MB3887
■METHOD OF SETTING THE SOFT-START TIME
For preventing rush current upon activation of IC, the IC allows soft-start using the capacitor (Cs) connected to the CS terminal (pin 22) .
When CTL terminal (pin 14) is placed under “H” level and IC is activated (V CC ≥ UVLO threshold voltage) , Q2 is turned off and the external soft-start capacitor (Cs) connected to the CS terminal is charged at 10 ∝A.
Error Amp output (FB3 terminal (pin 15) ) is determined by comparison between the lower voltage of the two non-reverse input terminals (4.2 V and CS terminal voltage) and reverse input terminal voltage (−INE3 terminal (pin 16) voltage) . Within the soft-start period (CS terminal voltage < 4.2 V) , FB3 is determined by comparison between −INE3 terminal voltage and CS terminal voltage, and DC/DC converter output voltage goes up propor- tionately with the increase of CS terminal voltage caused by charging on the soft-start capacitor.Soft-start time is found by the following formula :
Soft-start time : ts (time to output 100 %) tS (s) =: 0.42 ⋅ CS (∝F)
=4.9 V
=4.2 V
CS terminal voltage
Comparison with Error Amp block − INE3 voltage.
| VREF | | |
| 10 ∝A | | 10 ∝A |
FB3 | 15 | | |
| | | Error |
−INE3 | 16 | − | Amp3 |
|
| | + | |
CS | 22 | + | |
| | |
| | 4.2 V |
| Q2 | | |
Soft-start circuit
