Datasheet
MAX8650
4.5V to 28V Input Current-Mode Step-Down
Controller with Adjustable Frequency
______________________________________________________________________________________ 23
For the case where f
zMOD
is less than f
C
:
The power modulator gain at f
C
is:
The error-amplifier gain at f
C
is:
R
C
is calculated as:
where g
mEA
= 110µS.
C
C
is calculated from:
C
F
is calculated from:
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 = 1.2µH
R
DC
= 2.16mΩ
f
S
= 500kHz
g
mc
= 1 / (A
VCS
x R
DC
) = 1 / (12 x 0.00216) = 38.6S
V
OUT
= 3.3V
I
OUT(MAX)
= 15A
R
LOAD
= V
OUT
/ I
OUT(MAX)
= 3.3 / 15 = 0.22Ω
C
OUT
= 300µF
ESR = 3.5mΩ
3.23kHz << f
C
≤ 100kHz, select f
C
= 100kHz:
Since f
zMOD
> f
C
:
Select the nearest standard value: R
C
= 200kΩ:
Select the nearest standard value: C
C
= 270pF:
Since the calculated value for C
F
is very small (close to
the parasitic capacitance present at COMP), it is not
necessary:
R8 = R
C
= 200kΩ
C7 = C
C
= 270pF
C8 = C
F
= Not installed
C
Rf
p
F
F
C zMOD
=
××
=
××××
=
1
2
1
2 200 10 152 10
52
33
π
π ()()
.
C
RfLC
RfLR
p
C
LOAD S OUT
LOAD S C
=
×××
+×
()
×
=
××× ××
+×××
⎛
⎝
⎜
⎞
⎠
⎟
××
=
−−
−
0 22 500 10 1 2 10 300 10
0 22 500 10 1 2 10 200 10
241
36 6
36 3
.( )(. )( )
.( )(. )( )
R
V
gVG
k
C
OUT
mEA FB MOD fc
=
××
=
×
⎛
⎝
⎜
⎞
⎠
⎟
××
=
−
()
.
..
33
110 10 0 7 0 201
199
6
Ω
GG
f
f
MOD fc MOD dc
pMOD
C
() ( )
..=×=×
×
=622
3230
100 10
0 201
3
f
C ESR
kH
z
zMOD
OUT
=
××
=
×× ×
=
−
1
2
1
2 300 10 0 0035
152
6
π
π ().
ff
f
pMOD C
S
<< ≤
5
=
×× ×
××××
⎛
⎝
⎜
⎞
⎠
⎟
+×××
⎛
⎝
⎜
⎞
⎠
⎟
+
⎛
⎝
⎜
⎜
⎜
⎜
⎞
⎠
⎟
⎟
⎟
⎟
=
−
−
−
1
2 300 10
0 22 500 10 1 2 10
0 22 500 10 1 2 10
0 0035
323
6
36
36
π ()
.( ).
.( ).
.
. kHz
f
C
RfL
RfL
ESR
pMOD
OUT
LOAD S
LOAD S
=
××
××
+×
+
⎛
⎝
⎜
⎞
⎠
⎟
1
2π
Gg
RfL
RfL
MOD dc mc
LOAD S
LOAD S
()
.
.( ).
.( ).
.
=×
××
+×
=×
××××
⎛
⎝
⎜
⎞
⎠
⎟
+×××
⎛
⎝
⎜
⎞
⎠
⎟
=
−
−
38 6
0 22 500 10 1 2 10
0 22 500 10 1 2 10
622
36
36
C
Rf
F
C zMOD
=
××
1
2π
C
RfLC
RfLR
C
LOAD S OUT
LOAD S C
=
×××
+×
()
×
R
V
V
f
gG f
C
OUT
FB
C
mEA MOD fc zMOD
=×
××
()
GgR
f
f
EA fc mEA C
zMOD
C
()
=××
GG
f
f
MOD fc MOD dc
pMOD
zMOD
() ( )
=×