MAX15046_10 Datasheet, PDF (17/24 Pages) Maxim Integrated Products – 40V, High-Performance, Synchronous Buck Controller

40V, High-Performance, Synchronous
Buck Controller
The total loop gain, which is the product of the modulator
gain and the error-amplifier gain at fO, is:
1) GAIN_MOD × GAIN_EA = 1
So :
V_IN
V_OSC
×
(2π
×
ESR
f_O × L _OUT )
×
V_FB
V_OUT
× g_M
× R_F
=
1
Solving for R_F :
R_F
=
V_OSC × (2π × f_O × L _OUT ) × V_OUT
V_FB × V_IN × g_M × ESR
2) Set a midband zero (fZ1) at 0.75 x fPO (to cancel one
of the LC poles):
f_Z1 =
1
2π ×R_F
× C_F
= 0.75 × f_PO
Solving for CF:
C_F
=
2π
× R_F
1
× f_PO
× 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:
C _CF
=
π ×R_F
1
× f_SW
-1
C_F
VOUT
R1
R2
VREF
gM
RF
CF
COMP
CCF
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 two zeros
and three poles at the following frequencies:
f_Z1
=
2π
×
1
R_F
×
C_F
f_Z2
=
2π
×
C_I
1
× (R₁
+
R_I)
Two midband zeros (fZ1 and fZ2) cancel the pair of com-
plex poles introduced by the LC filter:
fP1 = 0
fP1 introduces a pole at zero frequency (integrator) for
nulling DC output-voltage errors:
f_P2
=
2π
1
× R_I
×
C_I
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:
f_P3
=
2π
× R_F
1
× ^CF
C_F
×
+
C _CF
C _CF
fP3 attenuates the high-frequency output ripple.
Place the zeros and poles such that 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 180N
phase shift is introduced to the response making the
loop unstable.
Use the following compensation procedures:
1) With RF >> 10kI, place the first zero (fZ1) at 0.8 x
fPO:
f_Z1 =
1
2π ×R_F
× C_F
= 0.8 × f_PO
Figure 3. Type II Compensation Network
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