Circuit description

AN2983

14/32

Doc ID 15679 Rev 1

Where, as already seen, the (nominal) LED peak current is:

Equation 23

On the other hand, if we fix I_Led_pk = 0 we can estimate the value of Va for which the LED

current is reduced to zero.

Equation 24

Equation 25

Equation 26

With our values of Ra = R19 = 10 k

as:

Equation 27

Equation 22

becomes:

Equation 28

And Equation 25:

Equation 29

In the plot shown in Figure 12 the LED current (I_Led) is expressed as a function of the trim

voltage V_trim (= Va) applied at the C_set module input, with the supply voltage (Vin) as

parameter (fixed at 36, 48 and 60 V).

As you can see, the linearity is quite good for currents in the range of 400 to 75 mA and

voltages in the range 0... 10 V, while with V_trim from 10 to 12 V the current still decreases

down to zero, but with some non-linearity.

This is due to the fact that the LED diodes are a non-linear load and their behavior at low

current levels may change from one device to another.

Another point that has to be taken into account is that when the average current is reduced,

the minimum LED current is reduced to zero, causing the converter to change from

continuous conduction mode (CCM) to discontinuous conduction mode (DCM) operation.

Vth

*

Rs

1

)

nom

(

pk

_

Led

_

I

⎟

⎠

⎞

⎜

⎝

⎛

=

⎥

⎦

⎤

⎢

⎣

⎡

⎟

⎠

⎞

⎜

⎝

⎛

−

⎟

⎠

⎞

⎜

⎝

⎛

+

⎟

⎠

⎞

⎜

⎝

⎛

=

Ra

Rb

*

Va

Ra

Rb

Ra

*

Vth

*

Rs

1

0

⎟

⎠

⎞

⎜

⎝

⎛

+

=

⎟

⎠

⎞

⎜

⎝

⎛

Ra

Rb

Ra

*

Vth

Ra

Rb

*

Va

⎟

⎠

⎞

⎜

⎝

⎛

+

=

Rb

Rb

Ra

*

Vth

Va

()

( )

[]

Va

*

1

.

0

Vth

*

1

.

1

*

Rs

1

pk

_

Led

_

I

−

⎟

⎠

⎞

⎜

⎝

⎛

=

.

1

*

)

nom

(

pk

_

Led

_

I

(max)

pk

_

Led

_

I

=

( )

11

*

Vth

Va

=