MB3778 Datasheet, PDF (14/29 Pages) Fujitsu Component Limited. – Switching Regulator Controller

MB3778
s HOW TO SET TIME CONSTANT FOR TIMER LATCH SHORT-CIRCUIT
PROTECTION CIRCUIT
Below Figure shows the configuration of the protection latch circuit.
Each error amplifier output is connected to the inverting inputs of the short-circuit protection comparator and is
always compared with the reference voltage (2.1 V) connected to the non-inverting input.
When the load condition of the switching regulator is stable, the error amplifier has no output fluctuation. Thus,
short-circuit protection control is also kept in balance, and the SCP terminal (pin 15) voltage is held at about 50 mV.
If the load changes drastically due to a load short-circuit and if the inverting inputs of the short-circuit protection
comparator go above 2.1 V, the short-circuit protection comparator output goes “Low” to turn off transistor Q1.
The SCP terminal voltage is discharged, and then the short-circuit protection comparator charges the protection
enable capacitor CPE according to the following formula :
VPE = 50 mV + tPE × 10 ^{− 6} / CPE
0.65 = 50 mV + tPE × 10 ^{− 6} / CPE
CPE = tPE / 0.6 (µF)
When the protection enable capacitor is charged to about 0.65 V, the protection latch is set to enable the under
voltage lockout circuit and the output drive transistor is turned off. The idle period is also set to 100% at the
same time.
Once the under voltage lockout circuit is enabled, the protection enable is released; however, the protection
latch is not reset if the power is not turned off.
The inverting inputs (pin 6 or 11) of the D.T.C. comparator are compared to the reference voltage (about 1.1 V)
connected to the non-inverting input.
To prevent malfunction of the short-circuit protection-circuit when the soft-start operation is done by using the
DTC terminal, the D.T.C. comparator outputs a “High” level while the DTC terminal goes up to about 1.1 V, and
then closes the SCP terminal by turning transistor Q2 on.
• Protection Latch Circuit
1 µA
S.C.P. Comp.
Error Amp. 1
−
Error Amp. 2
−
+
2.1 V
R1
15
Q1
Q2
Q3
CPE ^{S R}
Latch
2.46 V
U.V.L.O.
D.T.C. Comp.
1.1 V
6 DTC1
11 DTC2
14