MAX15021 datasheet(17/24 Pages) MAXIM

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Type II: Compensation when fCO > fZERO,ESR

When the fCO is greater than fESR, a Type II compensa-

tion network provides the necessary closed-loop com-

pensated response. The Type II compensation network

provides a midband compensating zero and a high-fre-

quency pole (see Figures 5a and 5b).

RFCF provides the midband zero fMID,ZERO, and

RFCCF provides the high-frequency pole, fHIGH,POLE.

Use the following procedure to calculate the compen-

sation network components.

Calculate the fESR and LC double pole, fLC:

where COUT is the regulator output capacitor and ESR

is the series resistance of COUT. See the Output-

Capacitor Selection section for more information on cal-

culating COUT and ESR.

Set the compensator’s leading zero, fZ1, at or below the

filter’s resonant double-pole frequency from:

Set the compensator’s high-frequency pole, fP1, at or

below one-half the switching frequency, fSW:

To maximize the compensator’s phase lead, set the

desired crossover frequency, fCO, equal to the geomet-

ric mean of the compensator’s leading zero, fZ1, and

high-frequency pole, fP1, as follows:

Select the feedback resistor, RF, in the range of 3.3k

to 30k

Ω.

Calculate the gain of the modulator (GainMOD)—com-

prised of the regulator’s pulse-width modulator, LC filter,

feedback divider, and associated circuitry—at the desired

crossover frequency, fCO, using the following equation:

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.

The gain of the error amplifier (GainE/A) in the midband

frequencies is:

The total loop gain is the product of the modulator gain

and the error amplifier gain at fCO and should be set

equal to 1 as follows:

GainMOD x GainE/A = 1

So:

20 log

0dB

4 ESR x V

L x V

4 ESR x V

L x V

OUT_

×+

××

⎡

⎣

⎢

⎤

⎦

⎥

××

⎡

⎣

⎢

⎤

⎦

⎥

Gain

R [k ]

E/A

Gain

4(V/V)

ESR [m ]

[kHz]

L[ H]

[V]

MOD

OUT_

=×

××

()

πμ

=×

≤

ESR C

2L C

ESR

OUT

××

≈

××

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

Regulator with Tracking/Sequencing Capability

MAX15021

Maxim Integrated

VREF

FB_

COMP_

VOUT_

CCF

Figure 5a. Type II Compensation Network

GAIN

(dB)

1ST ASYMPTOTE

(

ωR1CF)-1

2ND ASYMPTOTE

(RFR1)-1

3RD ASYMPTOTE

(

ωRFCCF)-1

ω(rad/sec)

1ST POLE

(AT ORIGIN)

2ND POLE

(RFCCF)-1

1ST ZERO

(RFCF)-1

Figure 5b. Type II Compensation Network Response

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