Purpose of Sense Lines

Recommended Design Rules and Strategies for BGA Devices User Guide (UG1099)

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As voltage regulators are often quite a distance away from the devices they supply power to, there is often a DC voltage IR drop in voltage between the regulator and the main points-of-load, especially if there is high current load. If the regulator was not accounting for this drop, the voltage at the point-of-load can possibly be quite a bit lower than what the regulator sees. The resulting lower voltage can thus be out of the required limits for proper device operation. The following figure illustrates the direction of current flow and the resulting voltage drop.

Figure 1. DC Voltage Drop between VRM and Point-of-Load

To overcome this voltage drop, regulators implement sense lines that connect directly from the regulator to the point-of-load. These sense lines are direct traces between the regulator and point-of-load and do not carry any current. Because they do not carry any current, the voltage at the regulator sense pins is exactly the same as seen at the point-of-load. The regulator can thus properly adjust its output voltage as necessary so that the voltage at the point-of-load is within the required specification. The following figure illustrates the placement and routing of sense lines from the VRM to the point-of-load.

Figure 2. Example of Sense Lines