MAX15118 Datasheet, PDF (19/23 Pages) Maxim Integrated Products – High-Efficiency, 18A, Current-Mode Synchronous Step-Down Regulator with Integrated Switches

MAX15118
High-Efficiency, 18A, Current-Mode Synchronous
Step-Down Regulator with Integrated Switches
Figure 3 shows a graphical representation of the asymp-
totic system closed-loop response, including the domi-
nant pole and zero locations.
The loop response’s fourth asymptote (in bold, Figure 3)
is the one of interest in establishing the desired crossover
frequency (and determining the compensation compo-
nent values). A lower crossover frequency provides for
stable closed-loop operation at the expense of a slower
load and line-transient response. Increasing the cross-
over frequency improves the transient response at the
(potential) cost of system instability. A standard rule of
thumb sets the crossover frequency P 1/5 to 1/10 of the
switching frequency.
Closing the Loop: Designing
the Compensation Circuitry
1) Select the desired crossover frequency. Choose f_CO
equal to 1/10th of f_SW, or f_CO @ 100kHz.
2) Select R_C using the transfer-loop’s fourth asymptote
gain equal to unity (assuming f_CO > f_P1, f_P2, and f_Z1).
R_C becomes:
R_C
=
R1+ R2
R2
×

¹⁺

R_LOAD
× K _S[(1-
L × f_SW
D)
-
0.5]


g_M × g_MC × R_LOAD


×
2π ×
f_CO
×
C_OUT
×

_{ESR +}


1
R_LOAD
+
1


K _S[(1- D) - 0.5]_
L × f_SW ^
where KS is calculated as:
K
S
=
1+
V_SLOPE ×
V_IN
f_SW × L
- V_OUT
×
g_MC
and g_M = 1.2mS, g_MC = 150A/V, and V_SLOPE = 130mV.
1ST ASYMPTOTE
dB
R2 x (R1 + R2)-1 x 10^AVEA(dB)/20 x g_MC x R_LOAD x {1 + R_LOAD x [K_S x (1 – D) – 0.5] x (L x f_SW)-1}-1
2ND ASYMPTOTE
R2 x (R1 + R2)-1 x g_M x (2GCC)-1 x g_MC x R_LOAD x {1 + R_LOAD x [K_S x (1 – D) – 0.5] x (L x f_SW)-1}-1
GAIN
3RD ASYMPTOTE
R2 x (R1 + R2)-1 x g_M x (2GCC)-1 x g_MC x R_LOAD x {1 + R_LOAD x [K_S x (1 – D) – 0.5] x (L x f_SW)-1}-1
x (2GC_OUT x {R_LOAD-1 + [K_S(1 – D) – 0.5] x (L x f_SW)-1}-1)-1
4TH ASYMPTOTE
R2 x (R1 + R2)-1 x g_M x R_C x g_MC x R_LOAD x {1 + R_LOAD x [K_S x (1 – D) – 0.5] x (L x f_SW)-1}-1
x (2GC_OUT x {R_LOAD-1 + [K_S(1 – D) – 0.5] x (L x f_SW)-1}-1)-1
UNITY
1ST POLE
[2GC_C(10^AVEA(dB)/20
x g_M^-1)]^-1
1ST ZERO
(2GC_CR_C)^-1 _fCO
3RD POLE 2ND ZERO
0.5 x fSW (2GC_OUTESR)-1
FREQUENCY
2ND POLE
fPMOD*
5TH ASYMPTOTE
R2 x (R1 + R2)-1 x g_M x R_C x g_MC x R_LOAD x {1 + R_LOAD x [K_S x (1 – D) – 0.5] x (L x f_SW)-1}-1
x [(2GC_OUT x {R_LOAD-1 + [K_S(1 – D) – 0.5] x (L x f_SW)-1}-1)-1 x (0.5 x f_SW)2 x (2Gf)-2
NOTE:
ROUT = 10^AVEA(dB)/20 x g_M^-1
*fPMOD = [2GCOUT x (ESR + {RLOAD^-1 + [KS(1 – D) – 0.5] x (L x fSW)^-1}^-1)]^-1
WHICH FOR
ESR << {RLOAD^-1 + [KS(1 – D) – 0.5] x (L x fSW)^-1}^-1
BECOMES
fPMOD = [2GCOUT x {RLOAD^-1 + [KS(1 – D) – 0.5] x (L x fSW)^-1}^-1]^-1
fPMOD = (2GCOUT x RLOAD)^-1 + [KS(1 – D) – 0.5] x (2GCOUT x L x fSW)^-1
6TH ASYMPTOTE
R2 x (R1 + R2)-1 x g_M x R_C x g_MC x R_LOAD x {1 + R_LOAD x [K_S x (1 – D) – 0.5] x (L x f_SW)-1}-1
x ESR x {R_LOAD-1 + [K_S(1 – D) – 0.5] x (L x f_SW)-1}-1 x (0.5·f_SW)2 x (2Gf)-2
Figure 3. Asymptotic Loop Response of Peak Current-Mode Regulator
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