RS2042 Orister Corporation, RS2042 Datasheet - Page 4

• Manufacturer Part Number: RS2042
• Description: Green-power Pwm Controller With Freq. Jiggling
• Manufacturer: Orister Corporation

Operation Description
Start-Up Current & Operating Current
The typical start-up current is only 8μA. This allows a high resistance, low-wattage start-up resistor to be used, to minimize power loss. A
1.5MOhms, 0.25W, start-up resistor and a 10μF/40V VDD hold-up capacitor would be sufficient for an AC/DC adapter with a universal input
range.
The required operating current has been reduced to 3.4mA. This results in higher efficiency and reduces the VDD hold-up capacitance
requirement.
Green-Power Mode Operation
The proprietary green-power mode function provides off-time modulation to continuously decrease the PWM frequency under light-load
conditions. To avoid acoustic-noise problem, the minimum PWM frequency set above 25KHz. This green-power mode function dramatically
reduces power consumption under light-load and zero-load conditions. Power supplies using a RS2042 controller can easily meet even the
most restrictive international regulations regarding standby power consumption.
Oscillator Operation
A resistor connected from the RI pin to GND pin generates a constant current source for the RS2042 controller. This current is used to
determine the center PWM frequency. Increasing the resistance will reduce PWM frequency. Using a 26KΩ resistor RI results in a
corresponding 67KHz PWM frequency. The relationship between RI and the switching frequency is:
RS2042 also integrates frequency Jiggling function internally. The frequency variation ranges from around 63KHz to 70KHz for a center
frequency 67kHz. The frequency jittering function helps reduce EMI emission of a power supply with minimum line filters.
Leading Edge Blanking
Each time the power MOSFET is switched on, a turn-on spike will inevitably occur at the sense-resistor. To avoid premature termination of
the switching pulse, a leading-edge blanking time is built in. During this blanking period, the current-limit comparator is disabled, and it
cannot switch off the gate drive.
Under-Voltage Lockout (UVLO)
The turn-on/turn-off thresholds are fixed internally at 12.4V/16.5V. To enable a RS2042 controller 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. VDD
must not drop below 12.4V during this start-up process. This UVLO hysteresis window ensures that the hold-up capacitor can adequately
supply VDD during start-up.
Gate Output / Soft Driving
The RS2042 output stage is a fast totem pole gate driver. Cross-conduction has been avoided to minimize heat dissipation, increase
efficiency, and enhance reliability. The output driver is clamped by an internal 18V Zener diode in order to protect the power MOSFET
transistors from any harmful over-voltage gate signals. A soft driving waveform is implemented to minimize EMI.
Slope Compensation
The sensed voltage across the current sense resistor is used for peak-current-mode control and cycle-by-cycle current limiting. The built-in
slope compensation function improves power supply stability and prevents peak-current-mode control from causing sub-harmonic
oscillations. Within every switching cycle, the RS2042 controller produces a positively sloped, synchronized ramp signal.
Constant Output Power Limit
When the SENSE voltage across the sense resistor RS reaches the threshold voltage, the output GATE drive will be turned off following a
small propagation delay TPD. This propagation delay will result in an additional current proportional to TPD*VIN/LP. The propagation delay
is nearly constant regardless of the input line voltage VIN. Higher input line voltages will result in larger additional currents. Thus, under high
input-line voltages the output power limit will be higher than under low input-line voltages.
The output power limit variation can be significant over a wide range of AC input voltages. To compensate for this, the threshold voltage is
adjusted by the current IIN. Since the pin VIN is connected to the rectified input line voltage through the start-up resistor, a higher line
voltage will result in a higher current IIN through the pin VIN.
DS-RS2042-03
Dec, 2007
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