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| MAX15026 Datasheet, PDF (16/23 Pages) Maxim Integrated Products – Low-Cost, Small, 4.5V to 28V Wide Operating Range, DC-DC Synchronous Buck Controller | |||
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Low-Cost, Small, 4.5V to 28V Wide Operating
Range, DC-DC Synchronous Buck Controller
Solving for CF:
CF
=
2Ï Ã RF
1
à fPO
à 0.75
3) Place a high-frequency pole at fP1 = 0.5 x fSW (to
attenuate the ripple at the switching frequency, fSW)
and calculate CCF using the following equation:
CCF
=
Ï
à RF
1
à fSW
â
1
CF
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 (fO). Use Type III com-
pensation. Type III compensation provides three poles
and two zeros at the following frequencies:
fZ1
=
2Ï
1
à RF
à CF
fZ2
=
2Ï
à CI
1
à (R1 + RI)
Two midband zeros (fZ1 and fZ2) cancel the pair of
complex poles introduced by the LC filter:
fP1 = 0
fP1 introduces a pole at zero frequency (integrator) for
nulling DC output voltage errors:
fP2
=
2Ï
1
à RI
à CI
Depending on the location of the ESR zero (fZO), use
fP2 to cancel fZO, or to provide additional attenuation of
the high-frequency output ripple:
fP3
=
2Ï
à RF
1
à CF
CF
Ã
+
CCF
CCF
fP3 attenuates the high-frequency output ripple.
Place the zeros and poles so the phase margin peaks
around fO.
Ensure that RF>>2/gM and the parallel resistance of R1,
R2, and RI is greater than 1/gM. Otherwise, a 180°
phase shift is introduced to the response making the
loop unstable.
Use the following compensation procedure:
1) With RF ⥠10kΩ, place the first zero (fZ1) at 0.8 x fPO.
fZ1 =
1
2Ï Ã RF Ã CF
= 0.8 Ã fPO
So:
CF
=
2Ï
à RF
1
à 0.8
Ã
fPO
2) The gain of the modulator (GAINMOD), comprises
the pulse-width modulator, LC filter, feedback
divider, and associated circuitry at the crossover
frequency is:
GAINMOD
=
VIN
VRAMP
Ã
(2Ï
Ã
fO)2
1
à LOUT
à COUT
VOUT
R1
R2
VREF
gM
RF
CF
COMP
CCF
VOUT
RI
R1
CI
R2
VREF
CCF
RF
CF
gM
COMP
Figure 3. Type II Compensation Network
Figure 4. Type III Compensation Network
16 ______________________________________________________________________________________
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