Datasheet

18 Maxim Integrated
High-Efficiency, 4A, Step-Down DC-DC
Regulators with Internal Power Switches
MAX15066/MAX15166
Figure 4. Asymptotic Loop Response of Peak Current-Mode Regulator
The dominant poles and zeros of the transfer loop gain
are shown below:
( )
( )
MV
P1
AVEA dB /20
C
P2
1
S
OUT
LOAD SW
SW
P3
Z1 Z2
C C OUT
g
f
2 C 10
1
f
K 1 D 0.5
1
2C
R fL
f
f
2
11
ff
2 C R 2 C ESR
−
<<
π× ×
=


×− −

π× +


×

=
= =
π× π×
The order of pole-zero occurrences is:
P1 P2 Z1 CO P3 Z2
ff ff f f<≤< <<
Note: Under heavy load, f
P2
can approach f
Z1
.
Figure 4 shows a graphical representation of the asymp-
totic system closed-loop response, including dominant
pole and zero locations.
The loop response’s fourth asymptote (in bold, Figure 4)
is the one of interest in establishing the desired cross-
over frequency (and determining the compensation
component values). A lower crossover frequency pro-
vides for stable closed-loop operation at the expense of
a slower load and line transient response. Increasing the
crossover 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.
First, select the passive power components that meet
the application’s requirements. Then, choose the small-
signal compensation components to achieve the desired
closed-loop frequency response and phase margin
as outlined in the Closing the Loop: Designing the
Compensation Circuitry section.
UNITY
GAIN
1ST ASYMPTOTE
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
MV
x (2GC
C
)
-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
3RD ASYMPTOTE
R2 x (R1 + R2)
-1
x g
MV
x (2
G
C
C
)
-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 (2
GC
OUT
x {R
LOAD
-1
+ [K
S
(1 – D) – 0.5] x (L x f
SW
)
-1
}
-1
)
-1
5TH ASYMPTOTE
R2 x (R1 + R2)
-1
x g
MV
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 [(2
GC
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:
R
OUT
= 10
AVEA(dB)/20
x g
MV
-1
WHICH FOR
ESR << {R
LOAD
-1
+ [K
S
(1 – D) – 0.5] x (L x f
SW
)
-1
}
-1
BECOMES
f
PMOD
= [2GC
OUT
x {R
LOAD
-1
+ [K
S
(1 – D) – 0.5] x (L x f
SW
)
-1
}
-1
]
-1
f
PMOD
= (2GC
OUT
x R
LOAD
)
-1
+ [K
S
(1 – D) – 0.5] x (2GC
OUT
x L x f
SW
)
-1
f
PMOD
= [2GC
OUT
x (ESR + {R
LOAD
-1
+ [K
S
(1 – D) – 0.5] x (L x f
SW
)
-1
}
-1
)]
-1
6TH ASYMPTOTE
R2 x (R1 + R2)
-1
x g
MV
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
4TH ASYMPTOTE
R2 x (R1 + R2)
-1
x g
MV
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 (2
GC
OUT
x {R
LOAD
-1
+ [K
S
(1 – D) – 0.5] x (L x f
SW
)
-1
}
-1
)
-1
1ST POLE
[2
GC
C
(10
AVEA(dB)/20
x g
MV
-1
)]
-1
2ND POLE
f
PMOD
*
1ST ZERO
(2
GC
C
R
C
)
-1
FREQUENCY
f
CO
3RD POLE
0.5 x f
SW
2ND ZERO
(2
GC
OUT
ESR)
-1