STMicroelectronics 250W TRANSITION-MODE PFC PRE-REGULATOR WITH L6563S EVL6563S-250W EVL6563S-250W Data Sheet
Product codes
EVL6563S-250W
Test results and significant waveforms
AN3119
22/32
Doc ID 16849 Rev 2
4.6
TBO (tracking boost option)
To achieve the TBO function on the L6563S a dedicated input of the multiplier is available on
pin #6 (TBO); the function can be realized by simply connecting a resistor (RT) between the
TBO pin and ground.
pin #6 (TBO); the function can be realized by simply connecting a resistor (RT) between the
TBO pin and ground.
Usually, in traditional PFC stages, the DC output voltage is regulated at a fixed value
(typically 400 V) but in some applications it may be advantageous to regulate the PFC
output voltage with the “tracking boost” or “follower boost” approach. In this way the circuit
with TBO function provides better efficiency, and thanks to the lower differential voltage
across the boost inductor the value of L2 can be reduced, compared to the same circuit
without TBO function.
(typically 400 V) but in some applications it may be advantageous to regulate the PFC
output voltage with the “tracking boost” or “follower boost” approach. In this way the circuit
with TBO function provides better efficiency, and thanks to the lower differential voltage
across the boost inductor the value of L2 can be reduced, compared to the same circuit
without TBO function.
The TBO pin presents a DC level equal to the peak of the MULT pin voltage and is then
representative of the mains RMS voltage. The resistor defines a current, equal to
V(TBO)/RT, which is internally 1:1 mirrored and sunk from the INV pin (#1) input of the error
amplifier. In this way, when the mains voltage increases, the voltage at the TBO pin also
increases, as does the current flowing through the resistor connected between TBO and
GND. Then a larger current is sunk by the INV pin and the output voltage of the PFC pre-
regulator is forced higher. Obviously the output voltage moves in the opposite direction if the
input voltage decreases.
representative of the mains RMS voltage. The resistor defines a current, equal to
V(TBO)/RT, which is internally 1:1 mirrored and sunk from the INV pin (#1) input of the error
amplifier. In this way, when the mains voltage increases, the voltage at the TBO pin also
increases, as does the current flowing through the resistor connected between TBO and
GND. Then a larger current is sunk by the INV pin and the output voltage of the PFC pre-
regulator is forced higher. Obviously the output voltage moves in the opposite direction if the
input voltage decreases.
To avoid an undesired output voltage rise, should the mains voltage exceed the maximum
specified value, the voltage at the TBO pin is clamped at 3 V. By properly selecting the
multiplier bias it is possible to set the maximum input voltage above which input-to-output
tracking ends and the output voltage becomes constant. If this application is not used and
the pin is left open, the device regulates at a fixed output voltage.
specified value, the voltage at the TBO pin is clamped at 3 V. By properly selecting the
multiplier bias it is possible to set the maximum input voltage above which input-to-output
tracking ends and the output voltage becomes constant. If this application is not used and
the pin is left open, the device regulates at a fixed output voltage.