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L6563
16/25
4.5 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 the L6563 this can be realized by connecting a resistor (R
T
) 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)/R
T
, that is internally 1:1 mirrored and
sunk from pin INV (#1) input of the L6563's 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 connect-
ed 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 di-
rection 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 con-
stant. If this function is not used, leave the pin open: the device will regulate a fixed output voltage.
Starting from the following data:
■
Vin
1
= minimum specified input RMS voltage;
■
Vin
2
= maximum specified input RMS voltage;
■
Vo
1
= regulated output voltage @ Vin = Vin1;
■
Vo
2
= 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 Vin
clamp
that produces Vo = Vox:
Vox
Vo
2
and choose a value Vin
x
such that Vin
2
= Vin
x
< Vin
clamp
. This will result in a limitation of the output volt-
age range below Vox (it will equal Vox if one chooses Vin
x
= Vin
clamp
)
2) Determine the divider ratio of the MULT pin (#3) bias:
and check that at minimum mains voltage Vin
1
the peak voltage on pin 3 is greater than 0.65V.
3) Determine R1, the upper resistor of the output divider:
.
4) Calculate the lower resistor R2 of the output divider and the adjustment resistor R
T
:
.
5) Check that the maximum current sourced by the TBO pin (#6) does not exceed the maximum specified (0.25 mA):
.
In the following Mathcad sheet, as an example, the calculation is shown for the circuit illustrated in
Figure
39.
Figure 40
shows the internal block diagram of the tracking boost function.
Vin
clamp
1
Vo
1
–
-----------–
Vin
2
Vox
Vo
2
2
Vo
1
–
-------------–
Vin
1
–
=
k
2 Vin
x
-----------------------
=
R1
20
----------
10
6
=
R2
2.5 R1
Vin
Vin
Vo
2
–
Vo
1
Vin
Vo
2
2.5
–
(
)
Vin
2
1
Vo
1
2.5
–
(
)
Vin
1
-----------------------------------------------–
=
R
T
2 k R1
–
-------------–
=
I
TBOmax
R
T
------
0.25 10
3
–
≤
=