MB3817 Datasheet, PDF (16/29 Pages) Fujitsu Component Limited. – Switching Regulator Controller

MB3817
s METHOD OF SETTING THE SHORT DETECTION TIME
The error amp. output is connected to the inverted input of the short detector comparator circuit (SCP Comp.),
where it is constantly compared to the reference voltage of approximately 1.5 V that is connected to the non-
inverted input.
If the switching regulator load conditions are stabilized, the short detector comparator output is at “H” level,
transistor Q3 is on, and the CSCP pin (pin14) holds the input standby voltage VSTB which is 50 mV.
If load conditions change rapidly due to a cause such as a load short, so that output voltage falls, the short
detector comparator circuit output changes to “L” level. When this happens, transistor Q3 turns off and the short
detector capacitor CSCP connected externally to the CSCP pin starts charging from the input source current II,
which is –1.0 µA.
Short detection time (tPE)
tPE [s] ^.=. 0.65 × CSCP [µF]
When the short detector capacitor CSCP has been charged to the threshold voltage VTH, which is 0.65 V, the SR
latch is set, and the external PNP transistor is turned off (setting dwell time to 100%). At this time, the SR latch
input is closed, and the CSCP pin is set to input latch voltage VI which is 50 mV.
• Short protection circuit
FB
5
−IN
4
+IN ₃
Error Amp.
−
+
SCP Comp.
−
+
1.5 V
PWM Comp.
OFF current
setting
block
Outside putting
PNP transistor
9
VCC
Q4
10
OUT
Q6
RE
D1
11
Q5
(0.5V)
VE
bias
RS
Latch
UVLO
1µ
Q2 Q3
14
CSCP
16