SG6846B Fairchild Semiconductor, SG6846B Datasheet - Page 11

• Manufacturer Part Number: SG6846B
• Description: The highly integrated PWM controller, SG6846B, provides several features to enhance the performance of fly-back converters
• Manufacturer: Fairchild Semiconductor

© 2008 Fairchild Semiconductor Corporation
SG6846B • Rev. 1.3.4
Operation Description
Startup Operation
The turn-on/turn-off thresholds are fixed internally at
16.5V and 10.5V. To enable the SG6846B during
startup, the hold-up capacitor must first be charged to
16.5V through the startup resistor.
If an open-circuit or short-circuit to ground occurs at the
RI pin, the internal protection circuit immediately shuts
down the controller.
The hold-up capacitor continues to supply V
energy can be delivered from the auxiliary winding of
the main transformer. The V
10.5V during this startup process. This UVLO hysteresis
window ensures that the hold-up capacitor can
adequately supply V
The typical startup current is only 8µA, which allows a
high-resistance, low-wattage startup resistor to be used.
For constant output power limit over a universal input-
voltage range, the peak-current threshold is adjusted by
the voltage of the VIN pin. Since the VIN pin is
connected to the rectified AC input line voltage through
the resistive divider, a higher line voltage generates a
higher V
the V
high line input voltage equal to that at low line input.
The value of R-C network should not be so large it
affects the power limit (shown in Figure 19). R and C
should put on less than 300Ω and 1000pF, respectively,
to minimize power loss. A 1.5MΩ/0.25W startup resistor
and a 10µF/25V V
a universal input range.
The required operating current has been reduced to
3.7mA, which enables higher efficiency and reduces the
V
Green-Mode Operation
The proprietary green-mode function provides off-time
modulation to continuously decrease the switching
frequency under light-load conditions. Maximum on-time
is limited to provide protection against abnormal
conditions. To further reduce power consumption under
zero-load condition, the PWM oscillator is completely
turned off and the power supply enters burst-mode.
Green mode dramatically reduces power consumption
under light-load and zero-load conditions. Power
supplies
international regulations regarding standby power
consumption.
Oscillator Operation
A resistor connected from the RI pin to GND generates
a internal reference current source used to determine
the
decreases the amplitude of the current source and
reduces the PWM frequency. Using a 26kΩ resistor
results in a corresponding 65kHz switching frequency.
The relationship between R
frequency is:
DD
hold-up capacitance requirement.
IN
PWM
increases, making the maximum output power at
IN
voltage. The threshold voltage decreases as
using
frequency.
DD
SG6846B
DD
hold-up capacitor are sufficient for
during startup.
Increasing
DD
can
I
must not drop below
and the switching
meet
the
resistance
DD
restrictive
before
11
If an open-circuit or short-circuit to ground occurs at the
RI pin, the internal protection circuit immediately shuts
down the controller.
Two-level Over-current Protection (OCP)
The cycle-by-cycle current limiting shuts down the PWM
immediately when the switching current is over the
peak-current threshold. Additionally, when the switching
current is higher than over-current threshold, the internal
counter counts down. When the total accumulated
counting time is more than ~200ms (R
controller is latched off and the internal counter counts
up. When the switching current is lower than over-
current threshold, the internal counter counts down.
When the total accumulated counting time is more than
~200ms (R
This two-level OCP protection and up/down counter are
especially designed for SMPS with surge current output,
such as those for printers, scanners, and motor drivers.
Constant Output Power Limit
For constant output power limit over universal input-
voltage range, the peak-current threshold is adjusted by
the voltage of the VIN pin. Since the VIN pin is
connected to the rectified AC input line voltage through
the resistive divider, a higher line voltage generates a
higher V
V
high-line input voltage equal to that at low-line input.
Brownout Protection
Since the VIN pin is connected through a resistive
divider to the rectified AC input line voltage, it can also
be used for brownout protection. If the V
less than 0.7V, the PWM output is shut off. If the V
over 0.9V, the PWM output is turned on again. The
hysteresis window for on/off is ~0.2V.
V
V
damage. If V
Over-Temperature Protection (OTP)
An external NTC thermistor can be connected from the
RT pin to GND. The impedance of the NTC decreases
at high temperatures. When the voltage of the RT pin
drops below 1.05V, the SG6846B is turned off. For
protection-mode options, see Ordering Information.
f
IN
DD
DD
PWM
increases, making the maximum output power at
Over-voltage Protection (OVP)
(kHz)
over-voltage protection is built in to prevent
IN
voltage. The threshold voltage decreases as
I
=
= 26kΩ), the controller is latched off.
DD
R
1690
I
is over 24V, SG6846B is latched off.
(k
Ω
)
I
= 26kΩ), the
IN
www.fairchildsemi.com
voltage is
IN
(2)
is