Datenblatt-Suchmaschine für elektronische Bauteile |
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UC1854J883B Datenblatt(PDF) 7 Page - Texas Instruments |
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UC1854J883B Datenblatt(HTML) 7 Page - Texas Instruments |
7 / 13 page UC1854 UC2854 UC3854 PROTECTION INPUTS ENA (Enable): The ENA input must reach 2.5 volts be- fore the REF and GT Drv outputs are enabled. This pro- vides a means to shut down the gate in case of trouble, or to add a time delay at power up. A hysteresis gap of 200mV is provided at this terminal to prevent erratic op- eration. Undervoltage protection is provided directly at pin 15, where the on/off thresholds are 16V and 10V. If the ENA input is unused, it should be pulled up to VCC through a current limiting resistor of 100k. SS (Soft start): The voltage at pin 13 (SS) can reduce the reference voltage used by the error amplifier to regu- late the output DC voltage. With pin 13 open, the refer- ence voltage is typically 7.5V. An internal current source delivers approximately -14 µA from pin 13. Thus a capaci- tor connected between that pin and ground will charge linearly from zero to 7.5V in 0.54C seconds, with C ex- pressed in microfarads. PKLIM (Peak current limit): Use pin 2 to establish the highest value of current to be controlled by the power MOSFET. With the resistor divider values shown in Figure 1, the 0.0V threshold at pin 2 is reached when the voltage drop across the 0.25 ohm current sense resistor is 7.5V*2k/10k=1.5V, corresponding to 6A. A bypass capaci- tor from pin 2 to ground is recommended to filter out very high frequency noise. CONTROL INPUTS VSENSE (Output DC voltage sense): The threshold voltage for the VSENSE input is 7.5V and the input bias current is typically 50nA. The values shown in Figure 1 are for an output voltage of 400V DC. In this circuit, the voltage am- plifier operates with a constant low frequency gain for minimum output excursions. The 47nF feedback capacitor places a 15Hz pole in the voltage loop that prevents 120Hz ripple from propagating to the input current. IAC (Line waveform): In order to force the line current waveshape to follow the line voltage, a sample of the power line voltage in waveform is introduced at pin 6. This signal is multiplied by the output of the voltage amplifier in the internal multiplier to generate a reference signal for the current control loop. This input is not a voltage, but a current (hence IAC). It is set up by the 220k and 910k resistive divider (see Figure 1). The voltage at pin 6 is internally held at 6V, and the two resistors are chosen so that the current flowing into pin 6 varies from zero (at each zero crossing) to about 400 µA at the peak of the waveshape. The following for- mulas were used to calculate these resistors: RAC = Vpk IACpk = 260 VAC × √2 400 µA = 910k RREF = R AC 4 = 220k (where Vpk is the peak line voltage) ISENSE/Mult Out (Line current): The voltage drop across the 0.25 ohm current-sense resistor is applied to pins 4 and 5 as shown. The current-sense amplifier also oper- ates with high low-frequency gain, but unlike the voltage amplifier, it is set up to give the current-control loop a very wide bandwidth. This enables the line current to follow the line voltage as closely as possible. In the present exam- ple, this amplifier has a zero at about 500Hz, and a gain of about 18dB thereafter. VRMS (RMS line voltage): An important feature of the UC3854 preregulator is that it can operate with a three-to- one range of input line voltages, covering everything from low line in the US (85VAC) to high line in Europe (255VAC). This is done using line feedforward, which keeps the input power constant with varying input voltage (assuming constant load power). To do this, the multiplier divides the line current by the square of the RMS value of the line voltage. The voltage applied to pin 8, proportional to the average of the rectified line voltage (and propor- tional to the RMS value), is squared in the UC3854, and then used as a divisor by the multiplier block. The multi- plier output, at pin 5, is a current that increases with the current at pin 6 and the voltage at pins 7, and decreases with the square of the voltage at pin 8. PWM FREQUENCY: The PWM oscillator frequency in Figure 1 is 100kHz. This value is determined by CT at pin 14 and RSET at pin 12. RSET should be chosen first be- cause it affects the maximum value of IMULT according to the equation: IMULTMAX = − 3.75 V RSET This effectively sets a maximum PWM-controlled current. With RSET=15k, IMULTMAX = − 3.75 V 15 k = −250µA Also note that the multiplier output current will never ex- ceed twice IAC. With the 4k resistor from Mult Out to the 0.25 ohm current sense resistor, the maximum current in the current sense resistor will be IMAX = − IMULTMAX ×4k 0.25 Ω = −4A Having thus selected RSET, the current sense resistor, and the resistor from Mult Out to the current sense resis- tor, calculate CT for the desired PWM oscillator frequency from the equation CT = 1.25 F × RSET APPLICATIONS INFORMATION (cont.) 7 |
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