DAP005

Application information

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6.5

Tracking Boost function

In some applications it may be advantageous to regulate the output voltage of the PFC pre-

regulator so that it tracks the RMS input voltage rather than at a fixed value like in

conventional boost pre-regulators. This is commonly referred to as "tracking boost" or

"follower boost" approach.

With this IC the function can be realized 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 will increase

as well and so will do the current flowing through the resistor connected between TBO and

GND. Then a larger current will be sunk by INV pin and the output voltage of the PFC pre-

regulator will be forced to get higher. Obviously, the output voltage will move 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 3V. 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 will regulate a fixed output voltage.

Starting from the following data:

●

Vin1 = minimum specified input RMS voltage;

●

Vin2 = maximum specified input RMS voltage;

●

Vo1 = regulated output voltage @ Vin = Vin1;

●

Vo2 = regulated output voltage @ Vin = Vin2;

●

Vox = absolute maximum limit for the regulated output voltage;

●

∆Vo = OVP threshold,

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

1.

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

Equation 6

and choose a value Vinx such that Vin2 = Vinx < Vinclamp. This will result in a limitation of the

output voltage range below Vox (it will equal Vox if one chooses Vinx = Vinclamp)

2.

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

Equation 7

Vin

clamp

Vox

Vo

1

–

Vo

2

Vo

1

–

--------------------------- Vin

2

Vox

Vo

2

–

Vo

2

Vo

1

–

--------------------------- Vin

1

⋅

–

⋅

=

k

3

2Vin

x

⋅

-----------------------

=