MB3882 Datasheet, PDF (18/30 Pages) Fujitsu Component Limited. – 2-channel DC/DC Converter IC with Synchronous Rectifier

MB3882
s SOFT START TIME SETTING
The soft start function prevents rush current events when the IC power is turned on, by connecting soft start
capacitors (Cs1, Cs2) to the CS1 terminal (pin 4) for channel 1, and the CS2 terminal (pin 21) for channel 2.
When the IC is activated (Vcc ≥ UVLO threshold voltage) , Q2 and Q3 are off and the CS1 and CS2 terminals
begin charging the externally connected soft start capacitors (Cs1, Cs2) at 10 µA.
Because the error Amp. output (FB1, FB2) is determined by the ratio of the lower of the two non-inverted input
terminals (1.25 V, CS terminal voltage) to the inverted input terminal voltage (−INE) , the soft start interval (when
CS terminal voltage < 1.25 V) FB is determined by the ratio of the −INE terminal voltage and CS terminal voltage.
Thus the DC/DC converter output voltage rises in proportion to the rise in the CS terminal voltage as the soft
start capacitor connected to the CS terminal charges. The soft start time is determined by the following formula.
Soft start time (time to output 100%)
ts (s) =: 0.125 × Cs (µF)
2.3 V
1.25 V
CS terminal voltage
Error Amp. block comparison voltage
to −INE voltage
0V
t
Soft start time ts
18