R5972D datasheet(28/35 Pages) STMICROELECTRONICS

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Application information

R5972D

28/35

DocID026873 Rev 1

Figure 17. Negative buck-boost regulator

8.7

Compensation network with MLCC at the output

MLCCs (multiple layer ceramic capacitor) with values in the range of 10 µF - 22 µF and

rated voltages in the range of 10 V - 25 V are available today at relatively low cost from

many manufacturers.

These capacitors have very low ESR values (a few m

) and thus are occasionally used for

the output filter in order to reduce the voltage ripple and the overall size of the application.

However, a very low ESR value affects the compensation of the loop (see Section 7 on page

13) and in order to keep the system stable, a more complicated compensation network may

be required. However, due to the architecture of the internal error amplifier the bandwidth

with this compensation is limited.

That is why output capacitors with a not negligible ESR are suggested. The selection of the

output capacitor have to guarantee that the zero introduced by this component is inside the

designed system bandwidth and close to the frequency of the double pole introduced by the

LC filter. A general rule for the selection of this compound for the system stability is provided

in Equation 41.

Equation 41

Figure 18 shows an example of a compensation network stabilizing the system with ceramic

capacitors at the output (the optimum component value depends on the application).

fZ ESR

 ESR C

OUT





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

bandwidth



fLC f



Z ESR

10 fLC





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