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SC4910AITSTRT Datasheet(PDF) 10 Page - Semtech Corporation |
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SC4910AITSTRT Datasheet(HTML) 10 Page - Semtech Corporation |
10 / 19 page 10 2005 Semtech Corp. www.semtech.com SC4910A/B POWER MANAGEMENT This is the minimum external slope required to avoid sub- harmonic oscillation at low input line. With SC4910, the external slope is very easy to implement. Referring to Figure 2, R12 is the current sense resistor. R10 and the internal 20K Ω resistor divide the internal slope 1.0V - 2.5V down to the required compensation slope. ) f N L R V Se ( ) 0 . 1 5 . 2 ( 20 ) f N L R V Se ( 10 R S S M S IN S S M S IN • • • − − − • • • • − = where: LM - Power transformer magnetizing inductance fS = Switching frequency In the example, Ω = • • − • − − − • • • − • − = 240 ) 3 E 250 100 6 E 450 21 36 085 . 0 ( ) 0 . 1 5 . 2 ( 20 ) 3 E 250 100 6 E 450 21 36 085 . 0 ( 10 R R10 C36 R22 C22 C13 U5 SC4910 18 17 11 16 6 4 10 20 19 3 1 15 2 5 FB COMP OUTB ISHARE PHASE CS OUTA RT1 RT2 SS -SENSE DELAY SY NC/EN VREF R21 R12 R25 Figure 2 Applications Information (Cont.) Closed-Loop Compensation The simplified control-to-output transfer function for the forward converter with current mode control, for small value of external slope ( Se ≤ Sn, Sn is on-time slope of sensed current waveform) is given by: p z vgo vg s 1 s 1 G G ω + ω + = where closed loop current with stage power of gain DC R I V nN G S O O S vgo = closed loop current with stage power of pole ant min Do RC 1 P = ω stage power of zero ESR C R 1 ESR Z = ω where R - Load resistance C - Output capacitance RESR - Output capacitors ESR For the given example above, at low line and R = 0.165 Ω, C = 2 x 680uF = 1360uF, R ESR = 17mΩ, therefore: dB 13 71 . 4 21 20 3 . 3 100 6 G vgo = = • • • = Hz 6887 s / rad 43253 6 E 1360 X 3 E 17 1 Hz 710 s / rad 4456 6 E 1360 X 165 . 0 1 Z P = = − − = ω = = − = ω |
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