FA5502P/M
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9. Design advice
Even after PFC starts up, Vcc may fall due to step
changes of the load or inputs. To prevent the IC from
stopping in those cases, the circuit shown in Fig.17 is
effective to prolong the hold time of the Vcc voltage.
After the PFC converter starts up, Vcc is supplied
through C6. Therefore, you can prolong the hold time
of Vcc by using a large capacity for C6.
(1) Vcc circuit
Vcc voltage can be supplied from an auxiliary
winding of the inductor. An example circuit is shown in
Fig.15.
L
D1
Vac
(sub)
D3
Co
R8
D3
R8
D4
C6
Q1
VCC
sub
Rs
C5
C5
10
VCC
FA5502 GND
Fig.17 Vcc circuit (2)
7
In some case, the Vcc voltage cannot be supplied
enough in light load condition. In this case, the circuit
shown in Fig.18 may be effective to improve the Vcc.
The appropriate value of C7 and R9 should be
determined by evaluation in actual circuit because
they depend on each circuit.
Fig.15 Vcc circuit (1)
In this circuit, R8 is a start up resistor. The start up
resistor R8 should be satisfied the following formula in
order to supply with at least 30µA of IC start up
current.
R8
D3
2 × Vac(min) −17.5
C7 R9
VCC
R8 <
• • • • • (11)
30×10−6
sub
Note that this formula is a minimum condition for
starting the IC. Practically, determine the value upon
taking into account the start up time required for
converter. The start up time must be determined upon
measurement at actual circuit operation.
C5
D5
Fig.18 Vcc circuit (3)
(2) Supplying Vcc from external power supply
If Vcc is not supplied from the auxiliary winding of
inductor but from an external power supply, pay
attention to the followings.
In steady state, Vcc is supplied from the auxiliary
winding (sub) of inductor. When the IC is just starting
up, however, it takes time for the voltage from auxiliary
winding to rise enough. The value of capacitor C5
connected to Vcc pin should be determined to prevent
Vcc from falling below the OFF threshold voltage of
UVLO during this period. The capacity of C5 should be
determined by evaluation in the actual circuit because
the time lag is different in each circuit.
• In order to start up the IC, Vcc must be above the ON
threshold voltage VTHUON (17.5V (max.)) of
undervoltage lockout circuit (UVLO). When starting
up, apply at least this VTHUON. After starting up,
the operation is available within the recommended
range of 10 to 28V.
• If a noise is applied to Vcc pin, it may cause
malfunction. To avoid a noise, connect a capacitor
near VCC pin even when Vcc is supplied from an
external power supply. To prevent a malfunction,
suppress the noise below about ±0.6V. And, make
sure there is no malfunction attributable to noise.
Vcc
UVLO
ON
UVLO
OFF
Vcc must not drop
below UVLO OFF.
Auxiliary winding voltage
t
Fig.16 Vcc voltage at start up
18
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