LM2633 datasheet(34/47 Pages) TI1

34 / 47 page

(D'm

c - 0.5

)

2SCR

2SLCf

Fh(s) =

(2Sfn)

2SfnQ

fz =

Fp(s) =

2Sfz

2Sfp

(D'mc - 0.5)

1 +

G(s) =

M Fp(s) Fh(s)

(D'mc - 0.5)

Q =

fn =

= koRe x

1 +

2SfnQ (2Sfn)

LM2633

SNVS139C – MAY 2004 – REVISED APRIL 2005

www.ti.com

α = LCsC(R + Re)

(38)

β = goLC(R + Re) + Cs(CRRe + L)

(39)

γ = C(R + Re) + go(CRRe + L) + CsR

(40)

δ = 1 + goR

(41)

For a reasonable design, the output filter has large attenuation at large complex frequencies (i.e. large s values).

At s values where 1/sC is smaller than Re, the power stage can be reduced to the one shown in Figure 13.

Figure 13. Simplified Power Stage at High Frequencies

The transfer function can be re-written as:

(42)

Where

(43)

(44)

All the Re terms are omitted in the denominator because their values are negligible compared to other terms.

Since the denominator of the control-output transfer function is a third-order polynomial, and its coefficients are

positive real numbers, the transfer function either has one real pole and two complex poles that are complex

conjugates or has three real poles. Thus it can be approximately written in the following format:

(45)

Where

(46)

and

(47)

where

(48)

and

(49)

The value of fp can be determined by comparing the denominators of Equation 45 and Equation 37. The result is:

(50)

From the above expressions, it can be seen that the control-output transfer function has three poles and one

zero. Of the three poles, one is a real pole (fp) that is located at low frequency, the other two are either complex

conjugates that are located at half the switching frequency (fn), or are separated real poles, depending on the Q

value. When Q value is less than 0.5, the two high frequency poles will become two real poles.

Submit Documentation Feedback

Product Folder Links: LM2633

LM2633 Datasheet(PDF) 34 Page - Texas Instruments

LM2633 Datasheet(HTML) 34 Page - Texas Instruments

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Datasheet Download

Link URL