an8049sh Panasonic Corporation of North America, an8049sh Datasheet - Page 18

• Manufacturer Part Number: an8049sh
• Description: 1.8-volt 3-channel Step-up, Step-down, And Polarity Inverting Dc-dc Converter Control Ic
• Manufacturer: Panasonic Corporation of North America

AN8049SH
I Application Notes (continued)
18
[4] Function descriptions (continued)
[5] Time constant setup for the timer latch short-circuit protection circuit
10. Output 3 block
11. CTL block
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 re-started by on/off control.
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 con-
verter 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.
9. Output 1 and output 2 blocks
can be set up freely by connecting current setting resistors to the RB pins (pins 10 and 11).
I
document.
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 in the shorted
1.26 V = I
∴ t
SI(OUT)
These output circuits have a totem pole structure. A constant-current source output with good line regulation
See the "[2] Main characteristics" section earlier in this document for details on the R
This output circuit has an open collector structure.
An output current of up to 50 mA can be provided, and the output pin has a breakdown voltage of 14.2 V.
This block controls the on/off state of each channel. See the "[9] Sequential operation" section later in this
PE
FB1
FB2
FB3
(s) = 1.15 × C
characteristics.
CHG
24
22
20
×
C
t
PE
SCP
On/off control
SCP
S.C.P. comp.
(µF)
SCP
0.9 V
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
Q1
V
CC
SDH00012BEB
1.1 µA
S.C.P.
High level detection comparator
V
REF
SCP
Latch
U.V.L.O.
R
S
connected to the S.C.P. pin will start to charge.
1.26
0.06
Q
V
SCP
Figure 5. S.C.P. pin charging waveform
Internal reference voltage
(V)
Short-circuit detection time t
Output shutoff
B
vs. I
SO(OUT)
PE
and R
t (s)
B
vs.