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FSFA2100 Datasheet(PDF) 10 Page - Fairchild Semiconductor |
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FSFA2100 Datasheet(HTML) 10 Page - Fairchild Semiconductor |
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10 / 16 page  © 2008 Fairchild Semiconductor Corporation www.fairchildsemi.com FSFA2100 • Rev. 1.0.0 10 Functional Description 1. Internal Oscillator : FSFA2100 employs a current- controlled oscillator as shown in Figure 17. Internally, the voltage of the RT pin is regulated at 2V and the charging/discharging current for the oscillator capacitor CT is determined by the current flowing out of the RT pin (ICTC). When the RT pin is pulled down to the ground with a resistor RSET, the switching frequency is fixed as: ) ( 100 27 kHz R k f SET S × Ω = (1) Figure 17. Current Controlled Oscillator 2. PWM Control : Figure 18 shows the typical control circuit configuration. The opto-coupler transistor should be connected to the VFB pin in parallel with the feedback capacitor to control the duty cycle. Figure 18. PWM Control Configuration Figure 19 shows the internal block diagram for PWM operation. Duty cycle is controlled by comparing the feedback voltage to the triangular signal with a range from 1V to 3V. Figure 19. Internal PWM Block Diagram 3. Protection Circuits : The FSFA2100 has Overload Protection (OLP), Abnormal Over-Current Protection (AOCP), Over-Voltage Protection (OVP), and Thermal Shutdown (TSD) self-protective functions. The OLP and OVP are auto-restart mode protections, while the AOCP and TSD are latch-mode protections, as shown in Figure 20. Auto-restart mode protection: Once the fault condition is detected, the switching is terminated and the MOSFETs remain off. When LVCC falls down to LVCC stop voltage of around 11V, the protection is reset. The FPS resumes normal operation when LVCC reaches the start voltage of about 14V. Latch-mode protection: Once this protection is triggered, the switching is terminated and the MOSFETs remain off. The latch is reset only when LVCC is discharged below 5V. Figure 20. Protection blocks Low-side MOSFET current should be sensed for Pulse- by-pulse current limit and AOCP. The FSFA2100 senses drain current as a negative voltage, as shown in Figure 21 and Figure 22. Half-wave sensing allows low-power dissipation in the sensing resistor, while full-wave sensing has less noise in the sensing signal. |
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