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MAX15058_11 Datasheet, PDF (16/21 Pages) Maxim Integrated Products – High-Efficiency, 3A, Current-Mode Synchronous, Step-Down Switching Regulator | |||
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High-Efficiency, 3A, Current-Mode
Synchronous, Step-Down Switching Regulator
1ST ASYMPTOTE
R2 Ã (R1 + R2)-1 Ã 10AVEA(dB)/20 Ã gMC Ã RLOAD Ã {1 + RLOAD Ã [KS Ã (1 - D) - 0.5] Ã (L Ã fSW)-1}-1
2ND ASYMPTOTE
GAIN
R2 Ã (R1 + R2)-1 Ã gMV Ã (2GCC)-1 Ã gMC Ã RLOAD Ã {1 + RLOAD Ã [KS Ã (1 - D) - 0.5] Ã (L Ã fSW)-1}-1
3RD ASYMPTOTE
R2 Ã (R1 + R2)-1 Ã gMV Ã (2GCC)-1 Ã gMC Ã RLOAD Ã {1 + RLOAD Ã [KS Ã (1 - D) - 0.5] Ã (L Ã fSW)-1}-1 Ã
(2GCOUT Ã {RLOAD-1 + [KS Ã (1 - D) - 0.5] Ã (L Ã fSW)-1}-1)-1
4TH ASYMPTOTE
R2 Ã (R1 + R2)-1 Ã gMV Ã RC Ã gMC Ã RLOAD Ã {1 + RLOAD Ã [KS Ã (1 - D) - 0.5] Ã (L Ã fSW)-1}-1 Ã
(2GCOUT Ã {RLOAD-1 + [KS Ã (1 - D) - 0.5] Ã (L Ã fSW)-1}-1)-1
UNITY
1ST POLE
[2GCC Ã (10AVEA(dB)/20 - gMV-1)]-1
3RD POLE (DBL) 2ND ZERO
0.5 Ã fSW (2GCOUTESR)-1
fCO
FREQUENCY
2ND POLE
fPMOD*
1ST ZERO
(2GCCRC)-1
5TH ASYMPTOTE
R2 Ã (R1 + R2)-1 Ã gMV Ã RC Ã gMC Ã RLOAD Ã {1 + RLOAD Ã [KS Ã (1 - D) - 0.5] Ã (L Ã fSW)-1}-1 Ã
(2GCOUT Ã {RLOAD-1 + [KS Ã (1 - D) - 0.5] Ã (L Ã fSW)-1}-1)-1 Ã (0.5 Ã fSW)2 Ã (2Gf)-2
NOTE:
ROUT = 10AVEA(dB)/20 Ã gMV-1
fPMOD = [2GCOUT Ã (ESR + {RLOAD-1 + [KS Ã (1 - D) - 0.5] Ã (L Ã fSW)-1}-1)]-1
WHICH FOR
ESR << {RLOAD-1 + [KS Ã (1 - D) - 0.5] Ã (L Ã fSW)-1}-1
BECOMES
fPMOD = [2GCOUT Ã {RLOAD-1 + [KS Ã (1 - D) - 0.5] Ã (L Ã fSW)-1}-1]-1
fPMOD = (2GCOUT Ã RLOAD)-1 + [KS Ã (1 - D) - 0.5] Ã (2GCOUT Ã L Ã fSW)-1
6TH ASYMPTOTE
R2 Ã (R1 + R2)-1 Ã gMV Ã RC Ã gMC Ã RLOAD Ã {1 + RLOAD Ã [KS Ã (1 - D) - 0.5] Ã (L Ã fSW)-1}-1 Ã
ESR Ã {RLOAD-1 + [KS Ã (1 - D) - 0.5] Ã (L Ã fSW)-1}-1 Ã (0.5 Ã fSW)2 Ã (2Gf)-2
Figure 3. Asymptotic Loop Response of Current-Mode Regulator
As previously mentioned, the power modulatorâs dominant
pole is a function of the parallel effects of the load resis-
tance and the current-loop gainâs equivalent impedance:
fPMOD
=
2Ï
à COUT
Ã

ESR

+
1

ï£
1
RLOAD
+
K S
à (1â D) â
fSW Ã L
0.5


â1



And knowing that the ESR is typically much smaller than
the parallel combination of the load and the current loop:

ESR << 
1
+
K S
Ã
(1â
D)
â
0.5


â1
ï£¬ï£ RLOAD
fSW Ã L

fPMOD
â
2Ï
à COUT
Ã

ï£
1
RLOAD
1
+
K S
à (1â D) â
fSW Ã L
0.5


â1
which can be expressed as:
fPMOD
â
2Ï
Ã
1
C OUT
à RLOAD
+
K S à (1â D) â 0.5
2Ï Ã fSW Ã L Ã COUT
Note: Depending on the applicationâs specifics, the
amplitude of the slope compensation ramp could have
a significant impact on the modulatorâs 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.
fZMOD
=
fZESR
=
2Ï
1
à COUT
à ESR
16ââ
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