FA7764 Datasheet, PDF (12/18 Pages) Fuji Electric – FUJI Power Supply Control IC

FA7764
8. Operation description of each block
(1) Reference voltage circuit
The reference voltage circuit generates the feed
back voltage reference (VIN) of 1.00V±1% which
compensated in temperature from VCC voltage, and
the regulated voltage (CREG) of 3.0V
The feedback voltage (VIN) is connected to the
non-inverting input of error amplifier (OTA) as a
reference voltage of the error amplifier.
The regulated voltage (CREG) is provided as a
regulated power supply for IC’s internal blocks, and
it has to connect capacitors in order to stabilize
voltages.
The CREG voltage is designed specifically for the
power supply for IC’s internal blocks. Therefore it
cannot be used as external stabilized power
supplies.
To determine capacitance of CREG pin, refer to
recommended operating conditions.
(2) Oscillator
The oscillator generates a triangular waveform by
charging and discharging the capacitor.
Oscillation frequency can be set by the value of
resistor connected to the RT pin
(Fig.1)
8765
VCC OUT VBIAS IN
(Operating frequency decreases
when RT increases, Operating
frequency increases when RT
CREG ENB RT GND
1234
decreases)
RT
Set the oscillation frequency
between 30 kHz and 400 kHz. The
RT pin outputs DC voltage of 1 V.
Fig.1
(3) Error amplifier (OTA)
The IN pin (5 pin) is the inverting input of the error
amplifier (OTA). The non-inverting input is internally
connected to the feedback reference voltage (VIN)
with 1.0 V ± 1%. The output of OTA has no external
pin and the phase compensations are built-in.
The feedback voltage of the DC-DC converter’s
output is connected to the IN pin by the divider
resister. The output voltage Vout of the DC-DC
converter can be calculated as follows;
Vout = ^{R1+ R2} ×VIN
R2
C1
R1
Vout
If it operates
without stability,
connect the
capacitor C1 in
order to adjust
the
phase
compensation.
8765
VCC OUT VBIAS IN
R2
CREG ENB RT GND
1234
Fig.2
(4) PWM comparator
The ON state of output (at the OUT pin) starts with
the ON trigger signal of the oscillator. It turns off
when the inductor current reaches to the specified
level by the error amplifier (OTA) output.
(5) Soft start circuit
In order to prevent the abnormal start up of DC-DC
converter such as rush current, the soft start circuit
is built-in.
The soft start is performed by raising the feedback
reference voltage of the error amplifier (OTA) step
by step. Therefore the output voltage of DC-DC
converter rises slowly.
The soft start time is fixed to 8msec (typ.)
internally.
The soft start stars after the input voltage reaches
the ON threshold (6.9V typ.) or more of under
voltage lock out (UVLO). (Fig. 3).
In power supplied condition, the soft start is
controlled by the ENB signal.
Start up by the power supply with the low level ENB signal
Startup by the ENB signal
ENB
VCC
ENB terminal low
Vout
Fig. 3
8msec typ.
(6) Timer and latch for short circuit protection
When the output voltage of the DC-DC converter
drops continues in constant time, the timer and latch
short circuit protection stops switching operation in
order to protect the DC-DC converter.
If the output voltage of DC-DC converter drops due
to output short-circuit or over load, the output
voltage feedback IN pin of the error amplifier will
also decreases. When the IN pin voltage drops
down to 0.75V (typ.), the timer latch starts its
counter.
If the drops continues and the counter operates
beyond delay time (90msec typ.), the protection
circuit regards the case as abnormal. Therefore it
stops switching, and protect the circuit (Fig. 4).
Fuji Electric Systems Co., Ltd.
AN-059E Rev.1.0
12
June.2010
http://www.fujielectric.co.jp/fdt/scd/