Rainbow Electronics MAX15026 - Manual (Page 16)

MAX15026
Solving for C F :
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:
Type III Compensation Network
(See Figure 4)
When using a low-ESR tantalum or ceramic type, the
ESR-induced zero frequency is usually above the tar-
geted zero crossover frequency (f
O
). Use Type III com-
pensation. Type III compensation provides three poles
and two zeros at the following frequencies:
Two midband zeros (f Z1 and f Z2 ) cancel the pair of
complex poles introduced by the LC filter:
f
P1
= 0
f P1 introduces a pole at zero frequency (integrator) for
nulling DC output voltage errors:
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:
f P3 attenuates the high-frequency output ripple.
Place the zeros and poles so 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 180°
phase shift is introduced to the response making the
loop unstable.
Use the following compensation procedure:
1) With R F
≥ 10kΩ, place the first zero (f
Z1 ) at 0.8 x f PO .
So:
2) The gain of the modulator (GAIN
MOD
), comprises
the pulse-width modulator, LC filter, feedback
divider, and associated circuitry at the crossover
frequency is:
GAIN
V
V f L C
MOD
IN
RAMP O OUT OUT
= Ч
Ч Ч Ч
1
2
2
( ) π
C
R f
F
F PO
=
Ч Ч Ч
1
2 0 8
π
.
f
R C
f
Z
F F
PO 1
1
2
0 8 =
Ч Ч
= Ч
π
.
f
R
C C
C C
P
F
F CF
F CF
3
1
2
=
Ч Ч
Ч
+
π
f
R C
P
I I
2
1
2
=
Ч Ч π
f
R C
f
C R R
Z
F F
Z
I I
1
2
1
1
2
1
2
=
Ч Ч
=
Ч Ч +
π
π
( )
C
R f
C
CF
F SW
F
=
Ч Ч −
1
1
π
C
R f
F
F PO
=
Ч Ч Ч
1
2 0 75 π .
Low-Cost, Small, 4.5V to 28V Wide Operating
Range, DC-DC Synchronous Buck Controller
16 ______________________________________________________________________________________
V REF
R 1
V OUT
R 2
g M
R F
COMP
C F C CF
Figure 3. Type II Compensation Network
V REF
g M
R 1
R 2
V OUT
R I
COMP
C I
C CF
R F
C F
Figure 4. Type III Compensation Network