UCC28019P datasheet(21/48 Pages) TI

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Voltage Error Amplifier

Non-linear Gain Generation

UCC28019

SLUS755 – APRIL 2007

APPLICATION INFORMATION (continued)

The transconductance error amplifier (gmv) generates an output current proportional to the difference between

the voltage feedback signal at VSENSE and the internal 5-V reference. This output current charges or

discharges the compensation network capacitors on the VCOMP pin to establish the proper VCOMP voltage for

the system operating conditions. Proper selection of the compensation network components leads to a stable

PFC pre-regulator over the entire ac-line range and 0-100% load range. The total capacitance also determines

the rate-of-rise of the VCOMP voltage at soft start, as discussed earlier.

The amplifier output VCOMP is pulled to GND during any Fault or Standby condition to discharge the

compensation capacitors to an initial zero state. Usually, the large capacitor has a series resistor which delays

complete discharge by their respective time constant (which may be several hundred milliseconds). If VCC bias

voltage is quickly removed after UVLO, the normal discharge transistor on VCOMP loses drive and the large

capacitor could be left with substantial voltage on it, negating the benefit of a subsequent Soft-Start. The

UCC28019 incorporates a parallel discharge path which operates without VCC bias, to further discharge the

compensation network after VCC is removed.

When output voltage perturbations greater than ±5% appear at the VSENSE input, the amplifier moves out of

linear operation. On an over-voltage, the OVP function acts directly to shut off the GATE output until VSENSE

returns within ±5% of regulation. On an under-voltage, the UVD function invokes EDR which immediately

increases the internal VCOMP voltage by 2 V and increases the external VCOMP charging current typically to

100

µA to 170 µA. This higher current facilitates faster charging of the compensation capacitors to the new

operating level, improving transient response time.

The voltage at VCOMP is used to set the current amplifier gain and the PWM ramp slope. This voltage is

buffered internally and is then subject to modification by the EDR function and the SOC function, as discussed

earlier.

Together the current gain and the PWM slope adjust to the different system operating conditions (set by the

ac-line voltage and output load level) as VCOMP changes, to provide a low-distortion, high-power-factor input

current wave-shape following that of the input voltage.

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UCC28019P Datasheet(PDF) 21 Page - Texas Instruments