L6911E datasheet(21/34 Pages) STMICROELECTRONICS

21 / 34 page

L6911E

Device description

21/34

To choose the other components of the compensation network, the transfer function of the

voltage loop is considered. To simplify the analysis is supposed that R3 << Rd,

where Rd = (R8//R9).

Figure 10. Compensation network definition

The transfer function may be evaluated neglecting the connection of R8 to PHASE because,

as will see later, this connection is important only at low frequencies. So R4 is considered

connected to VOUT. Under this assumption, the voltage loop has the following transfer

function:

Equation 17

Where ZC(s) and ZL(s) are the output capacitor and inductor impedance respectively.

The expression of ZI(s) may be simplified as follow:

|A v |

|R |

|G lo o p |

R 0

fLC

fCE

fEC

fCC

G 0

acitor

CeramicCap

Cceramic

Rceramic

Introduced

Cceramic

ESR

ESRzero

OUT

ESR

doublepole

ingularity

ConverterS

⋅

)

(

ingularity

onNetworkS

Compensati

⋅

Gloop s

()

Av s

() Rs

()

⋅

Av s

() Zf s

()

Zi s

()

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

⋅

where

Av s

()

Vin

osc

∆

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

C s

()

C s

() Z

L s

()

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

⋅

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