© 2008 Fairchild Semiconductor Corporation www.fairchildsemi.com
SG6846 • Rev. 1.3.1 11
SG6846 — Highly Integrated Green-Mode PWM Controller
Operation Description
Startup Operation
The turn-on/turn-off thresholds are fixed internally at
16.5V/10.5V. To enable the SG6846 during startup, the
hold-up capacitor must first be charged to 16.5V
through the startup resistor.
The hold-up capacitor continues to supply VDD before
energy can be delivered from the auxiliary winding of
the main transformer. The VDD must not drop below
10.5V during this startup process. This UVLO hysteresis
window ensures that the hold-up capacitor can
adequately supply VDD during startup.
The typical startup current is only 8μA. This allows a
high resistance, low-wattage startup resistor to be used
to minimize power loss. A 1.5MΩ/0.25W, startup
resistor and a 10µF/25V VDD hold-up capacitor is
sufficient for a universal input range.
The required operating current has been reduced to
3.7mA. This enables higher efficiency and reduces the
VDD hold-up capacitance requirement.
Green-Mode Operation
Power supplies using the SG6846 can easily meet even
the most restrictive international regulations regarding
standby power consumption. 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. After the PWM oscillator is turned off,
the IC’s supply voltage VDD drops gradually. Before the VDD
voltage drops below the UVLO threshold, the PWM
oscillator is turned on again. This green-mode function
dramatically reduces power consumption under light-load
and zero-load conditions.
Oscillator Operation
A resistor connected from the RI pin to GND generates
a reference-current source inside the SG6846. This
current is 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 RI results in a corresponding
65KHz switching frequency. The relationship between
RI and the switching frequency is:
)(k
1690
(KHz)
Ω
=
I
PWM R
f (1)
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)
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 2/3 of the peak-current threshold,
the internal counter starts counting up. When the
switching current is lower than 2/3 of the peak current
threshold, the internal counter counts down. When the
total accumulated counting time is more than about
96ms (RI = 26kΩ), the controller is latched off.
The 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 VIN
voltage. The threshold voltage decreases as the VIN
voltage increases, making the maximum output power at
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 VIN voltage is less than
0.72V, the PWM output is shut off. If the VIN voltage is
over 0.92V, the PWM output is turned on again. The
hysteresis window for ON/OFF is around 0.2V.
VDD Over-Voltage Protection (OVP)
VDD OVP is built in to prevent damage. If VDD is over
23.6V, SG6846 is 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 SG6846 is turned off. For
protection-mode options, see Ordering Information.