Datasheet

MAX15036/MAX15037
If f
ZESR
is lower than f
C
and close to f
LC
, use a Type II
compensation network where R
F
C
F
provides a midband
zero (f
mid,zero
) and R
F
C
CF
provides a high-frequency pole.
Calculate the modulator gain (G
M
) at the crossover
frequency.
where V
OSC
is the 1V
P-P
ramp amplitude and V
FB
= 0.6V.
The transconductance error amplifier gain at f
C
is:
G
E/A
= g
m
x R
F
The total loop gain at f
C
should be equal to 1:
G
M
= G
E/A
= 1
or
Place a zero at or below the LC double pole:
Place a high-frequency pole at f
P
= 0.5 x f
SW
. Therefore
C
CF
is:
Procedure 2 (see Figure 4)
When using a low-ESR ceramic-type capacitor as the
output capacitor, the ESR frequency is much higher
than the targeted unity-gain crossover frequency (f
C
).
In this case, Type III compensation is recommended.
Type III compensation provides a low-frequency pole
(≈DC) and two pole-zero pairs. The locations of the
zero and poles should be such that the phase margin
peaks at f
C
.
The is a good number to get approximate-
ly 60° of phase margin at f
C
. However, it is important to
place the two zeros at or below the double pole to
avoid conditional stability.
First, select the crossover frequency so that:
Calculate the LC double-pole frequency, f
LC
:
where:
with R
F
≥ 10kΩ.
Calculate C
A
for a target unity crossover frequency, f
C
:
Place a second zero, f
Z2
, at 0.2 x f
C
or at f
LC
, whichev-
er is lower.
C
C
(2 0.5 f R C ) -1
CF
F
SW F F
=
×× ××π
Place a second pole (f
1
2RC
at 1/2 the switchin
g
fre
q
uenc
y
.
P2
FCF
=
××π
)
R
1
2C
R
1
A
A
=
××
−
π f
Z2
R
A
=
××
1
2π fC
ZESR A
Place a pole f at f
P ZESR
() .
1
=
××
1
2R C
AA
π
C
2fLC V
A
C OUT OSC
=
××× ×
×
π
VR
IN F
C
F
=
×××
1
2 0.75 f R
LC F
π
Placeazerof
1
2RC
at 0.75 f
Z
FF
=
××
×
π
LC
f
1
2LC
LC
OUT
=
××π
f
f
20
C
SW
≤
f
f
f
f
5
C
Z
P
C
==
C
1
R
CF
=
××π
FSW
f
C
1
2R
F
F
=
××π f
LC
R
V (ESR 2 f L)V
V V g ESR
F
OSC C OUT
IN m
=
+××
×××
π
FB
G
V
IN
V
OSC
ESR
ESR 2 f
C
L)
V
FB
V
OUT
M
=×
+××
×
( π
2.2MHz, 3A Buck or Boost Converters
with an Integrated High-Side Switch
18 ______________________________________________________________________________________