For my design I am using DC-DC converters LTM4620 and LTM4644, which integrate the MOSFETs and the inductor in the chip.
For measuring the output current, I will add a sense resistor in each output with values from 1m to 5m (depending on current range). This measurement is independent of the DC-DC converter feedback loop.
My question is related to the position of this sense resistor, because in some articles, it is placed before the output capacitor, whereas in some designs (for example, KCU105 board), sense resistors are placed after output capacitor.
What should I consider for choosing the correct place for the sense resistor, or is this really not important?
Thank you very much!
Best Answer
This module is guaranteed to perform well, as long as you do not change the various components shown in the datasheet, NOR CHANGE THE SCHEMATIC.
Inserting a current-sense resistor in series with the energy-storage capacitors is CHANGING THE SCHEMATIC. Don't do that. The feedback loop will likely mis-behave, because you will have changed the high-frequency feedback.
For a safe way to monitor the output current, place your 0.001 ohm resistor (power of P = I^2 * R, or 30 * 30 * 0.001 = 0.9 watts) in series with the output pin, between the output pin and your load, and expect the voltage to your circuits to fall by 30 milliVolts as you increase the load current from 0 amps to 30 amps.
By the way, standard PCB copper foil is 1 ounce/square foot, yielding 1.4 mils or 35 microns thickness; the resistance of one square, measured from one edge to the opposite edge, with ohmmeter contact all along those 2 edges (not just at a small point), is 0.000500 ohms (0.5 milliOhms) at room temperature. The temperature coefficient of CU is 0.4% per degree Centigrade.
Thus 2 squares of CU foil might provide your 0.001 ohm resistor, if you come to understand 4-wire Kelvin sensing and temperature compensation.