Test results and significant waveforms

AN3119

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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.

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.

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.

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.