MAX15002_12 Datasheet, PDF(23/29 Page) Maxim Integrated Products – Dual-Output Buck Controller with Tracking/Sequencing

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MAX15002_12 Datasheet, PDF (23/29 Pages) Maxim Integrated Products – Dual-Output Buck Controller with Tracking/Sequencing

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MAX15002

Dual-Output Buck Controller with

Tracking/Sequencing

5) The gain of the modulator (GainMOD)âcomprised

of the regulatorâs pulse-width modulator, LC filter,

feedback divider, and associated circuitryâat

crossover frequency is:

GainMOD

(2Ï

fCO)2

Ã COUT

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

quencies is:

GainE/A = 2Ï x fCO x CI x RF

The total loop gain as the product of the modulator gain

and the error-amplifier gain at fCO should be equal to 1,

as follows:

GainMOD Ã GainE A = 1

So :

(2Ï

fCO )2

COUT

ÃL

Ã 2Ï Ã fCO Ã CI Ã RF = 1

Solving for CI :

(2Ï

fCO

ÃLÃ

Ã RF

COUT

)

6) For those situations where fLC < fCO < fESR <

fSW/2âas with low-ESR tantalum capacitorsâthe

compensatorâs second pole (fP2) should be used to

cancel fESR. This provides additional phase margin.

Viewed mathematically on the system Bode plot, the

loop gain plot maintains its +20dB/dec slope up to

1/2 of the switching frequency verses flattening out

soon after the 0dB crossover. Then set:

fP2 = fESR

If a ceramic capacitor is used, the capacitor ESR

zero, fESR, is likely to be located even above one-

half of the switching frequency, that is fLC < fCO <

fSW/2 < fESR. In this case, the frequency of the sec-

ond pole (fP2) should be placed high enough not to

significantly erode the phase margin at the

crossover frequency. For example, it can be set at 5

x fCO, so that its contribution to phase loss at the

crossover frequency fCO is only about 11Â°:

fP2 = 5 x fCO

Once fP2 is known, calculate RI:

2Ï

fP2

7) Place the second zero (fZ2) at 0.2 x fCO or at fLC,

whichever is lower and calculate R1 using the fol-

lowing equation:

2Ï

Ã fZ2

Ã CI

â RI

8) Place the third pole (fP3) at 1/2 the switching fre-

quency and calculate CCF from:

CCF

2Ï

0.5 Ã fSW

Ã RF

9) Calculate R2 as:

VFB

VOUT â VFB

where VFB = 0.6V.

Maxim Integrated

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