MAX15002 Datasheet, PDF (22/30 Pages) Maxim Integrated Products – Dual-Output Buck Controller with Tracking/Sequencing

Dual-Output Buck Controller with
Tracking/Sequencing
Type III: Compensation when f_CO < f_ESR
As indicated above, the position of the output capaci-
tor’s inherent ESR zero is critical in designing an appro-
priate compensation network. When low-ESR ceramic
output capacitors are used, the ESR zero frequency
(f_ESR) is usually much higher than unity crossover fre-
quency (f_CO). In this case, a Type III compensation net-
work is recommended (see Figure 7a).
V_OUT
C_CF
R_{I R1}
C_I
R2
R_F
C_F
FB
-
g_M
V_REF +
COMP
Figure 7a. Type III Compensation Network
GAIN
(dB)
3RD ASYMPTOTE
ωR_FC_I
1ST ASYMPTOTE
ωR_IC_F-1
2ND ASYMPTOTE
R_FR_I-1
4TH ASYMPTOTE
R_FR_IC
5TH ASYMPTOTE
ωR_IC_CF-1
1ST POLE
(AT ORIGIN)
1ST ZERO
2ND POLE
R_FC_F
R_IC_I
2ND ZERO
R_IC_I
3RD POLE ^{ω (rad/sec)}
R_FC_CF
Figure 7b. Type III Compensation Network Response
As shown in Figure 7b, the Type III compensation net-
work introduces two zeros and three poles into the con-
trol loop. The error amplifier has a low-frequency pole
at the origin, two zeros, and two higher frequency 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 (f_Z1 and f_Z2) are designed to can-
cel the pair of complex poles introduced by the LC filter.
f_P1 = at the origin (0Hz)
f_P1 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 (f_ESR), f_P2
can be used to cancel it, or to provide additional atten-
uation of the high-frequency output ripple.
( ) ^fP3
=
2π
× R_F
×
1
C_F
||
C_CF
=
2π
× R_F
1
× ^CF
C_F
× C_CF
+ C_CF
f_P3 attenuates the high-frequency output ripple.
The locations of the zeros and poles should be such
that the phase margin peaks around f_CO.
Set the ratios of f_CO-to-f_Z and f_P-to-f_CO equal to one
another, e.g., ^fCO ^{= f}P = 5 is a good number to get about
f_Z f_CO
60° of phase margin at f_CO. Whichever technique, it is
important to place the two zeros at or below the double
pole to avoid the conditional stability issue.
The following procedure is recommended:
1) Select a crossover frequency, f_CO, at or below one-
tenth the switching frequency:
f_CO
≤ ^fSW
10
2) Calculate the LC double-pole frequency, f_LC :
f_LC
=
2π×
1
L × COUT
3)
4)
Select R_F ≥ 10kΩ.
Place compensator’s first zero
at or below the output filter’s
f_Z1
=
2π
1
× R_F
× C_F
double pole, f_LC, as follows:
C_F
=
2π
× R_F
1
× 0.5
×
f_LC
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