SG6905SZ Fairchild Semiconductor, SG6905SZ Datasheet - Page 16

• Manufacturer Part Number: SG6905SZ
• Description: IC PWM CTLR PFC/FLYBACK 20-SOP
• Manufacturer: Fairchild Semiconductor

Specifications of SG6905SZ

• Mode: Average Current
• Frequency - Switching: 65kHz
• Current - Startup: 10µA
• Voltage - Supply: 16 V ~ 20 V
• Operating Temperature: -40°C ~ 105°C
• Mounting Type: Surface Mount
• Package / Case: 20-SOIC (7.5mm Width)
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

© 2007 Fairchild Semiconductor Corporation
SG6905 • Rev. 1.1.2
Functional Description
The highly integrated SG6905 is specially designed for
power supplies consisting of boost PFC and flyback
PWM. It requires very few external components to
achieve
protections. It is available in a 20-pin SOP package.
The
synchronizes the PFC and PWM stages and reduces
switching noise. At light loads, the switching frequency
is
consumption.
For the 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 shuts off PFC to
prevent extra-high voltage on output.
For
compensation ensures the stability of the current loop
under continuous-conduction-mode operation. Built-in
line-voltage compensation maintains constant output-
power limit. Hiccup operation during output overloading
is also guaranteed.
During startup, the RDY pin is pulled low until the PFC
output voltage reaches the setting levels. This signal
can be used to control the second power stage for
proper power-on sequence.
In addition, SG6905 provides complete protection
functions, such as brownout protection and RI pin
open/short.
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 the PFC controller. Once the
voltage on this pin is pulled below 2.45V, the OPFC is
enabled. Once the OPFC is enabled, the output voltage
of the PFC converter gradually increases to the
regulated voltage. To provide a proper power-on-
sequence control, a RDY pin is pulled high after the
PFC voltage reaches 90% (FBPFC>V
regulated voltage.
Startup
Figure 24 shows the startup circuit of the SG6905. A
resistor R
through S1. The turn-on and turn-off threshold of
SG6905 are fixed internally at 16V/10V. During startup,
the hold-up capacitor must be charged to 16V through
the startup resistor so that SG6905 is enabled. The
hold-up capacitor continues to supply V
energy can be delivered from auxiliary winding of the
main transformer flyback converter. V
below 10V during this startup process. This UVLO
hysteresis window ensures that the hold-up capacitor is
adequate to supply V
consumes less than 25µA startup current, the value of
R
10µF capacitor should hold enough energy for
successful startup. After startup, S1 switches so that
AC
can be large to reduce power consumption. One
continuously
the
proprietary
AC
green-mode
flyback
is utilized to charge the V
decreased
PWM, the synchronized slope
DD
interleave-switching
during startup. Since SG6905
operation
to
DD
FB-RDY-HIGH
reduce
and
must not drop
DD
DD
before the
capacitor
versatile
feature
) of its
power
the current I
helps
consumption.
Switching Frequency and Current Sources
The
programmed by the resistor R
RI and GND pins. The relationship is:
For example, a 24kΩ resistor R
switching frequency. Accordingly, a constant current I
flows through R
I
Line Voltage Detection (VRMS)
Figure 25 shows a resistive divider with low-pass
filtering for line-voltage detection on the VRMS pin. The
V
brownout protection.
For brownout protection, the SG6905 is disabled with
195ms delay time if the voltage V
For PFC multiplier, please refer to below section for
more details.
T
I
f
16
OSC
T
RMS
is used to generate internal current reference.
=
=
I R
Figure 25. Line Voltage Detection Circuit
switching
1.2V
voltage is used for the PFC multiplier and
I R
Figure 24. Startup Circuit of the SG6905
1560
(kΩ
reduce
(kΩ
)
AC
(mA)
)
is the input for the PFC multiplier. This
(KHz)
I
.
frequency
circuit
complexity
I
of
connected between the
RMS
I
results in a 65KHz
SG6905
drops below 0.8V.
and
www.fairchildsemi.com
can
power
(1)
(2)
be
T