FA551x ETC, FA551x Datasheet - Page 10

• Manufacturer Part Number: FA551x
• Description: CMOS IC For Switching Power Supply Control
• Manufacturer: ETC

FA551X
3. CS pin circuit
As shown in Figure 5, capacitor Cs is connected to the CS pin.
The CS pin voltage varies depending on the charging voltage of
this capacitor Cs.
When the power is turned on, the constant current source
(5.2µA) begins to charge capacitor. Accordingly, the CS pin
voltage rises as shown in Figure 6. The CS pin voltage is
connected to the PWM comparator, which is characterized to
make output based on the lowest of input voltages. The device
enters soft-start mode while the CS pin voltage is between 1.0V
and V
During normal operation, the CS pin is clamped at 4.0V by
internal zener diode. If the output voltage drops due to an
overload and the FB voltage rises to 3.5V or more, the clamp
voltage 4.0V is canceled and the CS pin voltage rises to 9.5V.
The CS pin is also connected to latch comparator C2. If the CS
pin voltage rises to 8.5V or more, comparator C2 toggles to turn
off the 5V REF circuit, thereby shutting the output down.
Since the CS pin is also connected to comparator C1, the 5V
REF circuit can be turned off to shut the output down by
dropping the CS pin voltage below 0.68V. In this way,
comparator C1 can be used for output on-off control.
As explained above, the CS pin can be used for soft-start,
overload output shutdown, and output on-off control by varying
the voltage. Further details on the above three major functions
of the CS pin are given below.
3.1 Soft start function
Figure 7 shows the soft start circuit. Figure 8 is a soft-start
operation timing chart. The CS pin is connected to capacitor
Cs. When the power is turned on, the constant current source
(5.2µA) begins to charge the capacitor. As shown in the timing
chart, the CS pin voltage rises slowly in accordance with the
capacitor Cs charging current. The CS pin is also connected to
the IC internal PWM comparator, which has such
characteristics that the voltage is determined to output on the
basis of the lowest of input voltages. The comparator output
pulse slowly widens to cause a soft start as shown in the timing
chart.
The soft start period can be approximately estimated by the
period t
pulse width widens to 30%. The period t
following equation:
t
Cs: Soft start capacitor [µF]
10
S
[ms]
THCSM
S
, from the time the IC is activated to the time the output
310
(FA5510/14: 1.92V, FA5511/15: 2.4V).
C
S
[ms]
..................................................... (3)
S
is given by the
FB pin voltage
Oscillator output
DT voltage
CS pin voltage
OUT pin output
REF
FB
FB
0.68/0.8
V
THCSM
2
7
2
9.5
8.5
4.0
1.0
0
Cs
5V VCC
Fig. 8
5V REF
OFF mode
Fig. 6
Fig. 7
ENB
3.5V
8
OSC
Fig. 5
CS
DMAX
1
Soft start
RT
Soft start timing chart
C3
5.2 A
CS pin waveform
Soft start circuit
6
CS pin circuit
OSC
VCC
1
RT
DMAX
C1
C2
Momentary
overload or
overvoltage
8.5/7.9V
4V
0.8/0.68V
PWM
Cs
Time t
PWM
8
CS
5.2 A
Output
circuit
Overload or
overvoltage
VCC
Output
circuit
UVLO
Shutdown
6