AN8014 データシートの表示（PDF） - Panasonic Corporation
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AN8014 Datasheet PDF : 18 Pages
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Voltage regulators
AN8014S
s Application Notes (continued)
[1] Function descriptions (continued)
4. Timer latch short-circuit protection circuit
This circuit protects external main switching devices, flywheel diodes, choke coils and so forth from breakdown
or deterioration when overload or short-circuit of power supply lasts a certain time.
Figure 3 shows the short-circuit protection circuit. The timer latch short-circuit protection circuit detects the
output level of the error amplifier.
If the output voltage of the DC-DC converter is stable, the output of the error amplifier from the FB pin is stable
and the short-circuit protection comparator is well balanced.
In that case, the transistor Q1 is conductive and the S.C.P. pin voltage is approximately 30 mV constantly.
If the load condition changes radically and output signal voltage of the error amplifier (FB) is 1.87 V or
higher, the short-circuit protection comparator outputs low-level voltage. Then, by cutting off the transistor Q1,
the external capacitor CS of S.C.P. pin (pin 5) starts charging with the current ICHG which is obtained from the
following formulas.
VPE = VSTBY + ICHG ×
tPE
CS
[V]
0.75 V = 0.03 V + ICHG ×
tPE
CS
CS = ICHG ×
tPE
0.72
[F]
ICHG is constant current which is determined by the timing resistor RT .
If RT is 15 kΩ, ICHG will be approximately 2.3 µA.
ICHG =
VRT
RT
1
× 11
[A]
When the external capacitor CS is charged up to approximately 0.75 V, the latch circuit will be turned on. Then
the totem-pole output pin will be set to low level and the dead-time will be set to 100%.
When the latch circuit is turned on, the S.C.P. pin will discharge electricity till the voltage on the S.C.P. pin
reduces to approximately 30 mV. The latch circuit cannot be, however, reset until power supply to the AN8014S
is turned off.
IN+ 6
IN− 7
FB 8
Error amp.
S.C.P.
comp.
1.82 V
ICHG
Q1
VREF
SQ
RQ
Latch
Cut output off
Q2
5
S.C.P.
CS
Figure 3. Short-circuit protection circuit
5. Low input voltage malfunction prevention circuit (U.V.L.O.)
This circuit protects system from breakdown or deterioration caused by malfunction in control circuit when
supply voltage is dropped during transient time at power on or off.
The low input voltage malfunction prevention circuit detects internal reference voltage which changes in
accordance with the supply voltage level. When the supply voltage is turned on, it sets the dead-time of Out pin
(pin 13) to 100% and keeps the DTC pin (pin 4) and S.C.P. pin (pin 5) low level until the supply voltage reaches
3.1 V. When the supply voltage falls, it will operate even below 2.96 V because of its hysteresis width of 140 mV.
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