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The correct placement of feedback sense pin in buck convertor
whether we need to keep the feedback pin of the buck convertor near the inductor or we can keep it near the load for better voltage regulation to avoid DC drop.
While devices will vary depending on their specific design, in general the Feedback / Output Voltage Sense pin can be connected anywhere along the output after the primary filtering capacitors. the closer the output voltage sense point is to the loading point, the more accurate the DC regulation will be over current as higher current will increase distribution / trace loses.
When the output voltage sense point is placed away from the inductor/ local output capacitors, and there are additional remote output capacitors at the output voltage sense point / input to the device(s) powered by the converter, it may be useful to add a place for a capacitor feedback path from the local output near the inductor back to feedback so that a pole introduced by the parasitic inductance in the power path, and the input capacitance at the remote sense point, can be compensated for with local sensing.
If the BUCK converter is powering a plane or several devices from the same voltage, it may be desirable to sense the output voltage at the average of several points. When doing this, a separate feedback trace with a small but finite resistance should be used to connect each of the desired sensing points to average those point, as weighted by the inverse of their resistances, before routing them back to the feedback sense pin. For example, a 100-Ω resistor from the local output and a 10-Ω resistor from the remote connection at a powered device would regulate the output voltage at about 9% (10/110) of the local voltage and 91% (100/110) of the remote voltage. Using 3 10-Ω resistors to connect the feedback sense point to the each of 3 devices running on the output would regulate their average voltage, allowing some to be slightly higher and some slightly lower, depending on the distribution losses to each.
Some devices will also provide "Remote Sensing" which is generally defined by allowing both the output voltage and ground voltage to be sensed at the load, rather than just the output voltage, which can reduce drops in the ground return voltage from affecting the regulation.
For specific constraints on the output voltage sense, refer to the design literature of the device used to implement the buck converter, some devices or control topologies may have specific limitations on the output voltage sense and require specific placement or routing of the output voltage sense. Specific guidance in a device's literature should always be considered priority over general guidance.
The "Feedback pin" is typically part of the IC, but the connection to the output, typically through a resistor, can generally be connected close to the decoupling capacitors of the load IC, yes.
Do you have a specific power device in mind? I might be able to provide more details and advice if I know the specific device we are talking about.
Thanks, we are using TPS548A29RWW for 12V to 1.2V generation, this power rail goes to main processor with ferrite bead in series, due to trace resistance and DCR of Ferrite bead the voltage drop is more than the allowable tolerance of the load. Thats Why I need to confirm whether we can place this sense pin after the Ferrite Bead and near the load to boost the voltage on source side to maintain the allowable tolearance on load.
Thats Why I need to confirm whether we can place this sense pin after the Ferrite Bead and near the load to boost the voltage on source side to maintain the allowable tolearance on load.
If that was your reasoning, why did you not mention that before?
Adding the resistance and inductance of a ferrite bead to the power path inside the feedback loop with additional local bypass capacitance at the load and after the bead complicates the frequency response of the feedback network, adding 1-2 additional poles and phase lag. This is often enough to make the loop unstable without taking additional measures to address the feedback.
Uses a moderate (10-50Ω) resistor to connect the feedback divider to the output at the loading IC chip. Connect a capacitor to the local output of the capacitors right next to the inductor and before the ferrite bead.
To size the capacitor set the cross-over frequency (1 / (2*π*R*C) ) with the series resistor to the loading point to be less than or equal to 1/2 the cross-over frequency of the ferrite bead, whose inductance you might have to calculate based on the impedance at frequency, (1 / (2*π*L*C) ) so that the loop transitions to local control before the ferrite bead and remote capacitance can introduce excessive lag.
That will allow the TPS548A29 to compensate for the additional IR drop while still maintaining stability.
