ZHCSCO9B June 2014 – January 2018 LM46001
PRODUCTION DATA.
To reduce noise sensitivity of the output voltage feedback path, it is important to place the resistor divider and CFF close to the FB pin, rather than close to the load. The FB pin is the input to the error amplifier, so it is a high impedance node and very sensitive to noise. Placing the resistor divider and CFF closer to the FB pin reduces the trace length of FB signal and reduces noise coupling. The output node is a low impedance node, so the trace from VOUT to the resistor divider can be long if short path is not available.
If voltage accuracy at the load is important, make sure voltage sense is made at the load. Doing so corrects for voltage drops along the traces and provide the best output accuracy. Route the voltage sense trace from the load to the feedback resistor divider away from the SW node path, the inductor and VIN path to avoid contaminating the feedback signal with switch noise, while also minimizing the trace length. This is most important when high value resistors are used to set the output voltage. It is recommended to route the voltage sense trace on a different layer than the inductor, SW node and VIN path, such that there is a ground plane in between the feedback trace and inductor / SW node / VIN polygon. This provides further shielding for the voltage feedback path from switching noises.