AN2653 datasheet(13/22 Pages) STMICROELECTRONICS

13 / 22 page

AN2653

In-the-loop compensation method

13/22

The second one occurs at higher frequencies where its impact on stability is limited. The

goal of the first pole is to decrease the loop gain to get closer to 0 dB, just before the zero,

occurring at

whose goal is to minimize the phase shift caused by the pole. The stability is increased as

the loop gain crosses the 0 dB axis with a limited phase shift. It minimizes the effect of the

second pole caused by the feedback, which is also pushed toward higher frequencies.

Although this compensation method may seem difficult to set up, it brings very good results,

as shown in Table 3 and Table 4, for the TS507 operational amplifier.

In a gain configuration, when considering the loop gain, the output is loaded by a resistive

load of RL // (Rf + Rg), where Rf and Rg are the resistors used for the gain. If Rf + Rg << RL,

the loop gain and therefore the stability parameters are the same whatever the value of RL.

This is visible in Table 4 where Rf + Rg = 1.1 kΩ with RL = 10 kΩ and RL = 100 kΩ..

π R

ILCIL

⋅

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

Table 3.

Results of in-the-loop compensation for different load conditions in the case of a

voltage follower configuration for TS507

RL =1kΩ

RL =10kΩ

RL =100 kΩ

RIL

(1)

(

Ω)

CIL

(nF)

Mg(2)

(dB)

ϕ(2)

(degree)

RIL

(1)

Ω)

CIL

(nF)

Mg(2)

(dB)

ϕ(2)

(degree)

RIL

(1)

Ω)

CIL

(nF)

Mg(2)

(dB)

ϕ(2)

(degree)

1nF

-4.1

-28.5 °

-5

-34.1 °

-5.1

-34.4 °

100

4.7

24.5 °

0.4

15.2

53.9 °

0.2

24.3

71.9 °

10 nF

-22.2

-78.4 °

-22.9

-79.5 °

-23

-79.6 °

100

21.9 °

1.26

13.6

61.2 °

1.26

13.3

79.2 °

100 nF

-34.1

-84.4 °

-34.4

-84.6 °

-34.5

-84.6 °

79.4

7.9

6.5

34.3 °

0.5

6.3

6.5

66.9 °

0.63

6.3

6.2

70.6 °

RIL cells are shaded when its value is clamped to RL/10.

Negative values indicate instability.

Table 4.

Results of in-of-the-loop compensation for different load conditions in the case of a

gain configuration of either -10 or +11 (Rg = 100 Ω and Rf = 1 kΩ) for TS507

RL =1kΩ

RL =10kΩ

RL =100 kΩ

RIL

(1)

(

Ω)

CIL

(pF)

Mg(2)

(dB)

ϕ(2)

(degree)

RIL

(1)

(

Ω)

CIL

(pF)

Mg(2)

(dB)

ϕ(2)

(degree)

RIL

(1)

(

Ω)

CIL

(pF)

Mg(2)

(dB)

ϕ(2)

(degree)

1nF

17.6

84.7 °

16.8

85.1 °

16.7

85.2 °

100

126

89 °

100

126

88.1 °

100

126

88 °

10 nF

-0.6

-16.1 °

-1.3

-25.7 °

-1.4

-25.9 °

39.8

251

40.2

78.6 °

31.6

316

40.3

84.9 °

31.6

316

40.3

84.8 °

100 nF

-13

-69.2 °

-13.3

-69.8 °

-13.3

-69.9 °

631

44.5

66.2 °

631

44.5

65.6 °

631

44.5

65.6 °

RIL cells are shaded when its value is clamped to (RL// Rf)/10.

Negative values indicate instability.

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