AN2823

The L5973AD in LED driving applications

Doc ID 15015 Rev 1

21/33

The gain block K(s) amplifies the voltage sensed on the resistor and thus decreases the

power lost across it. Its frequency behavior can also be useful to compensate the loop

function, introducing a pole or a zero if needed (see Figure 19).

The K(s) is implemented through an operational amplifier working in a non inverting

configuration.

In the schematic shown in Figure 19, the gain is

Equation 27

so leaving jp4 and jp5 open K(0)

≈10.01, otherwise closing both jumper K(0)≈5.7.

Equation 28

Equation 29

It can be considered negligible when compared to the amount of power delivered to the

load.

2.2.2

The dimming operation

The dimming operation is implemented as previously described in Section 2.1.2: switch Q1

clamps the output of the error amplifier to disable the switching activity.

Figure 20 and 21 show the dimming operation of the schematic in Figure 19, monitoring the

input dimming signal and the LED current.

Ks

()

1

R15//R16//R11//R13

R14//R12

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

+

⎝⎠

⎛⎞

=

I

LED1

V

FB

K

() R

SENSE

⋅

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

1.235

10.01 0.33

⋅

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

375 mA

≅

≅

=

I

LED2

V

FB

K

() R

SENSE

⋅

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

1.235

5.3 0.33

⋅

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

706 mA

≅≅

=

⎩

⎪

⎪

⎨

⎪

⎪

⎧

P

LOSS 1

R

SENSE

I

LED

()

2

46.4 mW

≅

⋅

=

P

LOSS 2

R

SENSE

I

LED

()

2

142 mW

≅

⋅

=

⎩

⎪

⎨

⎪

⎧