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SG6846A Datasheet(PDF) 11 Page - Fairchild Semiconductor |
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SG6846A Datasheet(HTML) 11 Page - Fairchild Semiconductor |
11 / 15 page © 2008 Fairchild Semiconductor Corporation www.fairchildsemi.com SG6846A • Rev. 1.1.4 11 Operation Description Startup Operation The turn-on/turn-off thresholds are fixed internally at 16.5V and 10.5V. To enable the SG6846A 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, which allows a high-resistance, low-wattage startup resistor to be used. 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 20). R and C should put on less than 300 Ω and 1000pF, respectively, to minimize power loss. A 1.5M Ω/0.25W startup 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 dramatically reduces power consumption under light-load and zero-load conditions. Power supplies using SG6846A can meet 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 SG6846A, used to determine the PWM frequency. Increasing the resistance decreases the amplitude of the current source and reduces the PWM frequency. Using a 26k Ω resistor results in a corresponding 65kHz switching frequency. The relationship between RI and the switching frequency is: ) (k R 1690 (kHz) I PWM f Ω = (2) 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 the over-current threshold, the internal counter counts down. When the total accumulated counting time is more than ~1600ms (RI = 26kΩ), the controller is latched off and the internal counter counts up. When the switching current is lower than the over-current threshold, the internal counter counts down. When the total accumulated counting time is more than ~1600ms (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 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 VIN 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 0.7V, the PWM output is shut off. If the VIN is 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. If VDD is over 23.6V, SG6846A 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 SG6846A is turned off. For protection-mode options, see Ordering Information. |
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