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FAN6861 Arkusz danych(PDF) 8 Page - Fairchild Semiconductor |
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FAN6861 Arkusz danych(HTML) 8 Page - Fairchild Semiconductor |
8 / 12 page AN-6861 APPLICATION NOTE © 2009 Fairchild Semiconductor Corporation www.fairchildsemi.com Rev. 1.0.1 • 6/9/09 8 (Design Example) 510k Ω resistor and 10μF capacitor are selected for RSTART and CDD1, respectively. Then, the current through the startup resistor for minimum line voltage is: 3 2 1 () 2 2 90 17.5 1 () 62 2510 10 MIN LINE DD ON RST START VV I R A π μ π − =− ⋅ =− = × Then, the maximum startup time is obtained as 1 6 6 17.5 10 10 3.7 sec (62 15) 10 MAX DD ON START DD MAX RST DD ST V TC II − − − − = − =× = −× The maximum power dissipation of RSTART is: 2 2 3 () 265 68 2 2 510 10 MAX LINE RST START V P mW R ≅= = ⋅× Leading-Edge Blanking (LEB) Each time the power MOSFET is switched on, a turn-on spike occurs across the sense resistor, caused by primary- side capacitance and secondary-side rectifier reverse recovery. To avoid premature termination of the switching pulse, a leading-edge blanking time is built in. During this blanking period (360ns), the PWM comparator is disabled and cannot switch off the gate driver. Thus, an RC filter with a small RC time constant is enough for current sensing (e.g. 200 Ω + 470pF). A non-inductive resistor is recommended for RCS. Figure 11. Current sensing Thermal Protection Figure 12 shows the internal blocks for thermal protection. A constant current, IRT, of 99μA is provided from the RT pin. For over-temperature protection, an NTC thermistor in series with a resistor can be connected between the RT and GND pins. As temperature increases, the impedance of NTC thermister decreases and RT pin voltage drops. When the voltage of the RT pin is less than 1V longer than 17ms (tDOTP-LATCH), OTP is triggered. When RT pin voltage is less than 0.7V, OTP is triggered after the 100 μs debounce time. If the thermal protection is not used, connect a small capacitor (around 0.47nF is recommended) from the RT pin to the GND pin to prevent noise interference. This RT capacitor should not be larger than 1nF; otherwise, the thermal protection is triggered before a successful build-up of output voltage. Figure 12. Thermal Protection Circuit |
Podobny numer części - FAN6861 |
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Podobny opis - FAN6861 |
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