AN2653 datasheet(12/22 Pages) STMICROELECTRONICS

12 / 22 page

In-the-loop compensation method

AN2653

12/22

In-the-loop compensation method

4.1

Theoretical overview

Figure 21 shows a commonly used compensation method, often called in-the-loop, because

the additional components (a resistor and a capacitor) used to improve the stability are

inserted in the feedback loop.

The loop gain in this configuration, corresponding to Figure 22, is the following:

It adds a zero and splits the pole caused by the capacitive load into two poles in the loop

gain. This compensation method allows, by a good choice of compensation components, to

compensate the original pole (caused by the capacitive load), and then to improve stability.

The main drawback of this circuit is the reduction of the output swing, because the isolation

resistor is in the signal path.

Note that, for the following cases, RIL is limited to 10% of RL (or Rf // RL in the case of a gain

configuration) even if better results can be obtained with higher RIL values.

But because the feedback loop is taken directly on Vout, the RIL / RL divider bridge does not

create inaccuracy on Vout as it does with the out-of-the-loop method.

4.2

Application on the TS507

In the case of the TS507, the first pole of the loop gain caused by the feedback occurs

around:

Figure 21.

In-the-loop compensation

schematics

Figure 22.

In-the-loop equivalent schematics

for loop gain calculation

------

–

()

1jR

ILCILω

[]

⋅

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

()C

Lω

IL 1

-------

⎝⎠

⎛⎞ C

ILω

oRILCILCLω

–

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

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

()C

IL 1

-------

⎝⎠

⎛⎞ C

⋅

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

() C

()

⋅

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

≈

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