AN2823 datasheet(13/33 Pages) STMICROELECTRONICS

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AN2823

The L5973AD in LED driving applications

Doc ID 15015 Rev 1

13/33

Relaxing the edge requirements or increasing the transient performance of the circuit, which

depend on VIN and the number of LEDs composing the string and inductor value, a lower D

can achieved.

2.1.3

Frequency compensation

The loop transfer function can be written as the product of meaningful terms.

Equation 9

where GPWM(fsw) is a specific parameter of the L5973AD and represents the gain of the

PWM stage, AOTA(s) is the transfer function of the error amplifier block, GPWR(s)

represents the power stage and

αLED(nLED) is the partition of the output voltage directly

applied to the FB pin.

The gain of the PWM stage GPWM(fsw) is:

Equation 10

so it is 26.3 at a switching frequency of 500 kHz.

When the system has reached working conditions, the LEDs composing the row are biased

and their equivalent circuit can be considered as a resistor for frequency << 1 MHz.

Typically, the manufacturer provides the typical equivalent dynamic resistance of the LED

biased with different DC current. This parameter is useful to study the behavior of the

system in small signal analysis. The equivalent dynamic resistance of the Luxeon III Star

from Lumiled measured with different bias current level is reported below:

Equation 11

Figure 12.

Load equivalent circuit

LOOP s

() G

PWM fSW

() A

OTA s

() G

PWR s

() α

LED nLED

()

⋅

PWM fSW

()

0.076

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

SW kHz

()

250

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

⋅

LED

1.3

LED

350mA

0.9

LED

700mA

⎩

⎨

⎧

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