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UCC28220QDRQ1 Datasheet(PDF) 11 Page - Texas Instruments |
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UCC28220QDRQ1 Datasheet(HTML) 11 Page - Texas Instruments |
11 / 26 page www.ti.com Slope Compensation PWM CS1 (4) + - REF CTRL (8) SLOPE (5) C_SC OUT 1 TO RESET of PWM LATCH ON OFF 2.5/(25*R_SLOPE) = I_SC R_SLOPE 0.5V 10 pF S1 S2 UCC28220-Q1 SLUS789 – MARCH 2008 The slope compensation circuit in the UCC28220 operates on a cycle-by-cycle basis. The two channels have separate slope compensation circuits. These are fabricated in precisely the same way so as current sharing is unaffected by the slope compensation circuit. For each channel, an internal capacitor is reset whenever that channel's output is off. At the beginning of the PWM cycle, a current is mirrored off the SLOPE pin into the capacitor, developing an independent ramp. Since the two channel's ramps start when the channel's output changes from a low to high state, the ramps are thus interleaved. These internal ramps are added to the voltages on the current sense pins (CS1 and CS2) and the result forms an input to the PWM comparators. Figure 3. Slope Compensation Detail for Channel 1 (Duplicate Matched Circuitry for Channel 2) To ensure stability, the slope compensation circuit must add between 1/5 and 1 times the inductor downslope to each of the current sense signals prior to being applied to the PWM comparator's input. Determining the value for the slope compensation resistor: Design Example NCT(p) = 1 VOUT = 12 Np = 7 RSENSE = 5.23 FS(out) = 500000 NCT(s) = 50 LOUT = 3.2 x 10 -6 Ns = 5 VEA(cl) = 1.98 Where NCT(p) = Number of primary turns on the Current Transformer (Turns) NCT(s) = Number of Secondary turns on the current transformer (Turns) VOUT = Nominal output voltage of the converter (V) LOUT = Inductance value of each output inductor (H) NP = Number of primary turns on the main transformer (Turns) NS = Number of secondary turns on the main transformer (Turns) RSENSE = Value of current sense resistor on secondary of current sense transformer (Ω) VEA(cl) = Maximum value of the E/A output voltage (V) FS(out) = Switching frequency of each output (Hz) Copyright © 2008, Texas Instruments Incorporated Submit Documentation Feedback 11 Product Folder Link(s): UCC28220-Q1 |
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