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LTC3111

3111fa

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APPLICATIONS INFORMATION

too high in frequency. The third pole at frequency fPOLE3

provides attenuation of high frequency switching noise.

The transfer function of the compensated Type III error

amplifier from the input of the resistor divider to the output

of the error amplifier, COMP, is:

VCOMP

= GCOMP •

2•

π • fZERO1









• 1

2•

π • fZERO2









s• 1

2•

π • fPOLE2









• 1

2•

π • fPOLE3









The compensation gain is given by the following equation.

The simpler approximate value is sufficiently accurate in

most cases since CFB is typically much larger in value

than CPOLE.

GCOMP ≅

R1• CFB +CPOLE

(

)

≅

R1•CFB

ThepoleandzerofrequenciesoftheTypeIIIcompensation

network can be calculated from the following equations

where all frequencies are in Hz, resistances are in ohms,

and capacitances are in farads.

fZERO1=

2•

π •RFB •CFB

fZERO2 =

2•

π R1+RFF

(

)•CFF

≅

2•

π •R1•CFF

fPOLE2 =

2•

π •

CFB •CPOLE

CFB +CPOLE

•RFB

≅

2•

π •RFB •CPOLE

fPOLE3 =

2•

π •RFF •CFF

Inmostapplicationsthecompensationnetworkisdesigned

so that the loop crossover frequency is above the resonant

frequency of the power stage, but sufficiently below the

boostmoderight-half-planezerotominimizetheadditional

phaseloss.Oncethecrossoverfrequencyisdecidedupon,

the phase boost provided by the compensation network

is centered at that point in order to maximize the phase

margin. A larger separation in frequency between the

zeros and higher order poles will provide a higher peak

phase boost but may also increase the gain of the error

amplifier which can push out the loop crossover to a

higher frequency.

The Q of the power stage can have a significant influence

on the design of the compensation network because it

determineshowrapidlythe180°ofphaselossinthepower

stage occurs. For very low values of series resistance, RS,

the Q will be higher and the phase loss will occur sharply.

In such cases, the phase of the power stage will fall rapidly

to –180° above the resonant frequency and the total phase

margin must be provided by the compensation network.

fZERO1

PHASE

GAIN

–20dB/DEC

fZERO2

3111 F08

fPOLE2 fPOLE3

Figure 8: Type III Compensation Bode Plot

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