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ISL8845AMBEPZ Datasheet(PDF) 8 Page - Intersil Corporation |
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ISL8845AMBEPZ Datasheet(HTML) 8 Page - Intersil Corporation |
8 / 10 page 8 FN6792.0 September 29, 2008 where D is the percent of on-time during a switching cycle. Setting Q = 1 and solving for Se yields Equation 10: Since Sn and Se are the on time slopes of the current ramp and the external ramp, respectively, they can be multiplied by tON to obtain the voltage change that occurs during tON. where Vn is the change in the current feedback signal (ΔI) during the on-time and Ve is the voltage that must be added by the external ramp. For a flyback converter, Vn can be solved for in terms of input voltage, current transducer components, and primary inductance, yielding where RCS is the current sense resistor, fSW is the switching frequency, Lp is the primary inductance, VIN is the minimum input voltage, and D is the maximum duty cycle. The current sense signal at the end of the on-time for CCM operation is: where VCS is the voltage across the current sense resistor, Ls is the secondary winding inductance, and IO is the output current at current limit. Equation 13 assumes the voltage drop across the output rectifier is negligible. Since the peak current limit threshold is 1.00V, the total current feedback signal plus the external ramp voltage must sum to this value when the output load is at the current limit threshold. Substituting Equations 12 and 13 into Equation 14 and solving for RCS yields Equation 15: Adding slope compensation is accomplished in the ISL884xAMBEPZ using an external buffer transistor and the RtCt signal. A typical application sums the buffered RtCt signal with the current sense feedback and applies the result to the CS pin, as shown in Figure 6. Assuming the designer has selected values for the RC filter (R6 and C4) placed on the CS pin, the value of R9 required to add the appropriate external ramp can be found by superposition. The factor of 2.05 in Equation 16 arises from the peak amplitude of the sawtooth waveform on RtCt minus a base-emitter junction drop. That voltage multiplied by the maximum duty cycle is the voltage source for the slope compensation. Rearranging to solve for R9 yields: The value of RCS determined in Equation 15 must be rescaled so that the current sense signal presented at the CS pin is that predicted by Equation 13. The divider created by R6 and R9 makes this necessary. Example: VIN = 12V VO = 48V Ls = 800µH Ns/Np = 10 Lp = 8.0µH IO = 200mA Switching Frequency, fSW = 200kHz Duty Cycle, D = 28.6% S e S n 1 π --- 0.5 + ⎝⎠ ⎛⎞ 1 1D – ------------- 1 – ⎝⎠ ⎛⎞ = (EQ. 10) V e V n 1 π --- 0.5 + ⎝⎠ ⎛⎞ 1 1D – ------------- 1 – ⎝⎠ ⎛⎞ = (EQ. 11) V e Dt ⋅ SW VIN RCS ⋅⋅ L p -------------------------------------------------- 1 π --- 0.5 + ⎝⎠ ⎛⎞ 1 1D – ------------- 1 – ⎝⎠ ⎛⎞ = V (EQ. 12) V CS N S RCS ⋅ N P ------------------------ I O 1D – () V O f ⋅⋅ SW 2L s ---------------------------------------------- + ⎝⎠ ⎜⎟ ⎛⎞ = V (EQ. 13) V e V CS + 1 = (EQ. 14) R CS 1 Df sw VIN ⋅⋅ L p ------------------------------- 1 π --- 0.5 + 1D – ------------------ 1 – ⎝⎠ ⎜⎟ ⎜⎟ ⎛⎞ ⋅ N s N p ------- I O 1D – () V O fsw ⋅⋅ 2L s -------------------------------------------- + ⎝⎠ ⎜⎟ ⎛⎞ ⋅ + ----------------------------------------------------------------------------------------------------------------------------------------------------- = (EQ. 15) CS RTCT R6 C4 R9 VREF FIGURE 6. SLOPE COMPENSATION V e 2.05D R 6 ⋅ R 6 R 9 + ---------------------------- = V (EQ. 16) R 9 2.05D V e – () R 6 ⋅ V e ---------------------------------------------- = Ω (EQ. 17) R ′ CS R 6 R 9 + R 9 --------------------- R CS ⋅ = (EQ. 18) ISL8840AMBEPZ, ISL8841AMBEPZ, ISL8842AMBEPZ, ISL8843AMBEPZ, ISL8844AMBEPZ, ISL8845AMBEPZ |
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