L7987 datasheet(24/37 Pages) STMICROELECTRONICS

24 / 37 page

Application information

L7987

24/37

DocID025589 Rev 3

The transfer function of the power section (i.e. the L-CO filters and the output load) is the

ratio of the parallel of CO and RO and the sum of L and the parallel of CO and RO, including

L and CO parasitics:

Equation 15

given L, RDC, CO, RES and RO the parameters shown in Figure 10. The power section

transfer function can be rewritten as follows:

Equation 16

Equation 17

Equation 18

with the assumption that the inductor parasitic resistance, RDC, is negligible compared to

RO. The closed loop gain is then given by:

Equation 19

GLOOP(s) = GLC(s) • GPWO(s) • GCOMP(s)

As noted in Section 5.2 on page 21, two different kinds of network can compensate the loop,

depending on the value of fzESR, lower or higher than the regulator required bandwidth.

In Section 5.4.1 and Section 5.4.2 the guidelines to select the type II and type III

compensation network are illustrated.







D C

E S

D C

E S

D C

E S

D C

E S

L C

s C

s L

s C



















































GLC s



GLCO

 f

zESR



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

 Q



fLC



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

 f



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



 2

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



;

GLCO

RO RDC

-------------------------- 1



fzESR

 C

ORES



--------------------------------;

fLC

 LC

RO RES

RO RDC

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

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

 LC

RO RES

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

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



LCO

RO RDC



RO RES



LCO  RORDC RORES RESRDC



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

LCO

RO  RO RES





LCORORES

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



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