MB3785A_03 Datasheet, PDF (23/30 Pages) Fujitsu Component Limited. – Switching Regulator Controller (4 Channels plus High-Precision, High-Frequency Capabilities)

MB3785A
s METHODS OF SETTING THE SOFT START TIME
To prevent surge currents when the IC is turned on, you can set a soft start using the DTC terminal (pin 4, 9, 28
and 33).
When power is switched on, channels 1 and 2 begin discharging the capacitor (Cdt) connected the DTC1 (DTC2)
terminal, channels 3 and 4 begin charging the capacitor (Cdt) connected the DTC3 (DTC4) terminal. The soft
start process operates by comparing the soft start setting voltage, which is proportional to the DTC terminal
voltage, with the triangular waveform, and varying the ON-duty of the OUT terminal (pin 46, 44, 40 and 39).
The soft start time until the ON duty reaches 50 % is determined by the following equation:
For figure 13
Soft start time (time until output ON duty = 50%)
ts (s) = −
Cdt (F) × Rdt (Ω) × ln (
1.6
2.5
)
0.446 × Cdt (F) × Rdt (Ω)
For figure 14
Soft start time (time until output ON duty = 50%)
ts (s) = − Cdt (F) × Rdt (Ω) × ln (1 −
1.6
2.5
)
1.022 × Cdt (F) × Rdt (Ω)
Figure 13. Setting Soft Start for CH1 and
CH2
Figure 14. Setting Soft Start for CH3 and
CH4
19 VREF
Cdt
Rdt
DTC1
(DTC2)
Rdt
Cdt
19 VREF
DTC3
(DTC4)
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