AN2823

The L5973AD in LED driving applications

Doc ID 15015 Rev 1

23/33

Figure 22.

Dimming signal

From Figure 20 and 21 (VIN = 18 V, string composed of two LEDs) the following data can be

inferred:

Equation 30

Considering a dimming frequency of 250 Hz, the minimum duty cycle of the PWM dimming

satisfies the equation:

Equation 31

so the minimum D is about 5%.

By relaxing the edge requirements or increasing the transient performance of the circuit,

which depend on VIN and the number of LED composing the string and inductor value, a

lower D can be achieved.

2.2.3

Frequency compensation

Most of the theory explained in Section 2.1.3 on page 13 is still valid for the circuit in

Figure 19. The equation of the loop transfer function is:

Equation 32

Here, the term

αLED(s) is slightly different and can be calculated as:

Equation 33

T

RISE

40

μs

=

T

FALL

6

μs

=

⎩

⎨

⎧

T

RISE

T

FALL

+

0.2 T

MIN_PULSE

⋅

0.2 D

MIN

T

DIMMING

⋅⋅

==

G

() G

() Ks

() A

() G

() α

()

⋅

⋅

⋅

⋅

=

α

()

R

SENSE

n

LED

r

LED

R

SENSE

+

⋅

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

=