MB3885 Datasheet, PDF (17/29 Pages) Fujitsu Component Limited. – 1-channel DC/DC Converter IC with Synchronous Rectifier

MB3885
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 (Cs) to the CS terminal (pin 6) .
When the IC is activated (Vcc ≥ UVLO threshold voltage) , Q2 is off and the CS terminals begin charging the
externally connected soft start capacitors (Cs) at 10 µA.
Because the error Amp. output (FB) 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 is 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
Soft start time ts
17