MAX8686 Maxim Integrated Products, MAX8686 Datasheet - Page 21

• Manufacturer Part Number: MAX8686
• Description: DC-DC Converter Delivers Up To 25A Per Phase
• Manufacturer: Maxim Integrated Products

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be less than or equal to 1/5 the switching frequency.
Select a value for f
The feedback voltage-divider gain (V
be included for an output voltage higher than 3.3V,
where V
At the crossover frequency, the total loop gain must
equal 1, and is expressed as:
For the case where f
Then R
where g
The error-amplifier compensation zero formed by R
and C
Calculate the value of C
If f
from COMP to GND. The value of C
As the load current decreases, the modulator pole also
decreases; however, the modulator gain increases
accordingly and the crossover frequency remains the
same.
For the case where f
The power modulator gain at f
DC-DC Converter Delivers Up to 25A Per Phase
PMOD
C
C
REFIN
mEA
should be set at the modulator pole f
is less than 5 x f
can be calculated as:
G
G
G
= 1.7mS.
R
is equal to 3.3V.
MOD fc
MOD fc
EA fc
C
C
C
( )
=
G
C
F
______________________________________________________________________________________
C
( )
( )
g
EA fc
f
pMOD
mEA
×
=
zMOD
=
zMOD
in the range:
( )
G
2π
=
2π
=
MOD fc
C
G
G
×
=
×
×
C
MOD dc
MOD dc
<<
as follows:
V
is greater than f
is less than f
R
, add a second capacitor C
g
f
REFIN
pMOD
( )
mEA
C
V
f
C
1
OUT
1
C
( )
( )
×
×
≤
f
is:
zMOD
×
V
×
f
V
×
SW
×
×
REFIN
R
5
OUT
G
R
F
C
f
f
f
pMOD
pMOD
zMOD
MOD fc
C
is:
REF
f
C
C
= 1
:
( )
/V
C
Single/Multiphase, Step-Down,
OUT
:
) should
PMOD
C
F
.
The error-amplifier gain at f
R
where g
C
C
The current-mode control model on which the above
design procedure is based requires an additional high-
frequency term, G
pling the peak inductor current. The term G
additional phase lag at crossover and should be modeled
to estimate the phase margin obtainable by the selected
compensation components. As a final step, it is useful to
plot the dB gain and phase of the following loop-gain
transfer function and check the obtained phase margin. A
phase margin of at least 45° is recommended:
where the sampling effect quality factor is:
G
C
C
F
LOOP
is calculated from:
is calculated as:
is calculated from:
( )
s
mEA
R
=
(
1
C
gmc
+
G s
s
S
=
/
= 1.7mS.
× +
G
( ) =
2
V
⎡
⎢
⎣
π
1
EA fc
V
OUT
×
Q
( (
FB
1
C
C
f
R
L f
+
pEA
( )
C
LOAD
⎛
⎜
⎜
⎝
×
C
F
S
s
1
(s), to account for the effect of sam-
/
=
SW
×
) (
+
=
2
=
× +
=
R
π
[
g
π
LOAD
1
π (
2π
2π
g
×
×
mEA
×
⎡ ⎣
mEA
f
×
zEA
(
s
Ks
Q
×
×
/
K
2
c
R
)
C
× −
s
f
π
S
pMOD
×
(
C
×
×
1
×
× −
is:
G
1
1
f
R
f
pdEA
×
SW
(
D
MOD fc
1
1
C
1
)
f
zMOD
)
f
−
×
C
×
)
D
+
0. .
×
( )
)
R
f
5
(
zMOD
g g
−
π
C
⎤ ⎦
mEA
⎤
⎥
⎦
f
C
0 5
×
×
×
. )
s
(
(
f
1
1
SW
f
×
2
zMOD
+
+
V
Ro V
]
OUT
s
s
S
/
/
(s) produces
)
×
2
2
2
π
π
⎞
⎟
⎟
⎠
REFIN
×
×
f
f
pMOD
zMOD
G s
S
)
)
( )
×
21