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SG3842DM Datasheet(PDF) 10 Page - Microsemi Corporation |
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SG3842DM Datasheet(HTML) 10 Page - Microsemi Corporation |
10 / 14 page C URRENT -M ODE PWM C ONTROLLER SG1842/SG1843 Series PRODUCT DA T ABOOK 1996/1997 Copyright © 2000 Rev. 1.6 4/00 10 P RODUCTION D ATA S HEET APPLICA TION INFORMA TION OSCILLATOR The oscillator of the 1842/43 family of PWM's is designed such that many values of R T and CT will give the same oscillator frequency, but only one combination will yield a specific duty cycle at a given frequency. FIGURE 13 — OSCILLATOR TIMING CIRCUIT A set of formulas are given to determine the values of R T and C T for a given frequency and maximum duty cycle. (Note: These formulas are less accurate for smaller duty cycles or higher frequencies. This will require trimming of R T or CT to correct for this error.) Given: Frequency ≡ f Maximum Duty Cycle ≡ D m Calculate: R T = 267 ( Ω) where .3 < D m < .95 C T = (µF) For Duty-Cycles above 95% use: Example: A Flyback power supply requires a maximum of 45% duty cycle at a switching frequency of 50kHz. What are the values of R T and CT? Given: f = 50kHz D m = 0.45 Calculate: R T = 267 = 674 Ω C T = = .025µF (1.76)1/.045 -1 (1.76).55/.45 - 1 1.86 * 0.45 50000 * 674 (1.76)1/Dm -1 (1.76)(1-Dm)/Dm - 1 1.86 * D m f * R T V REF R T/CT GND R T C T .001 1 C T Value - (µF) 1000 .002 .02 100 10 .005 .01 .05 0.1 R T = 680 W R T = 2k W R T = 5k W R T = 10k W R T = 20k W R T = 30k W R T = 50k W R T = 70k W R T = 100k W FIGURE 14 — OSCILLATOR FREQUENCY vs. R T FOR VARIOUS CT F ≈ where R T ≥ 5kΩ 1.86 R TCT |
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