SG6905 Fairchild Semiconductor, SG6905 Datasheet - Page 14

• Manufacturer Part Number: SG6905
• Manufacturer: Fairchild Semiconductor

Green mode PFC/Flyback-PWM Controller
OPERATION DESCRIPTION
The highly integrated SG6905 is specially designed for
power supplies consist of boost PFC and flyback PWM. It
requires very few external components to achieve
green-mode operation and versatile protections. It is
available in 20-pin SOP package.
The patented interleave-switching feature synchronizes
the PFC and PWM stages and reduces switching noise. At
light loads, the switching frequency is continuously
decreased to reduce power consumption.
For PFC stage, the proprietary multi-vector control
scheme provides a fast transient response in a
low-bandwidth PFC loop, in which the overshoot and
undershoot of the PFC voltage are clamped. If the
feedback loop is broken, the SG6905 will shut off PFC to
prevent extra-high voltage on output.
For the flyback PWM, the synchronized slope
compensation ensures the stability of the current loop
under continuous-conduction-mode operation. Built-in
line-voltage
output-power limit. Hiccup operation during output
overloading is also guaranteed.
During start-up, the RDY pin will be pulled low until the
PFC output voltage reaches to the setting level. This
signal can be used to control the second power stage for
proper power on sequence.
SG6905 provides complete protection functions such as
brownout protection and RI pin open/short.
Start Up
Figure 1 shows the start up circuit of the SG6905. A
resistor R
The turn-on and turn-off threshold of SG6905 are fixed
internally at 16V/10V. During start-up, the hold-up
capacitor must be charged to 16V through the start-up
resistor so that SG6905 will be enabled. The hold-up
capacitor will continue to supply V
can be delivered from auxiliary winding of the main
transformer flyback converter. V
10V during this start-up process. This UVLO hysteresis
window ensures that hold-up capacitor is adequate to
supply V
than 25µA startup current, the value of R
reduce power consumption. One 10uF capacitor should
hold enough energy for successful start-up. After start-up,
S1 will switch so that the current I
PFC multiplier. This helps reduce circuit complexity and
power consumption.
© System General Corp.
Version 1.1.1 (IAO33.0060.B1)
DD
AC
during start-up. Since SG6905 consumes less
is utilized to charge V
compensation
DD
DD
AC
maintains
DD
must not drop below
capacitor through S1.
will be the input for
before the energy
AC
can be large to
constant
- 14 -
I
PFC ON/OFF Control and RDY Signal for
Power ON Sequence Control
A PFC on/off control function is built-in to control the
power on and power off of PFC controller. Once the
voltage on this pin is pulled below 2.45V, the OPFC will
be enabled. Once the OPFC is enabled, the output voltage
of the PFC converter will gradually increase to the
regulated voltage. To provide a proper power on sequence
control, a RDY pin will be pulled high after the PFC
voltage reach 90% (FBPFC>V
voltage.
Switching Frequency and Current
Sources
The switching frequency of SG6905 can be programmed
by the resistor R
The relationship is:
For example, a 24KΩ resistor R
switching frequency. Accordingly, a constant Current I
will flow through R
T
is used to generate internal current reference.
FIG.1 Start up circuit of the SG6905
I
F
T
OSC
=
=
R
1.2V
I
R
(kΩ
www.sg.com.t w • www.fairchildsemi.com
1560
I
(kΩ
I
)
connected between RI pin and GND.
(mA)
I
.
)
(kHz)
---------------- (2)
----------- (1)
FB-RDY-HIGH
Product Specification
I
results in a 65 KHz
September 27, 2007
) of its regulated
SG6905
T