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SG6846

Highly-Integrated Green-Mode PWM Controller

Unspecified 

Highly-Integrated Green-Mode PWM Controller

Product Specification

SG6846

OPERATION DESCRIPTION

Start-up Operation

The turn-on/turn-off thresholds are fixed internally

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.

The hold-up capacitor will continue 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 start-up process. This UVLO hysteresis

window ensures that the hold-up capacitor can adequately

supply VDD during start-up.

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:

fPWM (kHz) = 1690 ---------------------------- (1)

Ri (kΩ)

If an open circuit or short circuit to ground occurs at the

RI pin, the internal protection circuit will immediately

shut down the controller.

The typical start-up current is only 8uA. This allows

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.

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

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 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.

Two-level Over-current Protection

The cycle-by-cycle current limiting will shut 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 will count down. When the

total accumulated counting time is more than about 96ms

(Ri = 26kohm), the controller will be 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, motor drivers,

etc.

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 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

Oscillator Operation

Since the VIN pin is connected through a resistive

divider to the rectified AC input line voltage, it can also

A resistor connected from the RI pin to GND be used for brownout protection. If the VIN voltage is less

generates a reference current source inside the SG6846. than 0.72V, the PWM output will be shut off. As the VIN

© System General Corp.

-9-

www.sg.com.tw

Version 1.0(IAO33.0008.B0)

Sep. 30, 2004

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