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RS2253SP Datasheet(PDF) 5 Page - Orister Corporation |
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RS2253SP Datasheet(HTML) 5 Page - Orister Corporation |
5 / 9 page Page No. : 5/9 DS‐RS2253‐11 May, 2010 www.Orister.com Detail Description The RS2253 is a low power off‐line SMPS Switcher optimized for off‐line flyback converter applications in sub 10W power range. The burst mode control greatly reduces the standby power consumption and helps the design easily to meet the international power conservation requirements. Startup Current and Start up Control Startup current of RS2253 is designed to be very low so that VDD could be charged up above UVLO threshold level and device starts up quickly. A large value startup resistor can therefore be used to minimize the power loss yet achieve a reliable startup in application. For AC/DC adaptor with universal input range design, a 2MΩ, 1/8W startup resistor could be used together with a VDD capacitor to provide a fast startup and yet low power dissipation design solution. Operation current The Operation current of RS2253 is low at 2mA. Good efficient is achieved with RS2253 low operating current together with the ‘Extended burst mode’ control features. Soft Start RS2253 features an internal 4 ms soft start to soften the electrical stress occurring in the power supply during startup. It is activated during the power on sequence. As soon as VDD reaches UVLO(OFF), the peak current is gradually increased from nearly zero to the maximum level of 0.77V. Every restart up is followed by a soft start. Frequency Jiggling Mode for EMI improvement The frequency Jiggling Mode (switching frequency modulation) is implemented in RS2253. The oscillation frequency is modulated so that the tone energy is spread out. The spread spectrum minimizes the conduction band EMI and therefore eases the system design. Extended Burst Mode Operation At light load or zero load condition, most of the power dissipation in a switching mode power supply is from switching loss on the MOSFET, the core loss of the transformer and the loss on the snubber circuit. The magnitude of power loss is in proportion to the switching frequency. Lower switching frequency leads to the reduction on the power loss and thus conserves the energy. The switching frequency is internally adjusted at no load or light load condition. The switch frequency reduces at light/no load condition to improve the conversion efficiency. At light load or no load condition, the FB input drops below burst mode threshold level and device enters Burst Mode control. The Gate drive output switches only when VDD voltage drops below a preset level and FB input is active to output an on state to minimize the switching loss and reduce the standby power consumption to the greatest extend. The switching frequency control also eliminates the audio noise at any loading conditions. Oscillator Operation The switching frequency of RS2253 is internally fixed at 50 KHz. No external frequency setting components are required for PCB design simplification. Current Sensing and Leading Edge Blanking Cycle‐by‐Cycle current limiting is offered in RS2253 current mode PWM control. The switch current is detected by sense resistor into the sense pin. An internal leading edge blank circuit chops off the sensed voltage spike at initial internal power MOSFET on state due to snubber diode reverse recovery and surge gate current of internal power MOSFET so that the external RC filtering on sense input is no longer needed. The current limiting comparator is disabled and cannot turn off the internal power MOSFET during the blanking period. The PWM duty cycle is determined by the current sense input voltage and the FB input voltage. |
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