Datasheet

Solving for CF:
F
F PO
1
C
2 R f 0.75
=
π× × ×
3) Place a high-frequency pole at f
P1
= 0.5 x f
SW
(to
attenuate the ripple at the switching frequency f
SW
) and
calculate C
CF
using the following equation:
CF
F SW
F
1
C
1
Rf -
C
=
π× ×
Type III Compensation Network (Figure 4)
When using a low-ESR tantalum or ceramic type, the
ESR-induced zero frequency is usually above the target-
ed zero crossover frequency (f
O
). Use Type III compensa-
tion. Type III compensation provides two zeros and three
poles at the following frequencies:
Z1
FF
Z2
I 1I
1
f
2R C
1
f
2 C (R R )
=
π× ×
=
π× × +
Two midband zeros (f
Z1
and f
Z2
) cancel the pair of com-
plex poles introduced by the LC filter:
f
P1
= 0
f
P1
introduces a pole at zero frequency (integrator) for
nulling DC output-voltage errors:
P2
II
1
f
2 RC
=
π× ×
Depending on the location of the ESR zero (f
ZO
), use f
P2
to cancel f
ZO
, or to provide additional attenuation of the
high-frequency output ripple:
P3
F CF
F
F CF
1
f
CC
2R
CC
=
×
π× ×
+
f
P3
attenuates the high-frequency output ripple.
Place the zeros and poles such that the phase margin
peaks around f
O
.
Ensure that R
F
>> 2/g
M
and the parallel resistance of
R
1
, R
2
, and R
I
is greater than 1/g
M
. Otherwise, a 180N
phase shift is introduced to the response making the loop
unstable.
Use the following compensation procedures:
1) With R
F
>> 10kΩ, place the first zero (f
Z1
) at 0.8 x f
PO
:
Z1 PO
FF
1
f 0.8 f
2R C
= = ×
π× ×
So:
F
F PO
1
C
2 R 0.8 f
=
π× × ×
2) The gain of the modulator (GAIN
MOD
), comprised of
the pulse-width modulator, LC filter, feedback divider,
and associated circuitry at crossover frequency is:
IN
MOD
2
RAMP
O OUT OUT
V
1
GAIN
V
(2 f ) L C
= ×
π× × ×
The gain of the error amplifier (GAIN
EA
) in midband
frequencies is:
EA O I F
GAIN 2 f C R= π× × ×
The total loop gain as the product of the modulator
gain and the error amplifier gain at f
O
is 1.
( )
MOD EA
IN
RAMP
O OUT OUT
OIF
RAMP O OUT OUT
IN F
GAIN GAIN 1
So :
(2 f ) C L
2 f CR 1
Solving for C :
V 2f L C
VR
×=
×
π× × ×
× π× × × =
× π× × ×
×
3) Use the second pole (f
P2
) to cancel f
ZO
when f
PO
<
f
O
< f
ZO
< f
SW/2
. The frequency response of the loop
gain does not flatten out soon after the 0dB crossover,
Figure 3. Type II Compensation Network
V
REF
R
1
V
OUT
R
2
g
M
R
F
COMP
C
F
C
CF
MAX15046 40V, High-Performance, Synchronous
Buck Controller
www.maximintegrated.com
Maxim Integrated │ 17