AN1895

Setting up FOT control with the L6562

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Setting up FOT control with the L6562

FOT control is implemented with the L6562 using the circuit shown in Figure 3, which shows

the maximum line voltage must be specified. Application note AN1792 shows that to reduce

Figure 3.

Circuit implementing FOT control with the L6562 and relevant timing waveforms.

Following the design procedure given in AN1792, with the aid of the diagram of Figure 4:

1.

2.

a horizontal line located at

ρ = 2.52 (on the left vertical axis) as long as it intercepts the

ρ curve relevant to the value V

K1=0.891.

3.

in P2. The ordinate of P2 (on the right vertical axis) gives the value K2=4.17.

4.

The required time constant is:

.

5.

A capacitor C = 560 pF is selected, then the associated resistance value will be:

.

6.

R1 and R2 will be respectively:

the standard values R1 = 12k

Ω and R2 = 1.5kΩ will be chosen.

ρ

810

6

–

⋅

3.18 10

6

–

⋅

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

2.52

==

τ

t

OFFmin

K2

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

3.18 10

6

–

⋅

4.17

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

0.76 10

6

–

s

⋅

==

=

R'

τ

C

----

0.76 10

6

–

⋅

560 10

12

–

⋅

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

1357

Ω

==

=

R1

R'

1K1

–

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

1357

10.891

–

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

12450

Ω

==

=

R2

R'

K1

-------

1357

0.891

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

1523

Ω

==

=