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UC2854B-EP датащи(PDF) 7 Page - Texas Instruments |
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UC2854B-EP датащи(HTML) 7 Page - Texas Instruments |
7 / 13 page UC2854BEP SGLS318 − NOVEMBER 2005 7 www.ti.com FUNCTIONAL DESCRIPTION The UC2854B is designed as a pin compatible upgrade to the industry standard UC2854 active power factor correction circuits. The circuit enhancements allow the user to eliminate in most cases several external components currently required to successfully apply the UC2854. In addition, linearity improvements to the multiply, square and divide circuitry optimizes overall system performance. Detailed descriptions of the circuit enhancements are provided below. For in-depth design applications reference data see the application notes, UC2854 Controlled Power Factor Correction Circuit Design (SLUA144) and UC2854A and UC2854B Advanced Power Factor Correction Control ICs (SLUA177). Multiply/Square and Divide The UC2854B multiplier design maintains the same gain constant K + * 1 V as the UC2854. The relationship between the inputs and output current is given as: I MOUT + IiAC V VAO * 1.5 V K V VRMS 2 This is nearly the same as the UC2854, but circuit differences have improved the performance and application. The first difference is with the IAC input. The UC2854B regulated this pin voltage to the nominal 500 mV over the full operating temperature range, rather than the 6 V used on the UC2854. The low offset voltage eliminates the need for a line zero crossing compensating resistor to VREF from IAC that UC2854 designs require. The maximum current at high line into Iac should be limited to 250 µA for best performance. Therefore, if VVAC(max) = 270 V, R IAC + 270 1.414 250 mA + 1.53 MW The VRMS pin linear operating range is improved with the UC2854B as well. The input range for VRMS extends from 0 V to 5.5 V. Since the UC2854A squaring circuit employs an analog multiplier, rather than a linear approximation, accuracy is improved, and discontinuities are eliminated. The external divider network connected to VRMS should produce 1.5 V at low line (85 VAC). This puts 4.77 V on VRMS at high line (27 VAC) which is well within its operating range. The voltage amplifier output forms the third input to the multiplier and is internally clamped to 6 V. This eliminated an external zener clamp often used in UC2854 designs. The offset voltage at this input to the multiplier has been raised on the UC2854A/B to 1.5 V. The multiplier output pin, which is also common to the current amplifier non-inverting input, has a −0.3 V to 5 V output range, compared to the −0.3 V to 2.5 V range of the UC2854. This improvement allows the UC2854B to be used in applications where the current sense signal amplitude is large. Voltage Amplifier The UC2854B voltage amplifier design is essentially similar to the UC2854 with two exceptions. The first is with the internal connection. The lower voltage reduces the amount of charge on the compensation capacitors, which provides improved recovery form large signal events, such as line dropouts, or power interruption. It also minimizes the dc current flowing through the feedback. The output of the voltage amplifier is also changes. In addition to a 6-V temperature compensated clamp, the output short circuit current has been lowered to 2 mA typical and an active pull down has replaced the passive pull down of the UC2854. (1) (2) |
Аналогичный номер детали - UC2854B-EP |
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Аналогичное описание - UC2854B-EP |
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