AN3027 datasheet(28/41 Pages) STMICROELECTRONICS

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Designing a TM PFC

AN3027

28/41

Doc ID 16134 Rev 4

With this IC the function can be implemented by connecting a resistor (RT) between the

TBO pin and ground. 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, that is internally 1:1 mirrored and sunk from pin INV (pin #1) input of the

error amplifier. In this way, when the mains voltage increases, the voltage at TBO pin

increases as well which increases 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 preregulator is forced to go higher. Obviously, the output voltage moves in the

opposite direction if the input voltage decreases. To avoid 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 function is not used, leave the pin open: the device regulates a fixed output

voltage.

Starting from the following data:

To set the output voltage at the desired values use the following design procedure:

Determine the input RMS voltage VinCLAMP that produces Vo = Vox:

Equation 62

and choose a value Vinx such that Vin2 = Vinx < VinCLAMP. This results in a limitation of the

output voltage range below Vox (it equals Vox if one chooses Vinx = VinCLAMP)

Determine the divider ratio of the MULT pin (pin #3) bias:

●

Minimum specified input RMS voltage

(18)

●

Maximum specified input RMS voltage

(19)

●

Regulated output voltage at Vin = Vin1

(20)

●

Regulated output voltage at Vin = Vin2

(21)

●

Absolute maximum limit for the

regulated output voltage

(22)

●

Maximum output voltage (Vdc) -

(7)

(23)

Vin1

Vin2

Vo1

Vo2

Vox

VOVP

inCLAMP

⋅

−

⋅

−

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