MB3882

18

s

s

s

s SOFT START TIME SETTING

The soft start function prevents rush current events when the IC power is turned on, by connecting soft start

When the IC is activated (Vcc

≥ UVLO threshold voltage) , Q2 and Q3 are off and the CS1 and CS2 terminals

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)

CS terminal voltage

2.3 V

1.25 V

0 V

Error Amp. block comparison voltage

to

−INE voltage

Soft start time ts

t