MAX15041 Maxim Integrated Products, MAX15041 Datasheet - Page 13

• Manufacturer Part Number: MAX15041
• Description: Step-Down DC-DC Regulator
• Manufacturer: Maxim Integrated Products

Figure 2. Asymptotic Loop Response of Peak Current-Mode Regulator
If C
resistance (large ESR), the circuit’s second zero may
come into play around the crossover frequency (f
ω
second (optional) small compensation capacitor (C
connected from COMP to SGND.
The loop response’s fourth asymptote (in bold, Figure
2) is the one of interest in establishing the desired
crossover frequency (and determining the compensa-
tion component values). A lower crossover frequency
provides for stable closed-loop operation at the
expense of a slower load and line transient response.
Increasing the crossover frequency improves the tran-
sient response at the (potential) cost of system instabili-
ty. A standard rule of thumb sets the crossover
frequency ≤ 1/10 of the switching frequency (for the
MAX15041, this is approximately 35kHz for the 350kHz
fixed switching frequency).
First, select the passive and active power components
that meet the application’s requirements. Then, choose
the small-signal compensation components to achieve
Step-Down DC-DC Regulator with Internal Switches
CO
OUT
/2π). In this case, a third pole may be induced by a
UNITY
GAIN
is large, or exhibits a lossy equivalent series
Low-Cost, 3A, 4.5V to 28V Input, 350kHz, PWM
gmV x (10
______________________________________________________________________________________
1ST POLE
AVEA[dB]/20
1ST ASYMPTOTE
V
FB
x V
C
OUT
C
)
-1
-1
x 10
AVEA[dB]/20
(C
OUT
2ND ASYMPTOTE
V
FB
x G
(ESR + R
2ND POLE
x V
MOD
OUT
x R
LOAD
-1
LOAD
x g
1ST ZERO
(C
))
MV
C
-1
R
C
x (C
)
-1
CO
CC
C
)
-1
V
FB
x G
=
),
x V
V
3RD ASYMPTOTE
V
MOD
FB
FB
OUT
x V
x V
x R
-1
OUT
OUT
LOAD
x g
the desired closed-loop frequency response and phase
margin as outlined in the Closing the Loop: Designing
the Compensation Circuitry section.
1) Select the desired crossover frequency. Choose f
2) Select R
therefore:
-1
-1
MV
x g
x g
equal to 1/10
gain (assuming f
the overall loop gain to unity) as follows:
x R
MV
MV
CO
R
C
C
4TH ASYMPTOTE
V
x (C
x (C
x G
FB
=
MOD
CC
C
x V
)
-1
1
)
×
V
5TH ASYMPTOTE
-1
OUT
x G
x (ESR || R
V
C
OUT
=
2
x G
FB
MOD
π
-1
V
using the transfer-loop’s fourth asymptote
MOD
V
x g
OUT
×
FB
x R
th
6TH ASYMPTOTE
×
MV
f
x (ESR || R
LOAD
CO
LOAD
of f
2π
x R
(C
×
)
CO
2ND ZERO
x (C
×
C
OUT
SW
Closing the Loop: Designing the
g
×
x G
MV
C
LOAD
ESR)
f
OUT
CO
g
MOD
, or f
OUT
> f
(ESR + R
MV
)
-1
×
x R
×
P1
R
1
CO
×
×
C
LOAD
C
(C
3RD POLE
, f
LOAD
Compensation Circuitry
OUT
G
(
CC
ES
×
P2
MOD
R
≈ 35kHz.
x (C
G
))
C
)
-1
www.DataSheet4U.com
R R R
-1
, and f
MOD
OUT
×
+
(
×
(ESR + R
ESR R
LOAD
R R
×
LOAD
Z1
R
+
LOAD
LOAD
)
and setting
LOAD
RAD/S
))
-1
)
CO
13