SNIU028D February 2016 – September 2020 UCD3138 , UCD3138064 , UCD3138064A , UCD3138128 , UCD3138A , UCD3138A64
For many topologies, it is useful to replace diodes with synchronous rectification (SR), a concept also known as Ideal Diode Emulation (IDE). In continuous conduction mode, the SR FET control is simple, because it can be turned on for the entire off time of the primary FET, minus a dead time. This is handled perfectly by Normal mode. The Sync FET Ramp and IDE calculation are only available in Normal mode. They are not compatible with the Cycle by Cycle Fault module.
However, in discontinuous mode, the SR FET needs to be turned off before the end of the period. The UCD3138 hardware provides an automatic function to make this easier. In this case, the falling edge of DPWMB is adjusted, as shown below:
The digital hardware implements the equation Db = Da * Kd. The firmware measures Vin and Vout and calculates Kd. For example, for a Buck topology, Kd = (Vin - Vout)/Vout, where Da is duty cycle of the control FET, Db is duty cycle of SR FET, Vin is input voltage and Vout is output voltage. The firmware periodically measures the slowly changing Vin and Vout, and puts the calculated result into the Kd register. The DPWM hardware adjusts Db every switching cycle, maintaining proper IDE even during transients which cause rapid changes in Da.
When starting up in prebias mode i.e. with a voltage already present on the outputs, it is difficult to accomplish precise diode emulation (IDE). One solution to this issue is to ramp the voltage up to the target without synchronous rectification (SR), and then to activate SR after the voltage is regulated. When activating or de-activating SR, in order to avoid glitches in the regulated voltage, it is best to gradually increase/decrease the Sync FET on-time. The UCD3138 provides what is termed as Sync FET Soft-On/Soft-Off (ramp) logic to accomplish this. This is documented in Section 3.4.8. The Ramp module in the Front End is used for this function. It will ramp up to either the limits imposed by normal mode, or to the limits imposed by the IDE logic.
With digital IDE enabled, as the system transitions from discontinuous mode into continuous conduction Mode (sync FET is on until end of the period) the IDE will stop reducing the Sync FET pulse width. This is because the reverse conduction through the Sync FET will keep Da and Db the same. It is necessary to detect DCM (Discontinuous Conduction Mode) current levels in the IDE approach. Once these levels are detected, the Sync FET should be ramped down, and then ramped back up with IDE enabled.