SLUSD12A October 2017 – February 2018 UCC28780
The RRDM resistor provides the power stage information to the tDM optimizer for auto-tuning the on-time of PWMH to achieve ZVS within a given tZ discharge time. The following equation calculates the resistance, based on the knowledge of the primary magnetizing inductance (LM), auxiliary-to-primary turns ratio (NA/NP), the values of the resistor divider (RVS1 and RVS2) from the auxiliary winding to VS pin, and the current sense resistor (RCS). Among those parameters, LM contributes the most variation due to its typically wider tolerance. The optimizer is equipped with wide enough on-time tuning range of PWMH to cover tolerance errors. Therefore, just typical values are enough for the calculation.
where KDM is equal to 5×109 (unit: F-1) for both VSET = 5 V and 0 V.