FA7703 Datasheet, PDF (14/21 Pages) Fuji Electric – FUJI Power Supply Control IC

FA7703/04
FA7703/7704
(4)PWM comparator
PWM comparator of FA7703/04 has 4 input
terminals. (Fig. 4) The oscillator output 2 is
compared with the CS terminal voltage 1, the
DT terminal voltage3, and the Error Amplifier
output 4. Among those 3 inputs of 1, 3, and
4, the one with the lowest voltage is chosen and
given priority. While the value of the chosen
voltage is lower than the value of the waveform
of oscillator output, PWM comparator output is
set to LOW. Similarly, while the value of the
chosen voltage is higher than the waveform of
oscillator output, PWM comparator output is set
to HIGH. (Fig. 5)
When FA7703/04 are turning ON, the soft start
function starts according to the CS terminal
voltage 1, then the output pulses begin to widen
gradually. The maximum pulse width is adjusted
by changing the DT terminal voltage. In steady
operation, the pulse width is determined with the
condition of the Error Amplifier output 4, and
then the output voltage of DC-DC converter is
stabilized. The operation flow chart of PWM
comparator and OUT terminal is shown in Fig. 5.
The output polarity of OUT1 terminal changes
according to the condition of SEL1 terminal. The
polarity of OUT 2 terminal is different between
FA7703 and FA7704; accordingly, select the type
for your desired circuit design.
1 CS term inal voltage
+
2 Oscillation voltage
-
3 DT terminal voltage
+
4 Error Amplifier output ₊
PW M output
Fig.4
4 _{Error Amplifier output}
3 _{DT terminal voltage}
2 _{Oscillation voltage}
1 _{CS terminal voltage}
PW M
output
OUT1
Nch driven
(SEL1:REG)
OUT1
Pch driven
(SEL1:GND)
OUT2
FA7703
OUT2
FA7704
Fig.5
Fuji Electric Systems Co., Ltd.
AN-057E Rev.1.0
Jun-2010
(5)Soft start function
As described in Fig. 6, a capacitor CCS is
connected to CS terminal. When the power
supply of FA7703/04 starts and the undervoltage
lockout protection circuit (UVLO) is cancelled,
the capacitor CCS is charged by the internal
current source (2.2μA. typ.), and the voltage of
CS terminal rises gradually. Since the voltage of
CS terminal is connected to PWM comparator,
the output pulses begin to widen gradually, and
then the soft start function starts as a result. (Fig.
5)
CS
2
CCS
VCC
UVLO
1.5V
FB
2.2μA
1.27V
+
-
-
S.C.DET
output off
S.C.P
2.0V
Fig.6
(6)Timer latch short-circuit protection circuit
The short-circuit protection circuit of FA7703/04
consists of two comparators. (S.C. DET
comparator, S.C.P comparator) In steady
operation, the output of Error Amplifier is
approximately 1V, accordingly, the output of
S.C.DET comparator is set to High, and the
voltage of CS terminal is clamped at 1.27V. If the
output voltage drops due to a short-circuit or the
like, the output voltage of Error Amplifiers rises.
When the output voltage of Error Amplifiers
exceeds 1.5V(typ.), the output of S.C.DET
comparator is set to Low, and the clamp action
at the CS terminal voltage of 1.27V is cancelled,
then the voltage of CS terminal rises up to the
VREG voltage. When the voltage of CS terminal
exceeds 2.0V(typ.), the output of S.C.P
comparator is set to High, accordingly, the
output of FA7703/04 is shut off. In this case,
FA7703/04 operate in OFF latch mode and the
current consumption in this mode is 1.3mA(typ.).
The operating waveform of the voltage of CS
terminal is shown in Fig. 7.
The approximate time (tp) between the
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