AN8049 Panasonic Semiconductor, AN8049 Datasheet - Page 18

• Manufacturer Part Number: AN8049
• Description: 1.8-volt 3-channel step-up/ step-down/ and polarity inverting DC-DC converter control IC
• Manufacturer: Panasonic Semiconductor

AN8049FHN
18
[5] Time constant setup for the timer latch short-circuit protection circuit
comparator continuously compares a 0.9 V reference voltage with the FB1, FB2, and FB3 error amplifier outputs.
average value since there are no fluctuations in the error amplifier outputs. At this time, the output transistor Q1 will
be in the conducting state, and the S.C.P. pin will be held at 0 V.
protection comparator from the error amplifier output, the short-circuit protection comparator will output a low level
and the output transistor Q1 will shut off. Then, the capacitor C
When the external capacitor C
will latch and the dead-time will be set to 100% with the output held fixed at the low level. Once the latch circuit has
latched, the S.C.P. pin capacitor will be discharged to about 0 V, but the latch circuit will not reset unless either power
is turned off or the power supply is restarted using on/off control.
in the shorted state, and the IC starts to charge the
S.C.P. pin capacitor. Therefore, users must select an
external capacitor that allows the DC-DC converter
output voltage to rise before the latch circuit in the
later stage latches. In particular, care is required if the
soft start function is used, since that function makes
the startup time longer.
Application Notes (continued)
Figure 6 shows the structure of the timer latch short-circuit protection circuit. The short-circuit protection
When the DC-DC converter output load conditions are stable, the short-circuit protection comparator holds its
If the output load conditions change rapidly and a high-level signal (0.9 V or higher) is input to the short-circuit
At power supply startup, the output appears to be
1.26 V
t
PE
(s)
I
FB1
FB2
FB3
CHG
1.15
22
20
18
C
C
t
PE
SCP
SCP
( F)
On/off control
SCP
S.C.P. comp.
is charged to about 1.26 V by the constant current of about 1.1 mA, the latch circuit
Figure 6. Short-circuit protection circuit
0.9 V
Q1
V
CC
1.1 A
S.C.P.
1.26
High level detection comparator
V
1.26 V
SCP
0
Figure 5. S.C.P. pin charging waveform
SCP
U.V.L.O.
Latch
(V)
Short-circuit detection time t
R
S
connected to the S.C.P. pin will start to charge.
Q
V
REF
Output shutoff
Voltage Regulators
PE
t (s)