MB3778 Datasheet, PDF (16/29 Pages) Fujitsu Component Limited. – Switching Regulator Controller

MB3778
s SETTING THE SOFT START TIME
When power is switched on, the current begins charging the capacitor (CDTC1) connected the DTC1 terminal (pin
6). The soft start process operates by comparing the soft start setting voltage, which is proportional to the DTC1
terminal voltage, with the triangular waveform, and varying the ON-duty of the OUT terminal (pin 7).
The soft start time until the ON duty reaches 50% is determined by the following equation:
Soft start time (time until output ON duty = 50%) .
ts (s) =: − CDTC1 × Ra × Rb / (Ra + Rb) × ln (1 − 1.6 (Ra + Rb) / (2.46 Rb) )
For example, if Ra = 4.7 kΩ and Rb = 10 kΩ, the result is:
ts (s) =: 0.1 × CDTC1 (µF)
• Soft Start on DCT1 terminal (DTC2 is similar)
VREF 16
DTC1 ⁶
Ra
Rb
CDTC1
16