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Detail Description
The RS2254 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 RS2254 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 RS2254 is low at 2mA. Good efficient is achieved with RS2254 low operating current
together with the ‘Extended burst mode’ control features.
Soft Start
RS2254 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 RS2254. 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 RS2254 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 RS2254 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.
DS‐RS2254‐11 May, 2010
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ORISTER [ ORISTER CORPORATION ]
