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DS-RS2042-01

May, 2007

www.Orister.com

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

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,

drive to terminate earlier, thus reducing total PWM turn-on time, and making the output power equal to that of the

low line input. This proprietary internal compensation feature ensures a constant output power limit over a wide

range of AC input voltages (90VAC to 264VAC).

VDD Over-voltage Protection

VDD over-voltage protection has been built in to prevent damage due to over voltage conditions. When the voltage

VDD exceeds the internal threshold due to abnormal conditions, PWM output will be turned off. Over-voltage

conditions are usually caused by open feedback loops.

Limited Power Control

The FB voltage will increase every time the output of the power supply is shorted or over-loaded. If the FB voltage

turned off, the supply voltage VDD will also begin decreasing.

When VDD goes below the turn-off threshold (eg, 12.4V) the controller will be totally shut down. VDD will be

charged up to the turn-on threshold voltage of 16.5V through the start-up resistor until PWM output is restarted.

This protection feature will continue to be activated as long as the over-loading condition persists. This will prevent

the power supply from overheating due to over loading conditions.

Thermal Protection

pin. Because the impedance of the NTC will decrease at high temperatures, when the voltage of the RT pin drops

below 1.065V, PWM output will be disabled. The RT pin output current is related to the PWM frequency

Noise Immunity

Noise from the current sense or the control signal may cause significant pulse width jitter, particularly in continuous-

conduction mode. Slope compensation helps alleviate this problem. Good placement and layout practices should

be followed. The designer should avoiding long PCB traces and component leads. Compensation and filter

components should be located near the RS2042. Finally, increasing the power-MOS gate resistance is advised.