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SI9118 数据表(PDF) 7 Page - Vishay Siliconix |
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SI9118 数据表(HTML) 7 Page - Vishay Siliconix |
7 / 8 page Si9118/Si9119 Vishay Siliconix New Product Document Number: 70815 S-60752—Rev. B, 05 Apr-99 www.Vishay Siliconix.com S FaxBack 408-970-5600 7 DETAILED OPERATIONAL DESCRIPTION Start-Up Si9118/Si9119 are designed with internal depletion mode MOSFET capable of powering directly from the high input bus voltage. This feature eliminates the typical external start-up circuit saving valuable space and cost. But, most of all, this feature improves the converter efficiency during full load and has an even greater impact on light load. With an input bus voltage applied to the +VIN pin, the VCC voltage is regulated to 9.3 V. The UVLO circuit prevents the controller output driver section from turning on, until VCC voltage exceeds 8.7 V. In order to maximize converter efficiency, the designer should provide an external bootstrap winding to override the internal VCC regulator. If external VCC voltage is greater than 9.3 V, the internal depletion mode MOSFET regulator is disabled and power is derived from the external VCC supply. The VCC supply provides power to the internal circuity as well as providing supply voltage to the gate drive circuit. Soft-Start/Enable The soft-start time is externally programmable with capacitor connected to the SS/EN pin. A constant current source provides the current to the SS/EN pin to generate a linear start-up time versus the capacitance value. The SS/EN pin clamps the error amplifier output voltage, limiting the rate of increase in duty cycle. By controlling the rate of rise in duty cycle gradually, the output voltage rises gradually preventing the output voltage from overshooting. The SS/EN pin can also be used to enable or disable the output driver section with an external logic signal. Synchronization The synchronization to external clock is easily accomplished by connecting the external clock into the SYNC pin (Si9119 only). The logic high to low transition synchronizes the clock. The external clock frequency must be at least 5% faster than the internal clock frequency. Reference Voltage The reference voltage for the Si9118/Si9119 are set at 4.0 V. The reference voltage is not connected to the non-inverting inputs of the error amplifier, therefore, the minimum output voltage is not limited to reference voltage. The VREF pin requires a 0.1- mF decoupling capacitor. Error Amplifier The error amplifier gain-bandwidth product is critical parameter which determines the transient response of converter. The transient response is function of both small and large signal responses. The small signal response is determined by the feedback compensation network while the large signal response is determined by the inductor di/dt slew rate. Besides the inductance value, the error amplifier gain-bandwidth determines the converter response time. In order to minimize the response time, Si9118/Si9119 is designed with a 2.7-MHz error amplifier gain-bandwidth product to provide the widest converter bandwidth possible. PWM Mode The converter operates in PWM mode if the PWM/PSM pin is connected to VREF pin or logic high. As the load current and line voltage vary, the Si9118/Si9119 maintain constant switching frequency until they reach minimum duty cycle. Once the output voltage regulation is exceeded with minimum duty cycle, the switching frequency will continue to decrease until regulation is achieved. The switching frequency is controlled by the external Rosc and Cosc as shown by the typical oscillator frequency curve. In PWM mode, output ripple noise is constant reducing EMI concerns as well as simplifying the filter to minimize the system noise. Pulse Skipping Mode If the PWM/PSM pin is connected to –VIN pin (logic low), the converter can operate in either PWM or PSM mode depending on the load current. The converter automatically transitions from PWM to PSM or vise versa to maintain output voltage regulation. In PSM mode, the MOSFET switch is turned on until the peak current sensed voltage reaches 100 mV and the output voltage meets or exceeds its regulation voltage. The converter is operating in pulse skipping mode because each pulse delivers excess energy into the output capacitor forcing the output voltage to exceed its regulation voltage. By forcing the output voltage to exceed the regulation voltage, succeeding pulses are skipped until the output voltage drops below the regulation point. Therefore, switching frequency will continue to reduce during PSM control as the demand for output current decreases. The pulse skipping mode cuts down the switching losses, the dominant power consumed during low output current, thereby maintaining high efficiency throughout the entire load range. With PWM/PSM pin in logic low state, the converter transitions back into PWM mode, if the peak current sensed voltage of 100 mV does not generate the required output voltage. In the region between pulse skipping mode and PWM mode, the controller may transition between the two modes, delivering spurts of pulses. This may cause the current waveform to look irregular, but this will not overly affect the ripple voltage. Even in this transitional mode, efficiency remains high. Programmable Duty Cycle Control The maximum duty cycle limit is controlled by the voltage on DMAX pin. A DMAX voltage of 3.2 V generates 80% duty cycle while 0.0 V generates 0% duty cycle. The 80% duty cycle is maximum default condition at 1-MHz switching frequency. The DMAX voltage can be easily generated using resistor divider from the reference voltage. |
类似零件编号 - SI9118 |
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类似说明 - SI9118 |
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