SG6846C 查看數據表（PDF） - Fairchild Semiconductor

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SG6846C

Highly Integrated Green-Mode PWM Controller

Fairchild Semiconductor 

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 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.

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 VDD hold-up capacitor are

sufficient for a universal input range.

The required operating current has been reduced to

3.7mA, which 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 is completely

turned off and the power supply enters burst-mode. This

green-mode function dramatically reduces power

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 RI and the

switching frequency is:

fPWM(kHz) = 1690

(2)

RI (kΩ)

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 half of the peak-current threshold,

the internal counter counts down. When the total

accumulated counting time is more than ~110ms

(RI = 26kΩ), the controller is latched off and the internal

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 (RI = 26kΩ), the

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 VIN voltage. The threshold voltage decreases as

the VIN increases, making the maximum output power at

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 VIN voltage is

less than 0.7V, the PWM output is shut off. As the VIN

over 0.9V, the PWM output is turned on again. The

hysteresis window for ON/OFF is ~0.2V.

VDD Over-voltage Protection (OVP)

VDD over-voltage protection is built in to prevent

damage. When VDD is over 23.6V, the SG6846C 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 SG6846C is turned off. For

protection-mode options, see Ordering Information.

© 2007 Fairchild Semiconductor Corporation

SG6846C • Rev. 1.1.2

10

www.fairchildsemi.com

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