value selected, the current limit will be triggered at lower loads because V _SENSE(MAX) can be reached sooner, resulting in poor load regulation.

Parasitic equivalent series inductance (ESL) is an inherent characteristic of surface mount device (SMD) resistors. Due to the low value of the sense resistor (milliohm range), ESL can have a significant impact on the sense architecture. Therefore, to eliminate the effects of parasitic ESL, RC filters must be added to the sense traces. Designers do not realize the benefit of omitting these components, but may omit them to reduce BOM size, reduce cost, or may simply forget to include them.

ESL includes not only the parasitic inductance of the sense resistor, but also the total inductance caused by the board layout and traces. ESL can be calculated using Equation 3:

V _ESL(step) is the additional voltage across the sense resistor. The filter needs to produce an RC time constant that is equal to or less than the calculated sense resistor time constant (ESL/R). With the filter removed, the sense resistor will exhibit inductive characteristics superimposed on its resistive characteristics. These can be observed as spikes (voltage steps) on the sense resistor waveform, as shown in Figure 6.

Additionally, due to the increased output ripple, the device incorrectly believes that it has reached its internal current limit at lower rated loads, resulting in poor load regulation.

This problem can be solved by adding a filter of appropriate size. This filter can be determined by the formula shown in Figure 8.

By doing this, the voltage delivered to the sensing architecture will increase. When compared to the signal across the sensing resistor without the filter, it is clear that the RC filter has smoothed the signal and eliminated the ESL step. As expected, the inductive spike disappears and the waveform becomes triangular. This can be seen in Figure 9.

This article serves as a guide to analyzing the design issues of sense resistors in buck converters. In addition, it provides designers with practical solutions to avoid any of the interfering behaviors described in the article. Although the sense resistor is often overlooked, its size is critical to maintaining a stable output voltage during load changes. If an inappropriately sized sense resistor is selected to save power, or layout resistor parasitics are not considered, the performance of the entire system may be degraded. In addition, neglecting the sense resistor filtering components will result in an inaccurate signal fed back to the sensing architecture and further degrade the system performance.