MB3885 Datasheet, PDF (15/29 Pages) Fujitsu Component Limited. – 1-channel DC/DC Converter IC with Synchronous Rectifier

MB3885
s SETTING THE TIMER LATCH SHORT CIRCUIT PROTECTOR TIME CONSTANT
The error Amp. output level constantly compares operation with the short circuit protection comparator as the
reference voltage.
When the DC/DC comparator load conditions are stable, the short circuit protection comparator output is at “H”
level, transistor Q1 is on, and the CSCP terminal (pin 3) is held at input standby voltage
(VSTB : = 50 mV) .
If load conditions change rapidly, such as during a load short, causing output voltage to drop, the short circuit
protection comparator output goes to “L” level. This causes the transistor Q1 to shut off, charging the short circuit
protection capacitor Cscp (connected to the CSCP terminal) at 1 µA.
Short detection time
tscp (s) =: 0.68 × Cscp (µF)
When the capacitor Cscp is charted to the threshold voltage (VTH : = 0.68 V) a latch is set, turning the external
FET off (setting the idle interval to 100%) . At this time the latch input is closed and the CSCP terminal is held
at the input latch voltage (VI : = 50 mV) .
FB
7
8
−INE
6
CS
CS
10 µA
Error
Amp.
−
+
+
1.25 V
Q2
Drive
Drive
13
OUT1
15
OUT2
1 µA
CSCP
3
CSCP
Q1
Q4
SCP Comp.
bias
SR
Latch
2.1 V
UVLO
< Timer Latch Short Circuit Protection Circuit >
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