ALED6000PHTR datasheet(21/45 Pages) STMICROELECTRONICS

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6.3

Compensation strategy

The compensation network must assure stability and good dynamic performance. The loop of the ALED6000 is

based on the voltage mode control. The error amplifier is an operational amplifier with high bandwidth. So, by

selecting the compensation network the E/A is considered as ideal, that is, its bandwidth is much larger than the

system one.

Figure 16. Switching regulator control loop simplified model

V S

PWM

V IN

COMP

V REF

Power

section

Compensation

network

Internal

switch

SNS

RD1

RD2

RES

RDC

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 feed-forward which generates a sawtooth with amplitude VS directly

proportional to the input voltage:

GPWO= 1VS= 1KFF⋅VIN

(13)

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

modulator gain (see Section 5.1 Oscillator and synchronization 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:

GLCs =VSNSsds =

RSNS⋅VIN

RSNS+s⋅L+RDC+N⋅Rd//1+s⋅C0⋅RES

s⋅C0

(14)

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

simplified model .

The power section transfer function can be rewritten as follows:

GLCs =GLC0⋅ 1+ s2π⋅fz

1+ s2π⋅Q⋅fLC+ s2π⋅fLC2;GLC0≅ RSNS⋅VIN

RSNS+N⋅Rd

(15)

ALED6000

Compensation strategy

DS13018 - Rev 4

page 21/45

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