SG6846CD ETC1 [List of Unclassifed Manufacturers], SG6846CD Datasheet - Page 9

• Manufacturer Part Number: SG6846CD
• Description: Highly-Integrated Green-Mode PWM Controller
• Manufacturer: ETC1 [List of Unclassifed Manufacturers]

Highly-Integrated Green-Mode PWM Controller
OPERATION DESCRIPTION
Start-up Operation
at 16.5V/10.5V. To enable the SG6846 during start-up,
the hold-up capacitor must first be charged to 16.5V
through the start-up resistor.
before energy can be delivered from the auxiliary winding
of the main transformer. The VDD must not drop below
10.5V during this start-up process. This UVLO hysteresis
window ensures that the hold-up capacitor can adequately
supply VDD during start-up.
a high resistance, low-wattage start-up resistor to be used,
to minimize power loss. A 1.5 MΩ/0.25W, start-up
resistor and a 10uF/25V VDD hold-up capacitor would be
sufficient for a universal input range.
3.7mA. This enables higher efficiency and reduces the
VDD hold-up capacitance requirement.
Green-Mode Operation
off-time modulation to continuously decrease the
switching
Maximum on time is limited to provide protection against
abnormal
consumption under zero-load condition, the PWM
oscillator will be completely turned off, and the power
supply will enter burst-mode. After the PWM oscillator is
turned off, the IC’s supply voltage VDD will drop
gradually. Before the VDD voltage drops below the
UVLO threshold, the PWM oscillator will be turned on
again. This green-mode function dramatically reduces
power consumption under light-load and zero-load
conditions. Power supplies using the SG6846 can easily
meet even the most restrictive international regulations
regarding standby power consumption.
Oscillator Operation
generates a reference current source inside the SG6846.
© System General Corp.
Version 1.0(IAO33.0008.B0)
The turn-on/turn-off thresholds are fixed internally
The hold-up capacitor will continue to supply VDD
The typical start-up current is only 8uA. This allows
The required operating current has been reduced to
The proprietary green-mode function provides
A resistor connected from the RI pin to GND
conditions.
frequency
under
To
further
light-load
reduce
conditions.
power
- 9 -
This current is used to determine the PWM frequency.
Increasing the resistance will decrease the amplitude of
the current source and reduce the PWM frequency. Using
a 26kΩ resistor Ri results in a corresponding 65kHz
switching frequency. The relationship between RI and the
switching frequency is:
If an open circuit or short circuit to ground occurs at the
RI pin, the internal protection circuit will immediately
shut down the controller.
Two-level Over-current Protection
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 will count down. When the
total accumulated counting time is more than about 96ms
(Ri = 26kohm), the controller will be latched off.
are especially designed for SMPS with surge current
output such as those for printers, scanners, motor drivers,
etc.
Constant Output Power Limit
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 will generate a
higher VIN voltage. The threshold voltage decreases as
the VIN voltage increases, making the max. output power
at high line input voltage equal to that at low line input.
Brownout Protection
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 will be shut off. As the VIN
The cycle-by-cycle current limiting will shut down
This two-level OCP protection and up/down counter
For constant output power limit over universal
Since the VIN pin is connected through a resistive
f
PWM
(kHz)
=
R
1690
i
(k Ω
)
---------------------------- (1)
Product Specification
www.sg.com.tw
Sep. 30, 2004
SG6846