L3751 datasheet(24/41 Pages) STMICROELECTRONICS

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GLOOPs =GPWM0∙GLCs ∙GCOMPs

(13)

The implemented compensation networks represented with GCOMP(s) terms depend on the characteristics of the

selected output capacitor, as explained in the following chapters.

7.2

Type II compensation network

If the equivalent series resistance (RES) of the output capacitor introduces a zero with a frequency inside the

desired bandwidth (that is: 2π

∗ RES ∗ CO > 1 / BW), this zero helps stabilize the loop. Electrolytic capacitors

show non-negligible ESR (> 30 mΩ typically), so with this kind of output capacitor the type II network combined

with the zero of the ESR allows the loop to be stabilized.

Figure 24. Type II compensation network

COMP

VREF

V OUT

The type II compensation network transfer function, from VOUT to COMP, is computed in the equation below:

GCOMPIIs =−ZFsRU =−1∙ 1+ s2π∙fZ1

s2π∙fP0∙ 1+ s2π∙fP1HF

(14)

fZ1= 12π∙RF∙CFfP0= 1

2π∙ CF+CP ∙RUfP1HF= 1

2π∙RF∙CF∥CP

(15)

The following suggestions can be followed for a quite common compensation strategy, assuming that CP << CF.

GLOOP(s) in case of type II compensation network and electrolytic output capacitors can be written as follows at

the at s = 2π * fBW to derive the RF / RU ratio as fixed:

GLOOPIIs=2π∙fBW ≅ 1KFF∙ fLC2fZESR∙RFRU∙ 1fBW=1

(16)

After choosing the regulator bandwidth (typically fBW < 0.2 * fSW) and a value for RU, usually between 1 kΩ and

50 kΩ in order to achieve CF and CP not comparable with parasitic capacitance of the board, the RF required

value is calculated as follows from the previous equation:

RF=KFF∙fZESR∙fBW

fLC2 ∙RU

(17)

Select CF in order to place fZ1 below fLC (typically 0.1 * FLC) and CP to place fP1HF at 0.5 * fSW

CF= 1

2π∙RF∙0.1∙fLC CP= 1

2π∙RF∙0.5∙fSW

(18)

L3751

Type II compensation network

DS13841 - Rev 1

page 24/41

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