AN2439

Board description

5/20

switching it off. As a result, as the controller is on, the current generator stops working, and

the controller is supplied by the auxiliary winding of the flyback converter.

1.2

Slope compensation

The circuit for slope compensation is shown in Figure 2. When the main MOSFET

(STP10NK70Z) is switched on, the GD pin (Gate Driver) of the L5991 goes high up to a

large enough, compared to the switching period, we can assume that the voltage across

Figure 2.

Slope compensation circuit

of the L5991 (Pin 10), when it is high. As a rule of thumb, in order to have approximately a

switching period.

Equation 3

Equation 4

Equation 5

M1

STP10NK70

R1

1.5k

RSENSE

C2

1.8 nF

D1

Bat49

C1

100 pF

R2 22k

R3

15k

RGATE

GD Pin of L5991

CS Pin of L5991

V

()

V

GD

R

1

R

3

R

SENSE

++

R

1

R

2

R

3

R

SENSE

+++

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

1e

t

–

C

2

R

T

•

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

–

⎝⎠

⎜⎟

⎛⎞

V

D

+

•

•

e

t

–

C

2

R

T

•

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

•

=

V

() V

GD

R

1

R

3

+

R

1

R

2

R

3

++

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

1e

t

–

C

2

R

T

•

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

–

⎝⎠

⎜⎟

⎛⎞

V

D

+

•

•

e

t

–

C

2

R

T

•

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

•

≅

R

2

R

1

R

3

–

R

SENSE

–

()

•

R

1

R

2

R

3

R

SENSE

+++

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

R

2

R

1

R

3

–

()

•

R

1

R

2

R

3

++

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

≅

=