MAX15053_11 datasheet(16/21 Pages) MAXIM

16 / 21 page

High-Efficiency, 2A, Current-Mode

Synchronous, Step-Down Switching Regulator

16 _____________________________________________________________________________________

As previously mentioned, the power modulator’s domi-

nant pole is a function of the parallel effects of the load

resistance and the current-loop gain’s equivalent imped-

ance:

(

)

PMOD

OUT

LOAD

1 D

0.5

ESR

−













× −

−









π ×

























And knowing that the ESR is typically much smaller than

the parallel combination of the load and the current loop:

(

)

LOAD

1 D

0.5

ESR

−









× −

−

















(

)

PMOD

OUT

LOAD

1 D

0.5

−

≈









× −

−





π ×













which can be expressed as:

(

)

PMOD

OUT

LOAD

OUT

1 D

0.5

L C





× −

−





≈

π ×

× ×

Note: Depending on the application’s specifics, the

amplitude of the slope compensation ramp could have

a significant impact on the modulator’s dominate pole.

For low duty-cycle applications, it provides additional

damping (phase lag) at/near the crossover frequency

(see the Closing the Loop: Designing the Compensation

Circuitry section). There is no equivalent effect on the

power modulator zero, fZMOD.

ZMOD

ZESR

OUT

ESR

π ×

GAIN

1ST ASYMPTOTE

R2 × (R1 + R2)-1 × 10AVEA(dB)/20 × gMC × RLOAD × {1 + RLOAD × [KS × (1 - D) - 0.5] × (L × fSW)-1}-1

2ND ASYMPTOTE

R2 × (R1 + R2)-1 × gMV × (2GCC)-1 × gMC × RLOAD × {1 + RLOAD × [KS × (1 - D) - 0.5] × (L × fSW)-1}-1

3RD ASYMPTOTE

R2 × (R1 + R2)-1 × gMV × (2GCC)-1 × gMC × RLOAD × {1 + RLOAD × [KS × (1 - D) - 0.5] × (L × fSW)-1}-1 ×

(2GCOUT × {RLOAD-1 + [KS × (1 - D) - 0.5] × (L × fSW)-1}-1)-1

4TH ASYMPTOTE

R2 × (R1 + R2)-1 × gMV × RC × gMC × RLOAD × {1 + RLOAD × [KS × (1 - D) - 0.5] × (L × fSW)-1}-1 ×

(2πCOUT × {RLOAD-1 + [KS × (1 - D) - 0.5] × (L × fSW)-1}-1)-1

5TH ASYMPTOTE

R2 × (R1 + R2)-1 × gMV × RC × gMC × RLOAD × {1 + RLOAD × [KS × (1 - D) - 0.5] × (L × fSW)-1}-1 ×

(2GCOUT × {RLOAD-1 + [KS × (1 - D) - 0.5] × (L × fSW)-1}-1)-1 × (0.5 × fSW)2 × (2Gf)-2

6TH ASYMPTOTE

R2 × (R1 + R2)-1 × gMV × RC × gMC × RLOAD × {1 + RLOAD × [KS × (1 - D) - 0.5] × (L × fSW)-1}-1 ×

ESR × {RLOAD-1 + [KS × (1 - D) - 0.5] × (L × fSW)-1}-1 × (0.5 × fSW)2 × (2Gf)-2

UNITY

1ST POLE

[2GCC × (10AVEA(dB)/20 - gMV-1)]-1

2ND POLE

fPMOD*

3RD POLE (DBL)

0.5 × fSW

2ND ZERO

(2GCOUTESR)-1

FREQUENCY

fCO

1ST ZERO

(2GCCRC)-1

NOTE:

ROUT = 10AVEA(dB)/20 × gMV-1

fPMOD = [2GCOUT × (ESR + {RLOAD-1 + [KS × (1 - D) - 0.5] × (L × fSW)-1}-1)]-1

WHICH FOR

ESR << {RLOAD-1 + [KS × (1 - D) - 0.5] × (L × fSW)-1}-1

BECOMES

fPMOD = [2GCOUT × {RLOAD-1 + [KS × (1 - D) - 0.5] × (L × fSW)-1}-1]-1

fPMOD = (2GCOUT × RLOAD)-1 + [KS × (1 - D) - 0.5] × (2GCOUT × L × fSW)-1

Figure 3. Asymptotic Loop Response of Current-Mode Regulator

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