Voltage Disparity
Because sense lines can only be placed at one location at the point-of-load, other areas in front of the sense points might show a higher voltage, while points beyond the point-of-load might see lower voltage. The following figure illustrates this voltage disparity. As the current flows from the VRM to the point-of-load, the voltage at point V1 is higher than the sense point at V2. Likewise, the voltage at point V3 is lower than at the sense point V2.
The recommendation in this case is to perform a simulation that shows the voltage levels at each BGA ball, and then to place the sense line at a ball with a voltage somewhere in-between. Care should be taken that the resulting sense line does not cause any associated BGA ball voltages to move beyond the datasheet specifications.
Managing Sense Lines with Multiple Connected Rails
It is a common practice to consolidate several adaptive SoC rails together to reduce the number of voltage regulators required. In general, the recommendation for sense line placement is the same as it is in Voltage Disparity, which is to perform simulations and choose a sense point between the lowest and highest BGA voltages.
However, multiple rail consolidation can present difficulties in determining the proper location for the associated sense line if one or more of the merged rails has excessive levels of current consumption and voltage drop compared to the other rails on the same plane. Placing the sense line at the point of the highest load or even an intermediate point can cause the regulator to increase its output to a point where lightly loaded BGA balls from other rails experience an increase in voltage that can affect timing with possible unpredictable circuit performance as a result. At worst, it can cause lightly loaded BGA balls to go higher than the datasheet specifications.
In the situation where one heavily loaded rail in a consolidated scheme can cause lightly loaded rails to be raised too high, it is recommended to place that heavily loaded rail on its own regulator. The other rails can still be consolidated. This is most often the case with high current rails such as VCCINT and VCC_SOC.
VRMs with Only One Sense Pin
Some VRMs do not include a sense pin for ground. The primary limitation of this arrangement is that fluctuations in the ground voltage at the point-of-load are not accounted for by the VRM, resulting in less precise voltage regulation.