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AN937 Datasheet(PDF) 5 Page - STMicroelectronics |
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AN937 Datasheet(HTML) 5 Page - STMicroelectronics |
5 / 29 page AN937 Power supply, UVLO and voltage reference Doc ID 5655 Rev 15 5/29 The oscillator peak-to-valley voltage is proportional to the supply voltage, and the voltage feed forward is operative from 8 V to 55 V of input supply. Also the ΔV/Δt of the sawtooth is directly proportional to the supply voltage. As Vcc increases, the Ton time of the power transistor decreases in such a way to provide to the choke, and finally also the load, the product Volt. sec constant. Figure 6 show how the duty cycle varies as a result of the change on the ΔV/Δt of the sawtooth with the Vcc. The output of the error amplifier doesn’t change to maintain the output voltage constant and in regulation. With this function on board, the output response time is greatly reduced in presence of an abrupt change on the supply voltage, and the output ripple voltage at the mains frequency is greatly reduced too. Figure 5. Device switching frequency vs Rosc and Cosc Figure 6. Voltage feed forward function ΔV osc V CC 1 – 6 -------------------- = 0 20 40 60 80 R2(K Ω) 5 10 20 50 100 200 500 fsw (KHz) D97IN630 0.82nF 1.2nF 2.2nF 3.3nF 4.7nF 5.6nF Tamb=25˚C V1 V2-3 D97IN684 Vi=30V Vi=15V Vc t Vi=30V Vi=15V t |
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