Datasheet

MAX8655
Highly Integrated, 25A, Wide-Input,
Internal MOSFET, Step-Down Regulator
20 ______________________________________________________________________________________
obtained phase margin. A phase margin of at least 45°
is recommended:
where the sampling effect quality factor:
,
Below is a numerical example to calculate R
C
and C
C
values of the typical operating circuit of Figure 3:
A
VCS
= 12
L = 0.56µH
R
L
= 1.8mΩ
f
S
= 600kHz
g
mc
= 1/(A
VCS
x R
L
) = 1/(12 x 0.0018) = 46.29S
V
OUT
= 1.2V
I
OUT(MAX)
= 20A
R
LOAD
= V
OUT
/I
OUT(MAX)
= 1.2/20 = 0.06Ω
C
OUT
= 4 x 100µF = 400µF
ESR = 2mΩ/4 = 0.5mΩ
D = V
OUT
/V
IN
= 1.2/12 = 0.1:
8.18kHz << f
C
≤ 120kHz, select f
C
= 60kHz.
Since f
zMOD
> f
C
:
R
C
= 44.7kΩ
Select the nearest standard value: R
C
= 40.2kΩ:
Select the nearest standard value: C
C
= 470pF:
R7 = R
C
= 40.2kΩ
C12 = C
C
= 470pF
C11 = C
F
= 5pF (not used)
C
Rf
x
pF
F
C zMOD
=
××
=
×××
=
1
2
1
2 40 2 10 884 2 10
5
33
π
π (. )( . )
C
fR
x
pF
C
pMOD C
=
××
=
××
=
1
2
1
2 8181 40 2 10
483 9
3
π
π (. )
.
R
V
Vg G
x
C
OUT
FB mEA MOD fc
=×
×
=×
−
1
12
07
1
110 10 0 307
6
()
.
.
()(.)
GG
f
f
MOD fc MOD dc
pMOD
c
() ( )
..=×=×=253
8118
60000
0 345
f
C ESR
kHz
zMOD
OUT
=
×× ×
=
×× × ×
=
−
1
209
1
2 0 9 400 10 0 0005
884 2
6
π
π
.
.( ).
.
ff
f
pMOD C
S
<< ≤
5
f
RC
Lf C
KD
x
xx
pMOD
LOAD OUT
S OUT
S
=
×××
+
×× × ×
××−−
[]
⎡
⎣
⎢
⎤
⎦
⎥
=
×
+
×
−−
()
⎡
⎣
⎢
−
−−
1
209
1
208
105
1
2 400 10 0 06 0 9
1
2 0 56 10 600000 400 10 0 8
1181 01 05
6
66
π
π
π
π
.
.
().
()(.).
( . )( )( ) .
.( .) .
⎢⎢
⎤
⎦
⎥
⎥
= 818.kHz
Gg
R
R
Lfs
KD
x
MOD dc mc
LOAD
LOAD
S
()
.
.
.
.
( . )( )
.( .) .
.
=×
+
×
××−
()
()
−
[]
⎡
⎣
⎢
⎤
⎦
⎥
=
×
+×−−
[]
=
−
1105
46 29
006
1
006
0 56 10 600000
1181 01 05
253
6
K
VLf
VVR
x
S
SCOMP S
IN O L
=+
××
×−×
=+
−
=
−
1
120
1
1 25 0 56 10 600000
120 12 1 2 0 0018
118
6
()
.(. )( )
(.)(.)
.
Q
KD
C
S
=
−−
[]
1
105π.( .( ) . )
Gs
s
Qf
s
f
S
cS
S
()
..
.
=
++
()
⎛
⎝
⎜
⎜
⎞
⎠
⎟
⎟
1
1
2
2
π
π
Gs
gR
R
Lf
KD
LOOP
mc LOAD
LOAD
S
S
()
().
=
×
+
×
××−
()
−11055
12
12
⎡
⎣
⎤
⎦
⎡
⎣
⎢
⎤
⎦
⎥
×
+×
()
()
+×
()
sf
sf
zMOD
pMOD
/
/
π
π
(()
×
+×
()
()
+×
()
()
×+ ×
12
12 12
sf
sf sf
zEA
pEA
/
//
π
ππ
ppdEA
mEA FB
OUT
S
gRoV
V
Gs
()
()
×
××
()