FA551 Datasheet, PDF (18/22 Pages) List of Unclassifed Manufacturers – CMOS IC For Switching Power Supply Control

FA551X
8. Simple voltage control on the primary side
In a flyback type power supply, the output voltages of the power
supply and auxiliary winding voltage are almost proportional to
the number of winding turns of the transformer. This
characteristic can be used in the circuit shown in Figure 30,
where the output voltage can easily be made constant by
detecting the auxiliary winding voltage.
However, this is an easy output voltage control method, and the
output voltage precision and regulation are therefore not as
good.
To reduce output pulse width completely to 0%, the FB pin
voltage must fall below 0.9V and R5 must be set below about
1kΩ from the characteristics of the FB pin voltage and source
current.
9. Disabling the overload shutdown function
As shown in Figure 31, connect a 10kΩ resistor R6 between
the FB pin and the ground. The FB pin voltage then does not
rise sufficiently high to reach the shutdown threshold voltage
when an overload occurs so that IC does not enter OFF latch
mode. Use a 5% or better-precision resistor for R6.
Even with this connection, the overvoltage shutdown function is
available.
10. Polarities for overcurrent detecting and their
characteristics
The FA5510/11 uses positive polarity detection for overcurrent
limiting (number 3 pin of IS pin) and the FA5514/15 uses
negative polarity detection. The characteristics of positive and
negative polarity detection are summarized below. Select one
in accordance with the circuit used. (See item 4 in “Description
of each circuit.”)
Positive detection (FA5510/11)
• Wiring is easy because the ground can be shared by the main
circuit and IC peripherals.
• It is easy to correct the current detected as overload against
the input voltage.
Negative detection (FA5514/15)
• The MOSFET drive current does not flow to the current
detection resistor and therefore it hardly affects overcurrent
detection.
11. Correcting overload detection current (FA5510/11 only)
If the power supply output is overloaded, the overcurrent
limiting function restricts the output power and the overload
shutdown function stops the IC. The output current when an
overload occurs varies depending on the input voltage; the
higher the input voltage, the more the overload detection
current may increase. If any problems occur as a result of the
appearance of this symptom, connect resistor R8 between
current detection resistor Rs and the IS (+) pin and add resistor
R7 for correction as shown in Figure 32. The standard
resistance of R8 is several hundred ohms, and that of R7 is
from several hundred kilohms to several megohms.
Note that the above correction slightly lowers the output current
when overload even where the input voltage is low. This
correction is available only for the FA5510/11 that uses positive
polarity for overcurrent detection.
C1 n1 n2
Vout
6
7
VCC _REF
FA551X
GND ₄ FB ² _R5
n3
C2
Fig. 30 Simple voltage control circuit
R6
10kΩ
FA551X
FB
2
GND
4
R3
C4
Fig. 31 Disabling overload shutdown function
DB
~+
AC input
~–
T1
C1
FA5510/11
R7
R8
IS(+) ³
Rs
Fig. 32 Correction of overload detection current
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