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SB380 датащи(PDF) 2 Page - Fairchild Semiconductor |
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SB380 датащи(HTML) 2 Page - Fairchild Semiconductor |
2 / 12 page AN-9719 APPLICATION NOTE © 2010 Fairchild Semiconductor Corporation www.fairchildsemi.com Rev. 1.0.0 • 11/2/10 2 2. Device Block Description 2.1 Startup Circuit For startup, the HV pin is connected to the line input or bulk capacitor through external resistor RHV, as shown in Figure 2. Typical startup current is 3.5mA and it charges the VDD capacitor (CDD) through resistor RHV. The startup current turns off when the VDD capacitor voltage reaches VDD-ON. The VDD capacitor maintains VDD until the auxiliary winding of the transformer provides the operating current. Figure 2. Startup Circuit 2.2 Soft-Start The FSL-series has internal soft-start circuit that slowly increases the SenseFET current during startup. The typical soft-start time is 5ms, during which the VLimit level is increased in six steps to smoothly establish the required output voltage, as shown in Figure 3. It also helps prevent transformer saturation and reduce stress on the secondary diode during startup. Figure 3. Soft-Start Function 2.3 Green Mode Operation The FSL-series uses feedback voltage (VFB) as an indicator of the output load and modulates the PWM frequency, as shown in Figure 4, such that the switching frequency decreases as load decreases. In heavy-load conditions, the switching frequency is 100kHz. Once VFB decreases below VFB-N (2.5V), the PWM frequency starts to linearly decrease from 100kHz to 18kHz to reduce the switching losses. As VFB decreases below VFB-G (2.4V), the switching frequency is fixed at 18kHz and FSL-series enters “deep” green mode to reduce the standby power consumption. As VFB decreases below VFB-ZDC (2.1V), FSL-series enters into burst-mode operation. When VFB drops below VFB-ZDC, FSL-series stops switching and the output voltage starts to drop, which causes the feedback voltage to rise. Once VFB rises above VFB-ZDC, switching resumes. Burst mode alternately enables and disables switching, thereby reducing switching loss to improve power saving, as shown in Figure 5. Figure 4. PWM Frequency Figure 5. Bust-Mode Operation |
Аналогичный номер детали - SB380 |
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Аналогичное описание - SB380 |
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