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FSDM0365RL Datasheet(PDF) 11 Page - Fairchild Semiconductor |
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FSDM0365RL Datasheet(HTML) 11 Page - Fairchild Semiconductor |
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11 / 20 page  FSDH321, FSDL321 11 5. Soft Start : The FPS has an internal soft start circuit that slowly increases the feedback voltage together with the Sense FET current after it starts up. The typical soft start time is 15msec, as shown in Figure 8, where progressive increments of the Sense FET current are allowed during the start-up phase. The pulse width to the power switching device is progressively increased to establish the correct working conditions for transformers, inductors, and capaci- tors. The voltage on the output capacitors is progressively increased with the intention of smoothly establishing the required output voltage. It also helps to prevent transformer saturation and reduce the stress on the secondary diode. Figure 8. Soft Start Function 6. Burst Operation : In order to minimize power dissipation in standby mode, the FPS enters burst mode operation. As the load decreases, the feedback voltage decreases. As shown in Figure 9, the device automatically enters burst mode when the feedback voltage drops below VBURH(500mV). Switching still continues but the current limit is set to a fixed limit internally to minimize flux density in the transformer. The fixed current limit is larger than that defined by VFB = VBURH and therefore, VFB is driven down further. Switching continues until the feedback voltage drops below VBURL(350mV). At this point switching stops and the output voltages start to drop at a rate dependent on the standby current load. This causes the feedback voltage to rise. Once it passes VBURH(500mV), switching resumes. The feedback voltage then falls and the process repeats. Burst mode operation alternately enables and disables switching of the power Sense FET thereby reducing switch- ing loss in Standby mode. Figure 9. Burst Operation Function 7. Frequency Modulation : Modulating the switching fre- quency of a switched power supply can reduce EMI. Fre- quency modulation can reduce EMI by spreading the energy over a wider frequency range than the bandwidth measured by the EMI test equipment. The amount of EMI reduction is directly related to the depth of the reference frequency. As can be seen in Figure 10, the frequency changes from 97KHz to 103KHz in 4ms for the FSDH321 (48.5KHz to 51.5KHz for FSDL321). Frequency modulation allows the use of a cost effective inductor instead of an AC input mode choke to satisfy the requirements of world wide EMI limits. Figure 10. Frequency Modulation Waveform 1ms 15steps Current limit 0.4A 0.7A t Drain current V BURH Switching OFF Current Waveform Burst Operation Normal Operation V FB V BURL Switching OFF Burst Operation 3 Vcc Vcc I DELAY I FB R 2.5R Vfb VBURL/VBURH PWM + - VBURH Vcc I BUR(pk) Burst Normal MOSFET Current t s f s=1/ts 100kHz 103kHz 97kHz 4ms t Drain Current |
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