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UCC3818DWTRG4 Datasheet(PDF) 6 Page - Texas Instruments |
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UCC3818DWTRG4 Datasheet(HTML) 6 Page - Texas Instruments |
6 / 32 page UCC2817, UCC2818, UCC3817, UCC3818 BiCMOS POWER FACTOR PREREGULATOR SLUS395J - FEBRUARY 2000 - REVISED MARCH 2009 6 www.ti.com pin descriptions (continued) IAC: (current proportional to input voltage) This input to the analog multiplier is a current proportional to instantaneous line voltage. The multiplier is tailored for very low distortion from this current input (IIAC) to multiplier output. The recommended maximum IIAC is 500 µA. MOUT: (multiplier output and current amplifier inverting input) The output of the analog multiplier and the inverting input of the current amplifier are connected together at MOUT. As the multiplier output is a current, this is a high-impedance input so the amplifier can be configured as a differential amplifier. This configuration improves noise immunity and allows for the leading-edge modulation operation. The multiplier output current is limited to 2 I IAC . The multiplier output current is given by the equation: I MOUT + I IAC (V VAOUT * 1) V VFF 2 K where K + 1 V is the multiplier gain constant. OVP/EN: (over-voltage/enable) A window comparator input that disables the output driver if the boost output voltage is a programmed level above the nominal or disables both the PFC output driver and resets SS if pulled below 1.9 V (typ). PKLMT: (PFC peak current limit) The threshold for peak limit is 0 V. Use a resistor divider from the negative side of the current sense resistor to VREF to level shift this signal to a voltage level defined by the value of the sense resistor and the peak current limit. Peak current limit is reached when PKLMT voltage falls below 0 V. RT: (oscillator charging current) A resistor from RT to GND is used to program oscillator charging current. A resistor between 10 k Ω and 100 kΩ is recommended. Nominal voltage on this pin is 3 V. SS: (soft-start) VSS is discharged for VVCC low conditions. When enabled, SS charges an external capacitor with a current source. This voltage is used as the voltage error signal during start-up, enabling the PWM duty cycle to increase slowly. In the event of a VVCC dropout, the OVP/EN is forced below 1.9 V (typ), SS quickly discharges to disable the PWM. Note: In an open-loop test circuit, grounding the SS pin does not ensure 0% duty cycle. Please see the application section for details. VAOUT: (voltage amplifier output) This is the output of the operational amplifier that regulates output voltage. The voltage amplifier output is internally limited to approximately 5.5 V to prevent overshoot. VCC: (positive supply voltage) Connect to a stable source of at least 20 mA between 10 V and 17 V for normal operation. Bypass VCC directly to GND to absorb supply current spikes required to charge external MOSFET gate capacitances. To prevent inadequate gate drive signals, the output devices are inhibited unless VVCC exceeds the upper under-voltage lockout voltage threshold and remains above the lower threshold. VFF: (feed-forward voltage) The RMS voltage signal generated at this pin by mirroring 1/2 of the IIAC into a single pole external filter. At low line, the VFF voltage should be 1.4 V. VSENSE: (voltage amplifier inverting input) This is normally connected to a compensation network and to the boost converter output through a divider network. VREF: (voltage reference output) VREF is the output of an accurate 7.5-V voltage reference. This output is capable of delivering 20 mA to peripheral circuitry and is internally short-circuit current limited. VREF is disabled and remains at 0 V when VVCC is below the UVLO threshold. Bypass VREF to GND with a 0.1-µF or larger ceramic capacitor for best stability. Please refer to Figures 8 and 9 for VREF line and load regulation characteristics. |
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