MB3886 Fujitsu Microelectronics, Inc., MB3886 Datasheet - Page 15

• Manufacturer Part Number: MB3886
• Description: 2-channel Dc/dc Converter Ic With Overcurrent Protection Symmetrical-phase Type
• Manufacturer: Fujitsu Microelectronics, Inc.

SETTING THE SOFT-START AND DISCHARGE TIMES
To prevent surge currents when the IC is turned on, you can set a soft-start by connecting soft-start capacitors
(C
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 (C
The error amplifier output (FB1 or FB2) is determined by comparison between the lower one of the potentials
at two noninverting input terminals (1.25 V CS terminal voltages) and the inverting 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 )
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 (C
discharge resistor (Rs : = 3.8 k ) and lowers the output voltage with a time constant of about 1/10 of the soft-
start time regardless of the DC/DC converter load current. The discharge time is obtained from the following
equation :
Discharge time : toff (time to output 10 )
ts (s) : = 0.125
toff (s) : = 0.0126
S1
and C
0.125 V
OPEN
CTL signal
1.25 V
S2
3.4 V
) to the CS1 terminal (pin 7) for channel 1 and the CS2 terminal (pin 24) for channel 2, respectively.
0 V
L
C
S
C
( F)
Soft-start time ts
S
( F)
Error Amp.
S1
Voltage compared to INE voltage
and C
CS terminal voltage
S2
S1
) connected to the CS1 and CS2 terminals at 10 A.
and C
S2
) charged at about 3.4 V using the internally set
Discharge time toff
MB3886
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