FA7700V Datasheet, PDF (9/13 Pages) List of Unclassifed Manufacturers – For Switching Power Supply Control

FA7700V, FA7701V
s Description of each circuit
1. Reference voltage circuit
This circuit consists of the reference voltage circuit using band
gap reference, and also serves as the power supply of the
internal circuit. The precision of output is 2.23V±3%.
It is stabilized under the supply voltage of 2.5V or over.
The precision of reference voltage of error amplifier circuit is
0.88V±2%, and the reference voltage circuit is connected to the
non-inverting input of the error amplifier circuit.
2. Oscillator
The oscillator generates a triangular waveform by charging and
discharging the built-in capacitor. A desired oscillation
frequency can be determined by the value of the resistor “RT”
connected to the RT terminal (Fig. 1).
The built-in capacitor voltage oscillates between approximately
0.66V and 1.1V with almost the same charging and discharging
gradients. You can set the desired oscillation frequency by
changing the gradients using the resistor connected to the RT
terminal. (Large RT: Low frequency, small RT: High frequency)
The oscillator waveform cannot be observed from the outside
because a terminal for this purpose is not provided. The
oscillator output is connected to the PWM comparator.
3. Error amplifier circuit
The IN(–) terminal (Pin 3) is an inverting input terminal.
The non-inverting input is internally connected to the reference
voltage (0.88V±2%; 25˚C). The FB terminal (Pin 4) is the
output of the error amplifier. Gain setting and phase
compensation setting is done by connecting a capacitance and
a resistor between the FB terminal and the IN(–) terminal. Vout
which is the output voltage of DC to DC converter can be
calculated by:
Vout = VB Ï« ^{R1 + R2}
R2
Gain AV between the Vout and the FB terminal can be
calculated by:
AV = – ^RNF
R1
4. PWM comparator
The PWM comparator has 4 input terminals. (Fig. 4)
The oscillator output ቢ is compared with the CS terminal voltage ባ,
and the error amplifier voltage ቤ, then, the lower voltage between ባ
and ቤ is preferred.
While the preferred voltage is lower than the oscillator output, the
PWM comparator output is Low. While the preferred voltage is higher
than the oscillator output, the PWM comparator output is High (Fig.
5). When the IC starts, the capacitor connected to the CS terminal is
charged through the resistor connected to the power supply, and
then the output pulses begin to widen gradually as the operation of
soft start.
In steady operation, the pulse width is determined based on the
voltage of the error amplifier ቤ, and then the output voltage is
stabilized. The Dead Time control voltage (ብ DT voltage) of FA7700
and FA7701 has different characteristics to adjust the ICs to various
types of power supply circuits being controlled and also to reduce
external discrete components as many as possible. FA7700 is
developed for fly-back circuits, and boost circuits, and the DT voltage
is set in the IC so that the maximum output duty cycle is fixed to 80%
min.. (Maximum output duty cycle changes according to operation
frequencies. ––See page 6 “Maximum output duty vs. temperature”.)
It prevents magnetic saturation of the transformer or the like when a
short-circuit in the output circuit occurs. FA7701 is developed for
buck circuits, and it is designed for the maximum output duty cycle of
100%. The timing chart of PWM comparator is described in Fig. 5.
OSC
RT
Fig. 1
RT value: small
RT value: large
1.1V
0.66V
Fig. 2
Vout
RNF
R1
3
IN(–)
R2
Er. AMP
4
FB
V_B
(0.88V)
PWM
Fig. 3
ቢ Oscillation output
–
ባ CS terminal voltage
+
ቤ Error amplifier output
+
ብ DT voltage
+
Fig. 4
PMW output
ቢ Oscillation output
ቤ Error amplifier output _{ባ CS terminal voltage}
ብ DT voltage
PWM
output pulse
Fig. 5
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