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UC2872 Fiches technique(PDF) 4 Page - Texas Instruments |
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UC2872 Fiches technique(HTML) 4 Page - Texas Instruments |
4 / 7 page 4 UC1872 UC2872 UC3872 AOUT, BOUT: These outputs provide complementary drive signals for the push-pull N-channel MOSFETs. Each one is high for 50% of the time, switching states each time a zero-detect is sensed. COMP: COMP is the output terminal of the error ampli- fier. Compensation components are normally connected between COMP and INV. Connecting a capacitor from this pin to ground limits turn on current and blanks the open lamp detect signal allowing the lamp to start. COUT: This output directly drives the bulk regulator P-channel MOSFET. COUT turn-on is synchronized to each zero-detect, and therefore switches at twice the fre- quency of AOUT and BOUT. The modulator controlling COUT is designed to provide smooth control up to 100% duty cycle. CT: A capacitor connected between this pin and GND ground sets the synchronization frequency range. The capacitor is charged with approximately 200 µA, creating a linear ramp which is used by COUT’s (buck regulator driver) PWM comparator. ENBL: When ENBL is driven high the device is enabled. When ENBL is pulled low, the IC is shut down and typi- cally draws 1 µA. GND: This pin is the ground reference point for the inter- nal reference and all thresholds. INV: This pin is the inverting input to the error amplifier and the input for the open lamp detect circuitry. If the voltage at INV is below the 1V open lamp detect thresh- old, the outputs are disabled. PGND: This pin is the high current ground connection for the three output drivers. REF: This pin is connected to the 3V reference voltage which is used for the internal logic. Bypass REF to ground with a 0.01 µF ceramic capacitor for proper opera- tion. VC: VC is the power supply voltage connection for the output drivers. Bypass it to ground with a 0.1 µF ceramic capacitor for proper operation. VCC: VCC is the positive supply voltage for the chip. Its operating range is from 4.2V to 24V. Bypass VCC to ground with a 0.1 µF ceramic capacitor for proper opera- tion. ZD: The zero-detect input senses when the trans- former’s primary center tap voltage falls to zero to syn- chronize the sawtooth voltage waveform on CT. The threshold is approximately 0.5V, providing a small amount of offset such that with propagation delay, zero-volt switching occurs. A resistor (typically 10k) should be connected between ZD and the primary cen- ter tap to limit input current at turn off. PIN DESCRIPTIONS Figure 1 shows a complete application circuit using the UC3872 Resonant Lamp Ballast Controller. The IC pro- vides all drive, control and housekeeping functions. The buck output voltage (transformer center-tap) provides the zero crossing and synchronization signals. The buck modulator drives a P-channel MOSFET di- rectly, and operates over a 0-100% duty-cycle range. The modulation range includes 100%, allowing operation with minimal headroom. The oscillator and synchronization circuitry are shown in Figure 2. The oscillator is designed to synchronize over a 3:1 frequency range. In an actual application however, the frequency range is only about 1.5:1. A zero detect comparator senses the primary center-tap voltage, gen- erating a synchronization pulse when the resonant wave- form falls to zero. The actual threshold is 0.5 volts, providing a small amount of anticipation to offset propa- gation delay. The synchronization pulse width is the time required for the 4mA current sink to discharge the timing capacitor to 0.1 volts. This pulse width limits the minimum linear con- trol range of the buck regulator. The 200 µA current source charges the capacitor to a maximum of 3 volts. A comparator blanks the zero detect signal until the capac- itor voltage exceeds 1 volt, preventing multiple synchro- nization pulse generation and setting the maximum frequency. If the capacitor voltage reaches 3 volts (a zero detection has not occurred) an internal clock pulse is generated to limit the minimum frequency. APPLICATION INFORMATION |
Numéro de pièce similaire - UC2872 |
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Description similaire - UC2872 |
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