A7985A datasheet(22/44 Pages) STMICROELECTRONICS

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Application information

A7985A

22/44

DocID023128 Rev 5

6.4.1

Type III compensation network

The methodology to stabilize the loop consists in placing two zeroes to compensate the

effect of the LC double pole, thereby increasing phase margin; then to place one pole in the

origin to minimize the DC error on the regulated output voltage; finally to place other poles

far from the zero dB frequency.

If the equivalent series resistance (ESR) of the output capacitor introduces a zero with

a frequency higher than the desired bandwidth (that is: 2

ESR COUT < 1 / BW), the Type

III compensation network is needed. Multi-layer ceramic capacitors (MLCC) have very low

ESR (< 1 m

), with very high frequency zero, so a Type III network is adopted to

compensate the loop.

In Figure 10, the Type III compensation network is shown. This network introduces two

zeroes (fZ1, fZ2) and three poles (fP0, fP1, fP2). They are expressed as:

Equation 23

Equation 24

Figure 10. Type III compensation network

fZ1

 C

R1 R3





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

fZ2

 R

4 C4



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



fP0 0

fP1

 R

3 C3



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

fP2

 R

C4 C5



C4 C5

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



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



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