SG6846C Fairchild Semiconductor, SG6846C Datasheet - Page 10

• Manufacturer Part Number: SG6846C
• Description: The highly integrated PWM controller, SG6846C provides several features to enhance the performance of flyback converters
• Manufacturer: Fairchild Semiconductor

© 2007 Fairchild Semiconductor Corporation
SG6846C • Rev. 1.1.2
Operation Description
Start-up Operation
The turn-on/turn-off thresholds are fixed internally at
16.5V/10.5V. To enable the SG6846C during start-up,
the hold-up capacitor must first be charged to 16.5V
through the start-up resistor.
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 start-up process. This UVLO
hysteresis window ensures that the hold-up capacitor
can adequately supply V
The typical start-up current is only 8µA, which allows a
high-resistance, low-wattage start-up resistor to be used
to minimize power loss. A 1.5MΩ/0.25W start-up
resistor and a 10µF/25V V
sufficient for 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. This
green-mode
consumption under light-load and zero-load conditions.
Power supplies using the SG6846C can meet even the
most restrictive international regulations regarding
standby power consumption.
Oscillator Operation
A resistor connected from the RI pin to GND generates
a reference current source, inside the SG6846C, used
to determine the PWM frequency. Increasing the
resistance 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
switching frequency is:
If an open-circuit or short-circuit to ground occurs at the
RI pin, the internal protection circuit immediately shuts
down the controller.
f
DD
PWM
hold-up capacitance requirement.
(kHz)
=
R
1690
function
I
(k
Ω
)
DD
dramatically
during start-up.
DD
DD
hold-up capacitor are
must not drop below
reduces
I
DD
and the
before
power
(2)
10
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 half of the peak-current threshold,
the internal counter counts down. When the total
accumulated counting time is more than ~110ms
(R
counter counts up. When the switching current is lower
than half of the peak current threshold, the internal
counter will count down. When the total accumulated
counting time is more than ~110ms (R
controller is latched off.
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 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 18). R and C
should put on less than 300Ω and 1000pF respectively.
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. As the V
over 0.9V, the PWM output is turned on again. The
hysteresis window for ON/OFF is ~0.2V.
V
V
damage. When V
latched off.
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 SG6846C is turned off. For
protection-mode options, see Ordering Information.
DD
DD
I
= 26kΩ), the controller is latched off and the internal
Over-voltage Protection (OVP)
IN
over-voltage protection is built in to prevent
increases, making the maximum output power at
IN
voltage. The threshold voltage decreases as
DD
is over 23.6V, the SG6846C is
I
= 26kΩ), the
IN
www.fairchildsemi.com
voltage is
IN