AN2983

Circuit description

Doc ID 15679 Rev 1

17/32

Going into further detail, we also have to consider the delay of the current sense comparator

(tdel) and modify Equation 33 as follows.

Equation 40

With:

Equation 41

Equation 42

Equation 43

And with Equation 38 that now becomes:

Equation 44

And Equation 40:

Equation 45

With the values of our application:

Equation 46

Equation 47

Equation 48

del

_

I

rip

_

I

2

1

pk

_

Led

_

I

avg

_

Led

_

I

+

−

=

()

VLed

Vin

*

L

tdel

del

_

I

−

⎟

⎠

⎞

⎜

⎝

⎛

=

⎟

⎠

⎞

⎜

⎝

⎛

−

⎥

⎦

⎤

⎢

⎣

⎡

⎟

⎠

⎞

⎜

⎝

⎛

−

⎟

⎠

⎞

⎜

⎝

⎛

⎟

⎠

⎞

⎜

⎝

⎛

+

+

⎟

⎠

⎞

⎜

⎝

⎛

+

⎥

⎦

⎤

⎢

⎣

⎡

⎟

⎠

⎞

⎜

⎝

⎛

−

⎟

⎠

⎞

⎜

⎝

⎛

+

⎟

⎠

⎞

⎜

⎝

⎛

=

L

tdel

*

VLed

L

toff

2

1

Ra

Rb

*

Rs

1

*

VLed

L

tdel

*

Vin

Ra

Rb

*

Vin

Ra

Rb

Ra

*

Vth

*

Rs

1

avg

_

Led

_

I

⎥

⎦

⎤

⎢

⎣

⎡

⎟

⎠

⎞

⎜

⎝

⎛

−

⎟

⎠

⎞

⎜

⎝

⎛

−

⎟

⎠

⎞

⎜

⎝

⎛

⎟

⎠

⎞

⎜

⎝

⎛

+

+

⎥

⎥

⎦

⎤

⎢

⎢

⎣

⎡

⎥

⎦

⎤

⎢

⎣

⎡

⎟

⎠

⎞

⎜

⎝

⎛

−

⎟

⎠

⎞

⎜

⎝

⎛

−

⎟

⎠

⎞

⎜

⎝

⎛

+

⎟

⎠

⎞

⎜

⎝

⎛

=

L

tdel

L

toff

2

1

Ra

Rb

*

Rs

1

*

VLed

Rs

/

L

tdel

Ra

Rb

*

Vin

Ra

Rb

Ra

*

Vth

*

Rs

1

avg

_

Led

_

I

⎟

⎠

⎞

⎜

⎝

⎛

+

⎟

⎠

⎞

⎜

⎝

⎛

=

⎟

⎠

⎞

⎜

⎝

⎛

Rs

/

L

tdel

Rs

/

L

toff

2

1

Ra

Rb

⎥

⎥

⎦

⎤

⎢

⎢

⎣

⎡

⎥

⎦

⎤

⎢

⎣

⎡

⎟

⎠

⎞

⎜

⎝

⎛

−

⎟

⎠

⎞

⎜

⎝

⎛

−

⎟

⎠

⎞

⎜

⎝

⎛

+

⎟

⎠

⎞

⎜

⎝

⎛

=

Rs

/

L

tdel

Ra

Rb

*

Vin

Ra

Rb

Ra

*

Vth

*

Rs

1

avg

_

Led

_

I

Ω

=

μ

=

μ

=

μ

=

2.8

Rs

H,

470

L

S,

0.2

tdel

S,

1.57

toff

()

S

10

*

68

.

1

8

.

2

10

*

470

Rs

L

4

6

−

−

=

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

=

⎟

⎠

⎞

⎜

⎝

⎛

( )

S

)

10

*

985

.

0

(

)

tdel

(

toff

2

1

6

−

=

+