FA551 Datasheet, PDF (15/22 Pages) List of Unclassifed Manufacturers – CMOS IC For Switching Power Supply Control

1.2 Connecting the startup resistor after rectification
(DC line)
When the startup resistor is connected after rectification (DC
line) as shown in Figure 19, the voltage applied to the startup
resistor becomes the peak value of the AC input voltage.
Startup resistor R1 must satisfy the three equations shown
below. Select a smaller-side value for R1 in consideration of
temperature characteristics.
(a) To supply startup current 30µA at ON threshold voltage
17.5V (max.) of UVLO:
R1
Ͻ
Ȉ˭2˭ ϫ
Vac –
0.03
17.5
...................................................... ₍₈₎
(b) To supply IC consumption current 80µA (max.) (Vcc =10V)
in latch mode:
R1 Ͻ ^{Ȉ˭2˭ϫ Vac – 10 ........................................................} (9)
0.08
(c) To supply IC consumption current 200µA (max.) (Vcc = 17V)
in the off state under the on/off function:
R1
Ͻ
Ȉ˭2˭
Ï«
Vac
0.2
–
17
...................................................... ₍₁₀₎
R1: Startup resistance [kΩ]
Vac: Effective value of AC input voltage [V]
If neither the latch nor the on/off functions are used, only the
expression in (8) needs to be satisfied. In this method, after
latch mode operation, smoothing capacitor C1 in the main
circuit supplies current to the IC via the startup resistor even if
the AC input is shut down. Therefore, some time must elapse
before the latch mode is reset.
2. Determining the Vcc capacitor value
To properly start the power supply, a certain value is required
for the capacitor connected to the VCC pin. Figure 20 shows
the Vcc voltage at start-up when a proper value is given to the
capacitor. When the input power is turned on, the capacitor
connected to the VCC pin is charged via the startup resistor
and the voltage increases. The IC is then in standby state and
almost no current is consumed. (Icc<2µA) Thereafter, Vcc
reaches the ON threshold voltage of UVLO and the IC begins
operation. When the IC begins operation to make output, the IC
operates based on the voltage from the auxiliary winding.
When the IC is just starting up, however, it takes time for the
voltage from the auxiliary winding to rise enough, and Vcc
drops during this period.
Determine the Vcc capacitor value so that Vcc will not drop
down to the OFF threshold voltage of UVLO during this period.
If the Vcc capacitor value is too small, Vcc will drop to the OFF
threshold voltage of UVLO before the auxiliary winding voltage
rises enough. If so, Vcc repeatedly goes up and down between
the UVLO threshold voltages, and the power supply cannot
start up. (Figure 21)
FA551X
DB
~+
AC INPUT
~–
T1
C1
R1
D1
VCC
6
FA551X
C2
5
OUT
MOSFET
Rs
Fig. 19 Startup circuit (2)
Vcc
UVLO
ON
UVLO
OFF
Vcc must not drop to
UVLO OFF.
Auxiliary winding
voltage
Time t
Fig. 20 Vcc voltage at startup with a adequate capacitor
Vcc
UVLO
ON
UVLO
OFF
Time t
Fig. 21 Vcc voltage at startup with a inadequate capacitor
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