MB3775
■
HOW TO SET TIME CONSTANT FOR TIMER LATCH SHORT PROTECTION CIRCUIT
TIMING CHART shows the configuration of the protection latch circuit.
Error amplifier outputs, are internally connected to the non-inverting inputs of the short-circuit protection com-
parator and are compared with the reference voltage (1.1 V) connected to the 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 protection enable terminal (pin 15) voltage is kept
at about 50 mV.
If the load condition drastically changes due to a load short-circuit and if low-level signals (1.1 V or lower) are
input to the non-inverting inputs of the short-circuit protection comparator from the error amplifiers, the short-
circuit protection comparator outputs a “Low” level to turn transistor Q
1
off. The protection enable terminal voltage
is discharged, and then the short-circuit protection comparator charges the externally connected protection
enable capacitor C
PE
according to the following formula:
V
PE
= 50 mV + t
PE
x 10
-6
/C
PE
0.65 = 50 mV + t
PE
x 10
-6
/C
PE
C
PE
= t
PE
/0.6 (µF)
When the protection enable capacitor charges to about 0.65 V, the protection latch is set to enable the under
voltage lockout protection circuit and to turn the output drive transistor off. The dead time is set to 100 %.
Once the under voltage lockout protection circuit is enabled, the protection enable is released; however, the
protection latch is not reset if the power is not turned off.
The non-inverting inputs of the D.T.C. comparator are connected to the D.T.C. terminals (pins 6 and 11) through
the power supply (about 0.9 V) and are compared with a reference voltage (about 1.8 V) connected to the
inverting input.
To prevent malfunction of the short protection circuit in soft-start mode (using D.T.C. terminals), the D.T.C.
comparator outputs a “High” level to turn Q
2
on until the D.T.C. terminals (pins 6 and 11) voltage drops to about
0.9 V.
Fig. 7 - Protection Latch Circuit
2.5 V
1
µA
S.C.P.Comp.
R
1
Error Amp1
Error Amp2
1.1 V
+
+
−
SCP
15
C
PE
S
R
U.V.L.O.
Q
1
Q
2
Q
3
Latch
+
+
−
0.9 V
0.9 V
1.8 V
6
11
D.T.C.1
D.T.C.2
D.T.C.Comp.
11
FUJITSU [ FUJITSU COMPONENT LIMITED. ]
