LED6000 datasheet(28/49 Pages) STMICROELECTRONICS

28 / 49 page

Application notes - step-down conversion

LED6000

28/49

DocID027777 Rev 2

The transfer function of the PWM modulator, from the error amplifier output (COMP pin) to

the LX pin results in an almost constant gain, due to the voltage feedforward which

generates a sawtooth with amplitude VS directly proportional to the input voltage:

Equation 13

For the LED6000 the feedforward gain is KFF = 1/30.

The synchronization of the device with an external clock provided through the SYNCH pin

can modify the PWM modulator gain (see Section 4.1 on page 11 to understand how this

gain changes and how to keep it constant in spite of the external synchronization).

The transfer function of the power section (i.e.: the voltage across RSNS resulting as

a variation of the duty cycle) is:

Equation 14

given L, RDC, CO, RES, RSNS and Rd the parameters shown in Figure 16.

The power section transfer function can be rewritten as follows:

Equation 15

Equation 16

Equation 17

with the assumption that the inductor parasitic resistance, RDC, and the output capacitor

parasitic resistance, RES, are negligible compared to LED dynamic resistance, Rd.

The closed loop gain is then given by:

Equation 18





SNS

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)

(

)

(

)

(

SNS





































;

)

(





SNS

















;

SNS













)

(

)

(

)

(

)

(

COMP

LOOP





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