MB3785A_03 Datasheet, PDF (17/30 Pages) Fujitsu Component Limited. – Switching Regulator Controller (4 Channels plus High-Precision, High-Frequency Capabilities)

MB3785A
s METHOD OF SETTING TIME CONSTANT
FOR TIMER/LATCH-ACTUATED SHORT-CIRCUTING PROTECTION CIRCUIT
Figure 5 schematically shows the protection latch circuit.
The outputs from the output-shorting detection comparators 1 to 4 are respectively connected to the inverted
inputs of the SCP comparator. These inputs are always compared with the reference voltage of approximately
2.1 V which is fed to the non-inverted input of the SCP comparator.
While the switching regulator load conditions are stable, there are no changes in the outputs of the comparators
1 to 4 so that short-circuit protection control keeps equilibrium state. At this time, the voltage on the SCP terminal
(pin 23) is held at approximately 50 mV.
When load conditions change rapidly due to a short-circuiting of load, for example, the output voltage of the
comparator for the relevant channel goes “H” (2.1 V or more). Consequently, the SCP comparator outputs a “L”,
causing the transistor Q1 to turn off, and the short-circuit protection capacitor CPE (externally fitted to the SCP
terminal) begins to charge.
VPE = 50 mV + tPE × 10^–6/CPE
0.65 = 50 mV + tPE × 10^–6/CPE
CPE = tPE/0.6 (s)
When the external capacitor CPE is charged to approximately 0.65 V, the SR latch is set and the output drive
transistor is turned off. Simultaneously, the dead time is extended to 100% and the output voltage on the SCP
terminal (pin 23) is held “L”. As a result, the S-R latch input is closed and CPE is discharged.
Figure 5. Protection Latch Circuit
2.5 V
1 µA
Comparator 1
–
Comparator 2
–
Comparator 3
–
Comparator 4
–
+
2.1 V
23
Q2
Q1
CPE
S
R
Latch
PWM
U.V.L.O ^comparator
OUT
17