FUJITSU MB39A104 - Manual (Page 19)

MB39A104
19
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 (3.1 V Typ).
While DC/DC converter load conditions are stable on all channels, the short-circuit detection comparator output
remains at “L” level, and the CSCP terminal (pin 8) is held at “L” level.
If the load condition on a channel changes rapidly due to a short-circuit of the load, causing the output voltage
to drop, the output of the short-circuit detection comparator goes to “H” level. This causes the external short-
circuit protection capacitor C
SCP
connected to the CSCP terminal to be charged at 1
µA.
Short-circuit detection time (t
SCP
)
t
SCP
(s) := 0.73 × C
SCP
( µF)
When the capacitor C
SCP
is charged to the threshold voltage (V
TH
:= 0.73 V), the latch is set and the external
FET is turned off (dead time is set to 100%). At this time, the latch input is closed and the CSCP terminal (pin
8) is held at “L” level. If a short-circuit is detected on either of the two channels, both channels are shut off.
When the power supply is turned on back or VREF terminal (pin 17) voltage is less than 2.4 V (Min) by setting
CTL terminal (pin 24) to “L” level, the latch is released.
−
+
+
−
+
8
10
15
V O
R1
R2
( −INE2)
−INE1
9
16
(FB2)
FB1
CSCP
Error
Amp
SCP
Comp.
(3.1 V)
(1.24 V)
(1 µA)
VREF
UVLO
S
Latch
R
To each channel
Drive
• Timer-latch short-circuit protection circuit