To maintain a controlled power up, the UCC28070A has been designed with an adaptive soft-start function that overrides the internal reference voltage with a controlled voltage ramp during power up. On initial power up, once V_VSENSE exceeds the 0.75V enable threshold (V_EN), the internal pulldown on the SS pin is released, and the 1.5mA adaptive soft-start current source is activated. This 1.5mA pullup almost immediately pulls the SS pin to 0.75V (V_VSENSE) to bypass the initial 25% of dead time during a traditional 0V to V_REGULATION SS ramp. Once the SS pin has reached the voltage on VSENSE, the 10μA soft-start current (I_SS) takes over. Thus, through the selection of the soft-start capacitor (C_SS), the effective soft-start time (t_SS) may be easily programmed based on Equation 21.

Equation 21.

Often, a system restart is desired following a brief shutdown. In such a case, VSENSE may still have substantial voltage if V_OUT has not fully discharged or if high line has peak charged C_OUT. To eliminate the delay caused by charging C_SS from 0V up to the precharged V_VSENSE with only the 10μA current source and minimize any further output voltage sag, the adaptive soft start uses a 1.5mA current source to rapidly charge C_SS to V_VSENSE, after which time the 10μA source controls the V_SS rise at the desired soft-start ramp rate. In such a case, t_SS is estimated as follows:

Equation 22.

where

• V_VSENSE0 is the voltage at VSENSE at the moment a soft start or restart is initiated

Figure 6-4 Soft-Start Ramp Rate