L6986ITR datasheet(24/60 Pages) STMICROELECTRONICS

24 / 60 page

Sn represents the on time slope of the sensed inductor current, Se the on-time slope of the external ramp (VPP

peak-to-peak amplitude) that implements the slope compensation to avoid sub-harmonic oscillations at duty cycle

over 50%.

Se can be calculated from the parameter VPP × gCS given in .

The sampling effect contribution FH(s) is:

FHs= 1

1+ s

ωn∙Qp+s2ωn2

(11)

where

ωn=π∙fSW

Qp= 1

π∙mC∙ 1−D −0.5

(12)

5.2

Error amplifier compensation network

The typical compensation network required to stabilize the system is shown in figure below:

Figure 27. Transconductance embedded error amplifier

RC and CC introduce a pole and a zero in the open loop gain. CP does not significantly affect system stability but it

is useful to reduce the noise at the output of the error amplifier.

The transfer function of the error amplifier and its compensation network is:

A0s=

AV0∙1+s∙RC∙CC

s2∙R0∙C0+Cp∙RC∙CC+s∙R0∙CC+R0∙C0+Cp+RC∙CC+1

(13)

where

AV0=Gm∙R0.

The poles of this transfer function are (if CC >> C0 + CP)

fPHF= 12∙π∙R0∙CC

(14)

L6986I

Error amplifier compensation network

DS13647 - Rev 1

page 24/60

L6986ITR Datasheet(PDF) 24 Page - STMicroelectronics