MB3889 Datasheet, PDF (18/35 Pages) Fujitsu Component Limited. – 2-Channel DC/DC Converter IC with Overcurrent Protection Symmetrical-Phase Type

MB3889
s SETTING THE SOFT-START AND DISCHARGE TIMES
To prevent rush currents when the IC is turned on, you can set a soft-start by connecting soft-start capacitors
(CS1 and CS2) to the CS1 terminal (pin 3) for channel 1 and the CS2 terminal (pin 13) for channel 2, respectively.
Setting the each control terminals (CTL1 and CTL2) from “L” to “OPEN” switches SW1 and SW2 from B to A to
charge the external soft-start capacitors (CS1 and CS2) connected to the CS1 and CS2 terminals at 3 µA.
The error amplifier output (FB1 or FB2) is determined by comparison between the lower one of the potentials
at two noninverted input terminals (1.23 V, CS terminal voltages) and the inverted input terminal voltage (-INE).
The FB terminal voltage during the soft-start period is therefore determined by comparison between the -INE
terminal and CS terminal voltages. The DC/DC converter output voltage rises in proportion to the CS terminal
voltage as the soft-start capacitor connected to the CS terminal is charged.
The soft-start time is obtained from the following equation:
Soft-start time: ts (time to output 100%)
ts (s) =: 0.41 × CS (µF)
Setting the each control terminals (CTL1 and CTL2) from “OPEN” to “L” switches SW1 and SW2 from A to B.
Then the IC discharges the soft-start capacitors (CS1 and CS2) charged at about 3.4 V using the internally set
discharge resistor (Rs: = 6 kΩ) and lowers the output voltage regardless of the DC/DC converter load current.
The discharge time is obtained from the following equation:
Discharge time: toff (time to output 10%)
toff (s) =: 0.020 × CS (µF)
=: 3.4 V
CS terminal voltage
=: 1.23 V
=: 0.123 V
=: 0 V
Error amp block comparison
voltage to −INE voltage
Soft-start time (ts)
OPEN
Discharge time (toff)
CTL signal
L
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