SG6846 Ver la hoja de datos (PDF) - Fairchild Semiconductor

Número de pieza

componentes Descripción

Fabricante

SG6846

Highly Integrated Green-Mode PWM Controller

Fairchild Semiconductor 

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:

fPWM(KHz) =

1690

RI (kΩ)

(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.

© 2008 Fairchild Semiconductor Corporation

SG6846 • Rev. 1.3.1

11

www.fairchildsemi.com

Share Link: