MB3889 Fujitsu Microelectronics, Inc., MB3889 Datasheet - Page 21

• Manufacturer Part Number: MB3889
• Description: 2-channel Dc/dc Converter Ic With Overcurrent Protection Symmetrical-phase Type
• Manufacturer: Fujitsu Microelectronics, Inc.

2. Setting Time Constant for Timer-Latch Short-Circuit Protection Circuit
Each channel uses the short-circuit detection comparator (SCP Comp.) to always compare the error amplifier’s
output level to the reference voltage.
While the DC-DC converter load conditions are stable on both channels, the short-circuit detection comparator
keeps its output at the “H” level and the CSCP terminal (pin 11) remains at the “L” level.
If a load condition changes rapidly due to a short-circuit of the load, causing the output voltage to drop, the short-
circuit detection comparator changes its output to the “L” level. This causes the external short-circuit protection
capacitor Cscp connected to the CSCP terminal to be charged at 10 A.
When capacitor Cscp is charged to the threshold voltage (VTH : = 0.70 V), the protection circuit sets the latch
and turns off the external FET (setting the dead time to 100%). At this time, the latch input is closed and the
CSCP terminal is held at the “L” level. The protection circuit closes both channels even when a short-circuit is
detected on only either.
To reset the actuated protection circuit, either the power supply turn off and on again or set the CTL terminal
(pin 6) to the “L” level to lower the VREF terminal (pin 8) voltage to 1.7 V (Min) or less. It can also be reset by
setting both of the CTL1 terminal (pin 9) and CTL2 terminal (pin 10) to the “L” level.
Short-circuit detection time (t
t
SCP
(s) : = 0.070
C
SCP
( F)
R1
R2
V
O
<Timer-latch short-circuit protection circuit>
SCP
( INE2)
CSCP
(FB2)
INE1
FB1
)
14
15
11
2
1
10 A
1.23 V
Error
Amp
Latch2
S
Latch
CTL
CTL1
CTL2
R
SCP
Comp.
VREF
UVLO
3.1 V
12
PWRGOOD
MB3889
21