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UCC38084DG4 Datasheet(PDF) 6 Page - Texas Instruments |
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UCC38084DG4 Datasheet(HTML) 6 Page - Texas Instruments |
6 / 27 page UCC28083, UCC28084, UCC28085, UCC28086 UCC38083, UCC38084, UCC38085, UCC38086 SLUS488E − SEPTEMBER 2002 − REVISED JULY 2009 6 www.ti.com detailed pin descriptions CTRL: The error voltage is typically generated by a secondary-side error amplifier and transmitted to the primary-side referenced UCC3808x by means of an opto-coupler. CTRL has an internal divider ratio of 0.45 to maintain a usable range with the minimum VDD of 4.1 V. The UCC38083/UCC38084 family features a built-in full-cycle soft start while the UCC38085/6 does not. For the UCC38083/4, soft-start is implemented as a clamp at the input to the PWM comparator. This causes the output pulses to start near 0% duty cycle and increase until the clamp exceeds the CTRL voltage. ISET: Program the slope compensation current ramp by connecting a resistor, RSET, from ISET to ground. The voltage of the ISET pin tracks the 1.5-V internal oscillator ramp, as shown in Figure 1. ISET IRAMP OUTA OUTB I RAMP, peak = 5 x ISET, peak 1 2 3 4 UCC38083 8 7 6 5 1 mF VDD RT 165 k W 220 mF 10 k W RF 1 k W I RAMP VCS CTRL VDD ISET CS RT OUTA OUTB GND Figure 1. Full Duty Cycle Output The compensating current source, ISLOPE, at the CS pin is proportional to the ISET current, according to the relation: I SLOPE + 5 I SET The ramping current due to ISLOPE develops a voltage across the effective filter impedance that is normally connected from the current sense resistor to the CS input. In order to program a desired compensating slope with a specific peak compensating ramp voltage at the CS pin, use the RSET value in the following equation: RSET + V OSC(peak) 5 RF RAMP VOLTAGE HEIGHT Where V OSC(peak) + 1.5 V Notice that the PWM Latch drives an internal MOSFET that will discharge an external filtering capacitor on the CS pin. Thus, ISLOPE will appear to terminate when the PWM comparator or the cycle-by-cycle current limit comparator sets the PWM latch. The actual compensating slope is not affected by premature termination of the switching cycle. (1) (2) |
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