Circuit description

AN2983

12/32

Doc ID 15679 Rev 1

Equation 11

Equation 12

Neglecting the current that flows though the CS pin (1 µA max):

Equation 13

And then:

Equation 14

Equation 15

Vth = Va + Ib*Ra

Then:

Equation 16

Vs = Vth + Ib*Rb

and:

Equation 17

Equation 18

Equation 19

Equation 20

Equation 21

If we consider the limit condition Va = 0, which is equivalent to connecting Ra directly to

ground, the maximum peak LED current I_Led_pk (max) can be obtained by:

Rb

*

Ib

Vs

Vth

−

=

Ra

*

Ia

Va

Vth

−

=

Ia

Ib

−

=

Rb

*

Ib

Vs

Vth

−

=

)

Va

Vth

(

*

Ra

1

Ib

−

⎟

⎠

⎞

⎜

⎝

⎛

=

Rb

*

)

Va

Vth

(

*

Ra

1

Vth

Vs

−

⎟

⎠

⎞

⎜

⎝

⎛

+

=

Va

*

Ra

Rb

Vth

*

Ra

Rb

Vth

Vs

⎟

⎠

⎞

⎜

⎝

⎛

−

⎟

⎠

⎞

⎜

⎝

⎛

+

=

⎟

⎠

⎞

⎜

⎝

⎛

−

⎟

⎠

⎞

⎜

⎝

⎛

+

=

Ra

Rb

*

Va

Ra

Rb

Ra

*

Vth

Vs

⎥

⎦

⎤

⎢

⎣

⎡

⎟

⎠

⎞

⎜

⎝

⎛

−

⎟

⎠

⎞

⎜

⎝

⎛

+

⎟

⎠

⎞

⎜

⎝

⎛

=

Ra

Rb

*

Va

Ra

Rb

Ra

*

Vth

*

Rs

1

pk

_

Led

_

I