AN2653 datasheet(9/22 Pages) STMICROELECTRONICS

9 / 22 page

AN2653

Out-of-the-loop compensation method

9/22

3.2

Application on the TS507

This compensation method brings very good results in terms of stability, improving strongly

the phase and gain margins. Table 1 and Table 2 show the results obtained for different load

conditions, in the case of voltage follower and gain configurations. Note that ROL is limited to

1% of RL even though better results can be obtained with higher values of ROL.

As expected, Table 1 and Table 2 show that the higher the value of ROL, the better the

compensation (because the best ROL is always its maximum value RL/100).

These results also show that, for a voltage follower configuration, this compensation method

does not work with low RL (and low CL), because the zero frequency cannot be one decade

before the unity-gain frequency of the open loop gain. In the case of the TS507, it works well

only if the ROL.CL product is above 10

-6.

Table 1.

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

voltage follower configuration for TS507

RL =1kΩ

RL =10kΩ

RL =100 kΩ

ROL

(

Ω)

fu/fz

(1)

Mg(2)

(dB)

ϕ(2)

(degree)

ROL

(

Ω)

fu/fz

(1)

Mg(2)

(dB)

ϕ(2)

(degree)

ROL

Ω)

fu/fz

Mg(2)

(dB)

ϕ(2)

(degree)

1nF

-4.1

-28.5

-5

-34.1

-5.1

-34.4

0.11

-2.5

-16.8

100

1.13

26.9

11.3

22.4

52.1

10 nF

-22.2

-78.4

-22.9

-79.5

-23

-79.6

1.13

-14

-32.4

100

11.3

112.3

22.6

52.3

100 nF

-34.1

-84.4

-34.4

-84.6

-34.5

-84.6

11.3

17.1

6.8

100

113.3

23.4

39.4

1126

22.6

52.3

fu/fz cells are shaded when the value is lower than 10, which is not the best case due to ROL limitation.

Negative values indicate instability.

Table 2.

Results of out-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

ROL

(

Ω)

fu/fz

(1)

Mg(2)

(dB)

ϕ(2)

(degree)

ROL

(

Ω)

fu/fz

(1)

Mg(2)

(dB)

ϕ(2)

(degree)

ROL

Ω)

fu/fz

Mg(2)

(dB)

ϕ(2)

(degree)

1nF

17.6

84.7

16.8

85.1

16.7

85.2

0.11

84.7

100

1.13

36.9

85.1

11.3

43.4

10 nF

-0.6

-16.1

-1.3

-25.7

-1.4

-25.9

1.13

7.2

81.2

100

11.3

43.9

81.4

112.6

43.4

84.8

100 nF

-13

-69.2

-13.3

-69.8

-13.3

-69.9

11.3

100

113.3

44.3

80.6

1126

43.4

84.8

fu/fz cells are shaded when the value is lower than 10, which is not the best case due to ROL limitation.

Negative values indicate instability.

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