MAX15021 datasheet(18/24 Pages) MAXIM

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Solving for R1:

where VFB is the 0.6V (typ) FB_ input-voltage set-point,

L is the value of the regulator inductor, ESR is the

series resistance of the output capacitor, and VOUT_ is

the desired output voltage.

1) CF is determined from the compensator’s leading

zero, fZ1, and RF as follows:

2) CCF is determined from the compensator’s high-fre-

quency pole, fP1, and RF as follows:

3) Calculate R2 using the following equation:

where VFB = 0.6V (typ) and VOUT_ is the output voltage

of the regulator.

Type III: Compensation when fCO < fESR

As indicated above, the position of the output capaci-

tor’s inherent ESR zero is critical in designing an appro-

priate compensation network. When low-ESR ceramic

output capacitors (MLCCs) are used, the ESR zero fre-

quency (fESR) is usually much higher than the desired

crossover frequency (fCO). In this case, a type III com-

pensation network is recommended (see Figure 6a).

As shown in Figure 6b, the Type III compensation net-

work introduces two zeros and three poles into the con-

trol loop. The error amplifier has a low-frequency pole

at the origin, two zeros, and two higher frequency poles

at the following frequencies:

Two midband zeros (fZ1 and fZ2) are designed to com-

pensate for the pair of complex poles introduced by the

LC filter.

fP1 introduces a pole at zero frequency (integrator) for

nulling DC output voltage errors.

fP1= at the origin (0Hz)

Depending on the location of the ESR zero (fESR), fP2

can be used to cancel it, or to provide additional atten-

uation of the high-frequency output ripple.

fP3 attenuates the high-frequency output ripple.

Since CCF << CF then:

FCF

××

C C

FCF

××

()

××

R )

××

R [k] R [k]

[V]

[V] V

[V]

OUT_

ΩΩ

=×

−

CF]

R [k ] f

[kHz]

FP1

[

××

C[ F]

R [k ] f

[kHz]

FZ1

××

R [k ]

R [k ] 4 ESR[m ] V

[V]

[kHz] L[ H] V

[V]

FFB

OUT_

ΩΩ

××

πμ

Dual, 4A/2A, 4MHz, Step-Down DC-DC

Regulator with Tracking/Sequencing Capability

MAX15021

Maxim Integrated

VREF

FB_

COMP_

VOUT_

CCF

Figure 6a. Type III Compensation Network

GAIN

(dB)

1ST ASYMPTOTE

(

ωR1CF)-1

3RD ASYMPTOTE

(

ωRFCI)-1

5TH ASYMPTOTE

(

ωRICCF)-1

ω(rad/sec)

1ST POLE

(AT ORIGIN)

2ND POLE

(RICI)-1

3RD POLE

(RFCCF)-1

1ST ZERO

(RFCF)-1

2ND ASYMPTOTE

( )

4TH ASYMPTOTE

( )

2ND ZERO

(R1CI)-1

Figure 6b. Type III Compensation Network Response

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