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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 R
I
results in a corresponding 67KHz PWM frequency. The relationship between R
I
and the
switching frequency is:
f
PWM
=
1742
(
KHz
)
R
I
(
K
Ω
)
RS2042 also integrates frequency jittering 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.
I
RT
=
Leading Edge Blanking
70
μ
A
* 26
R
I
(
K
Ω
)
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.
DS-RS2042-01
May, 2007
www.Orister.com
ORISTER [ ORISTER CORPORATION ]
