Datasheet
MAX8543/MAX8544
Step-Down Controllers with Prebias Startup,
Lossless Sensing, Synchronization, and OVP
24 ______________________________________________________________________________________
The crossover frequency, f
C
, should be much higher
than the power-modulator pole f
PMOD
. Also, f
C
should
be less than or equal to 1/5th the switching frequency.
Select a value for f
C
in the range:
At the crossover frequency, the total loop gain must
equal 1, and is expressed as:
For the case where f
zMOD
is greater than f
C
:
then R
C
can be calculated as:
where g
mEA
= 110µS.
The error-amplifier compensation zero formed by R
C
and C
C
should be set at the modulator pole f
PMOD
. C
C
is calculated by:
If f
zMOD
is less than 5 x f
C
, add a second capacitor C
F
from COMP to GND. The value of C
F
is calculated as
follows:
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
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 1
(MAX8544):
A
VCS
= 11 (for ILIM1 = GND)
R
DC
= 2.5mΩ
g
mc
= 1 / (A
VCS
x R
DC
) = 1 / (11 x 0.0025) = 36.7S
V
OUT
= 2.5V
I
OUT(MAX)
= 15A
R
LOAD
= V
OUT
/ I
OUT(MAX)
= 2.5 / 15 = 0.167Ω
C
OUT
= 360µF
ESR = 5mΩ
Gg
RfL
RfL
MOD dc mc
LOAD S
LOAD S
()
()
.
.( ).
.( ).
.
=×
××
+×
=
××××
()
+×××
()
=
−
−
36 36
0 167 600 10 0 8 10
0 167 600 10 0 8 10
450
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
() ( )
=×
C
Rf
F
C zMOD
=
××
1
2π
C
RfLC
RfLR
C
LOAD S OUT
LOAD S C
=
×××
+×
()
×()
R
V
gVG
C
OUT
mEA FB MOD fc
=
××
()
GG
f
f
MOD fc MOD dc
pMOD
C
() ( )
=×
GgR
EA fc mEA C()
=×
GG
V
V
EA fc MOD fc
FB
OUT
() ()
××=1
ff
f
pMOD C
S
<< ≤
5
f
CR
pEA
FC
=
××
1
2π
f
CR
zEA
CC
=
××
1
2π