GCOMPIIIs =− ZF(s)

RU//ZS(s)=−1⋅ 1+ s2π⋅fZ1 ⋅ 1+ s2π⋅fZ2

s2π⋅fP0⋅1+ s2π⋅fP1 ⋅1+ s2π⋅fP2

(24)

In addition to what shown in Eq. (21), two more singularities are proper of this compensation network:

fZ2= 1

2π⋅CS(RU+RS);fP2= 12π⋅CSRS

(25)

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

and RS<<RU

•

Starting from Eq. (19), in case of type III compensation network and MLCC ceramic output capacitors, the

control loop gain module at s=2π*FBW allows the RF/RU ratio to be fixed:

GLOOP,III(s=2π⋅fBW) ≅ 1kFF⋅fLCfBW⋅RFRU=1

(26)

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 a parasitic capacitance of the board, the RF required value

is computed by Eq. (26).

•

Select CF in order to place FZ1 below FLC (typically 0.1*FLC)

•

Select CP in order to place FP1 at 0.5*FSW

CF= 1

2π⋅RF⋅0.1⋅fLC;CP= 1

2π⋅RF⋅0.5⋅fSW

(27)

•

Select CS in order to place FZ2 at FLC

•

Select RS in order to place FP2 at 0.5*FSW

CS= 1

2π⋅RU⋅fLC;RS= 1

2π⋅CS⋅0.5⋅fSW

(28)

The resultant control loop and other transfer functions gain are shown in Figure 14. Type III compensation - Bode

plot

A7987

Compensation network

DS12928 - Rev 3

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