Rainbow Electronics MAX5099 - Manual (Page 22)

MAX5099
Dual, 2.2MHz, Automotive Synchronous Buck
Converter with 80V Load-Dump Protection
22 ______________________________________________________________________________________
Procedure 2 (See Figure 4)
If the output capacitor used is a low-ESR ceramic type,
the ESR frequency is usually far away from the targeted
unity crossover frequency (f
C ). In this case, Type III
compensation is recommended. Type III compensation
provides two-pole zero pairs. The locations of the zero
and poles should be such that the phase margin peaks
around f
C . It is also important to place the two zeros at
or below the double pole to avoid the conditional stabil-
ity issue.
1) Select a crossover frequency:
2) Calculate the LC double-pole frequency, f LC :
3) Place a zero
where
and R F ≥ 10kΩ.
4) Calculate C I for a target unity crossover frequency, f C .
5) Place a pole
6) Place a second zero, f Z2 , at 0.2 x f C or at f LC ,
whichever is lower.
7) Place a second pole at 1/2 the switching frequency.
Boost Converter Compensation
The boost converter compensation gets complicated
due to the presence of a right-half-plane zero
f
ZERO,RHP
. The right-half-plane zero causes a drop in
phase while adding positive (+1) slope to the gain
curve. It is important to drop the gain significantly below
unity before the RHP frequency. Use the following pro-
cedure to calculate the compensation components:
1) Calculate the LC double-pole frequency, f LC , and
the right-half-plane-zero frequency.
where
Target the unity-gain crossover frequency for:
f
f
C
ZERO RHP ≤ ,
5
D
V
V
R
V
I
IN
OUT
MIN
OUT
OUT MAX
=
=
−
( )
1
( )
f
D R
L
ZERO RHP
MIN
OUT
,
=
( )
×
− ( ) 1
2
2
π
f
D
L C
LC
OUT OUT
=
Ч Ч
− 1
2
π
C
C
f R C
CF
F
SW F F
=
Ч Ч Ч Ч
( )
− 2 0 5 1
π
.
R
f C
R
Z I
I
1
1
2
2
=
Ч Ч
−
π
R
f C
I
ZERO ESR I
=
Ч Ч
1
2 π
,
f
R C
at f
P
I I
ZERO ESR 1
1
2
=
Ч Ч π
.
,
C
f L C V
V R
I
C OUT OUT OSC
IN F
=
Ч Ч Ч Ч
Ч
2 π
C
f R
F
LC F
=
Ч Ч Ч
1
2 0 75 π .
f
R C
at f
Z
F F
LC 1
1
2
0 75
=
Ч Ч
Ч
π
. .
f
L C
LC
OUT OUT
=
Ч Ч
1
2 π
f
f
SW
SW ≤
20
R1 R F
COMP_
V OUT
V REF
R2
R
I
C I
C F
C CF
-
+
g M
FB_
Figure 4. Type III Compensation Network