FA551 Datasheet, PDF (14/22 Pages) List of Unclassifed Manufacturers – CMOS IC For Switching Power Supply Control

FA551X
■ Design advice
1. Deciding the startup circuit
These ICs, which use CMOS process, consume less current,
and therefore can use larger startup resistance than the
conventional bipolar type of IC. To decide the startup
resistance, the following conditions must be satisfied:
(a) The IC is started when the power is turned on.
(b) The IC consumption current is supplied during latch mode
operation to maintain the latch state.
(c) The IC consumption current is supplied during the off state
under the on/off function to maintain the off state.
However, these are the minimum conditions for using the IC.
The startup time required for the power supply must also be
decided on.
1.1 Connecting the startup resistor before rectification
(AC line)
When the startup resistor is connected before rectification (AC
line) as shown in Figure 18, the voltage applied to the startup
resistor forms a half-wave rectified waveform 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 the temperature characteristics.
(a) To supply startup current 30µA at ON threshold voltage
17.5V (max.) of UVLO:
R1
Ȉ2˭˭
Ͻ^π
Ï«
Vac
–
17.5
...................................................... ₍₅₎
0.03
(b) To supply IC consumption current 80µA (max.) (Vcc=10V) in
latch mode:
R1
Ͻ
Ȉ2˭˭
π
Ï«
Vac
–
10
........................................................ ₍₆₎
0.08
(c) To supply IC consumption current 200µA (max.) (Vcc=17V)
in the off state under the on/off function:
R1
Ͻ
Ȉ2˭˭
π
Ï«
Vac
0.2
–
17
........................................................ ₍₇₎
R1: Startup resistance [kΩ]
Vac: Effective value of AC input voltage [V]
If neither the latch mode operation nor the on/off functions are
used, only the expression in (5) needs to be satisfied. In this
method, the supply current to the IC via the start-up resistor is
stopped when AC input is shut down. Therefore, after latch
mode operation, shutting the AC input down resets the latch
mode in a very short period of time.
AC INPUT
DB
~+
T1
C1
~
R1
D1
VCC
6
FA551X
C2
5
OUT
MOSFET
Rs
Fig. 18 Startup circuit (1)
14