SNVSBJ0B December 2019 – August 2021 LM5170
PRODUCTION DATA
TI strongly recommends employing a non-inductive resistor for RCS. Even a few nH of inductance cause the current sense signal to be remarkably distorted, as shown in Figure 9-4. The adversary consequences include reduced peak current limit than actually programmed and false current zero-crossing detection well above 0 A. The former can reduce the available maximum current to be delivered. The latter terminates the sync FET gate early and the body diode is used to conduct the remaining current, thereby reducing the efficiency as well as the accuracies of the channel DC current regulation and IOUT monitors under light load.
When the current sense resistor has some parasitic inductance, it is necessary to compensate the effects of inductance with an RC circuit, as shown in Figure 9-5. Place a 1-Ω resistor in each of the current sense signal path. The selection of CCS must satisfy Equation 37, assuming the inductance of the current sense resistor is LCS:
For instance, if RCS =1 mΩ and LCS = 1 nH, the required compensation capacitor CCS must be approximately 0.5 µF.
Note that selecting CCS greater than the value given by Equation 37 overcompensates the inductance and consequently defers the current zero crossing detection point to a negative current. Excessively larger capacitor must not be used to prevent malfunction of the controller.