Detailed derivation and calculation of DCDC power supply output line loss compensation circuit based on current sampling op amp

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Detailed derivation and calculation of DCDC power supply output line loss compensation circuit based on current sampling op amp [Copy link]

Author: TI Engineer Kevin Zhang When the output of a DCDC power supply needs to pass through a long cable to reach the load, the voltage drop caused by the impedance of the cable will cause the voltage at the load end to be lower than the actual DCDC output voltage. In order to ensure that the voltage at the load end maintains the fixed value we want under different load currents, we can collect the load current in real time and dynamically increase the output voltage of the DCDC power supply according to the wire load to compensate for the voltage drop of the cable so that the voltage at the load end remains unchanged. TI's official website has relevant reference designs such as PMP4425 and PMP10282, but none of them give the derivation of relevant circuit parameters. Customers can only refer to the values in the design. This article takes LMR14030 and INA213 as examples to give a detailed derivation of line loss compensation. When customers need to change circuit components (such as using different DCDC or current sampling op amps) or change circuit parameters (such as requiring different output voltages), they can also use the formulas in this article to quickly calculate and determine other parameters in the circuit. In the following example, once the voltages required by the DCDC, current sampling amplifier and load are determined, the relevant resistance values can be calculated using the last equations (7) and (8). Parameter Description: VOUT is the output voltage of LMR14030. VFB = 0.75V, the FB pin voltage of the DCDC power supply. IOUT is the USB output current. VUSB = 5V, the voltage on the USB socket of the central control panel after passing through the USB cable. VINA is the output voltage of INA213. is the voltage on the REF pin of INA213. GINA = 50 is the gain of INA213. RSENSE = 0.01Ω is the current sampling resistor. RDS(on) = 0.073Ω is the on-resistance of the internal switch current limiting MOSFET of TPS2546. If there is no TPS2546 in the circuit, this item can be ignored. RCABLE is the total resistance of the 5V line and the GND line in the USB cable. From the schematic diagram, we can get the following formula: (1) (2) (3) From formulas (1), (2), and (3), we can get: (4) The design goal of line loss compensation is: regardless of the value of IOUT, VUSB must be maintained at 5V. Therefore, from formula (4), we can get: (5) (6) Substituting the relevant parameters, we can get (7) (8) First select one of the values of R1, R2, and R3, and then calculate the other two values through formulas (7) and (8). Reprinted from》》》》》