TEA2028 datasheet(13/47 Pages) STMICROELECTRONICS

13 / 47 page

Thus, a variable (24

o to + 135o) phase lead with a

gain higher than 10dB, must be implemented on-

chip so as to enable the system to enter into

oscillation.

The frequencydead points correspond to the maxi-

mum internal phase variations. This phase shift is

controlled by voltage V22 whose value of 5.6V

± 0.7

is determined by two diodes.

From the Figure 21, the non-linearity of phase-fre-

quency characteristics is clearly apparent. If linear

voltage-frequency response is required for a sym-

metrical gain of

φ1 loop, it would then be necessary

to implement a non-linearity, on the phase control

amplifier A4, but in the opposite direction.

D. - STUDY OF THE INTERNAL AMPLIFIER

Let’s study the gain and phase response of

V18

VIN

a function of V22.

V22 =

where K is a non-linear coefficient

To start with, the ”VC” voltage of comparator ”A3”

is taken as reference parameter.

The dynamic representation of the output stage

can be depicted as below (Figure 22).

with : I2’=

+ jω R1 C1

(at f = 500kHz)

R1 C1

ω =1 ⇒ i2’=

+ j

and Z = R1 +

ω C

<< R

⇔ i ≈ i2’

R1C1 network produces -45

o phase lag of ”i” with

respect to ”i2”, around 500kHz.

V18

≈ -R ⋅ (i1 +I2’)

i1 and i2 calculation as a function of ”VIN” on Pin 19

- A1 Amplifier :

VS1

VIN

dr1

1200

=21

dr : dynamic resistance =

- A2 Amplifier :

VS2

2dr2

⇔

VIN

VS1

VIN

⋅

VS1

= 0.395

⇒ i2 = 0.39 VIN,i2 is in phase with VIN therefore :

i3 =-i2 = -0.39 VIN

- A3 Amplifier :

i1 =i3







− VC

λ +







= - 0.39 VIN







− VC

λ +







V18

i18

V18

i 18

i’

Figure 22

V18 @i1 =0

VIN

VOUT

phase variation = f(VC)

V18 @i1 (Max.)

Ri1

Ri2

VIN

-45

R( i + i )

R(i + i )

+136

Figure 23 : Vector Representation of V18/VIN

”VIN” always leads the ”i1” by 180, only the ampli-

tude of i1 is a function of VC (see Figure 23).

VOUT

VIN

=- R

i1 (1 + jR1C1ω

) + i2

+ j

(R1 + R) C1ω

-i1 = - 0.39 VIN







−







and i2 = 0.39 VIN

The Figure 24 illustrates the characteristics of

V18/VIN phase versus VC.

- Phase variation determined by VC falls between

+24

o and +135o range

- The gain is higher than 10dB. The Pin 18 output

signal of 30 to 40dB has a rectangular component

(see Figure 24).

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