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WS3252 Datasheet(PDF) 7 Page - Shenzhen Winsemi Microelectronics Co., Ltd |
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WS3252 Datasheet(HTML) 7 Page - Shenzhen Winsemi Microelectronics Co., Ltd |
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7 / 8 page  WS3252 WS3252 WS3252 WS3252 7/8 Steady, Steady, Steady, Steady, keep keep keep keep you you you you advance advance advance advance Operation Operation Operation Operation switching switching switching switching frequency frequency frequency frequency The switching frequency of WS3252 is adaptively controlled according to the load conditions and the operation modes. The operation switching frequency at maximum output power is set to 60KHz internally. In DCM mode, the maximum output power is given by 2 2 1 P SW P oMAX I F L P = Where Lp indicate the inductance of primary winding and Ip is peak current of primary winding. The change of the primary winding inductance results in the change of the maximum output power and constant output current in CC mode. To compensate the change from viariations of primary winding inductance, the switching frequency is locked by an internal loop such that the switching frequency is Demag SW T F 2 1 = Since TDemag is inversely proportional to the inductance, as a result, the product Lp and Fsw is constant, thus the maximum output power and constant current in CC mode will not change as primary winding inductance changes. Up to ±10% variation of the primary winding inductance can be compensation. Frequency Frequency Frequency Frequency Shuffling Shuffling Shuffling Shuffling Excellent EMI performance is achieved by using frequency jittering in WS3252 Current Current Current Current Sensing Sensing Sensing Sensing and and and and Leading Leading Leading Leading Edge Edge Edge Edge Blanking Blanking Blanking Blanking Cycle-by-Cycle current limiting is offered in WS3252. The switch current is detected by a sense resistor into the sense pin. An internal leading edge blanking circuit chops off the sense voltage spike at initial MOSFET on state due to snubber diode reverse recovery so that the external RC filtering on sense input is no longer required. The current limit comparator is disabled and thus cannot turn off the internal MOSFET during the blanking period. PWM duty cycle is determined by the current sense input voltage and the EA output voltage. Programmable Programmable Programmable Programmable Cable Cable Cable Cable Drop Drop Drop Drop Compensation Compensation Compensation Compensation Cable drop compensation is implemented to achieve good load regulation inWS3252. An offset voltage is generated at INV by an internal current flowing into the resister divider. The current is inversely proportional to the voltage across pin COMP, as a result, it is inversely proportional to the output load current, thus the drop due to the cable loss can be compensated. As the load current decreases from full-load to no-load, the offset voltage at INV will increase. It can also be programmed by adjusting the resistance of the divider to compensate the drop for various cable lines used. Gate Gate Gate Gate Drive Drive Drive Drive The gate drive strength which is too weak leads to over switch loss of MOSFET while too strong gate drive output compromises in the over EMI. A good tradeoff between output strength and dead time control is achieved through the design of the built-in totem pole gate. The low standby dissipation and good EMI system design is easier to achieve through this dedicated devise. For MOSFET gate protection, an internal 12V clamp is added at higher than expected VCC input. Protection Protection Protection Protection Controls Controls Controls Controls Excellent system stability is achieved by the comprehensive protection of WS3252. Including Cycle-by-Cycle current limiting (OCP), VCC Clamp, Under Voltage Lockout on VCC (UVLO). Device restarts when VCC voltage drops below UVLO limit. It is clamped when VCC is higher than threshold value. The power MOSFET is shut down when VCC drops below UVLO limit and device enters power on start-up sequence thereafter. |
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