FA5550 Fuji Electric holdings CO.,Ltd, FA5550 Datasheet - Page 14

• Manufacturer Part Number: FA5550
• Description: Power Factor Correction Converter
• Manufacturer: Fuji Electric holdings CO.,Ltd

11. Directions for design
(1-1) Output voltage setting
In order to detect FB pin open, pull-up current (Ipullup) is fed
through FB pin. This current flows into GND via R2. When setting
output voltage (Vout), therefore, set resistors R1 and R2 in
consideration of this current.
VREF
Ipullup
(For reference)
The current IR2 flowing through R2 is given as:
Voltage at the FB pin, VREF is given as:
The integrated result of Vout is given by the above formula.
(1-2) Slope compensation
Sub-harmonic oscillation may be generated in the current mode
control in case of current continuous mode operation with the duty
cycle of 50 percent or more. To avoid this phenomenon,
FA5550/51 have slope compensation circuits built-in. In addition, it
is possible to enhance the slope compensation by connecting a
resistor (Rslope) between No.5 pin (SLOPE pin) and GND. The
following formula shows the amount of slope compensation in
case of connecting the resistor. See P.14 for regulation of the
slope compensation.
Rslope
For direct connection of SLOPE pin to GND, the slope
compensation value of 24.5mV/us is obtained.
(1-3) Brownout protection
FA5550/51 detect the AC input voltage using MUL pin voltage.
When the AC input voltage decreases, protection circuit is
activated to stop the output of the IC. For resistors connected to
MUL pin, 10kΩ and 790kΩ are recommendable for R
R
voltages become 70Vac and 63Vac respectively.
With MUL pin, AC input voltage compensation is made on
over-current detection level. Changing the resistance values of
R
of the compensation values. Therefore, the above resistance
values are recommendable.
Rev.1 Feb-2008
Vout
VREF
IR
SLOPE
MUL2
MUL1
2
=
=
=
respectively
and R
(
=
Vout
(
VREF
R
: Reference voltage = 2.5V (typ)
: Pull-up current in FB pin = 5uA (typ)
=
: Resistance (kΩ) between SLOPE pin and GND
R
2
24
2
×
MUL2
×
−
{(
5 .
IR
VREF
/
Vout
+
R
2
(
.
connected to MUL pin may result in difference
2
. 0
In this case, the brownout protection operating
−
12
−
/ )
Ipullup
VREF
×
R
1
Rslope
+
Ipullup
/ )
)
×
R
)
R
1
1
+
+
(
Ipullup
mV
VREF
/
us
}
)
FA5550 / 51
14
MUL1
and
R1
R2
Vout
Fig.17 Output voltage setting circuit
SLOPE
Fuji Electric Device Technology
Fig.18 Slope compensation circuit
1
Fig.19 Brownout setting circuit
R
R
FB
5
MUL2
MUL1
Ipullup
Pull Up Current
Rslope
SLOPE
MUL
ERRAMP
4
3
IS
VREF(2.5V)
MUL
ERRAMP
COMP
CUR.comp
Rs