MAX15002_12 Datasheet, PDF (21/29 Pages) Maxim Integrated Products – Dual-Output Buck Controller with Tracking/Sequencing

MAX15002
Dual-Output Buck Controller with
Tracking/Sequencing
Type II: Compensation When f_CO > f_ESR
V_OUT
R₁
FB
-
g_m
R₂
V_REF +
R_F
C_F
COMP
C_CF
Figure 6a. Type II Compensation Network
GAIN
(dB)
1ST ASYMPTOTE
G_MODV_REFV_OUT-1(ωC_F)^-1
2ND ASYMPTOTE
G_MODV_REFV_OUT-1R_F
3RD ASYMPTOTE
G_MODV_REFV_OUT-1(ωC_CF)^-1
1ST POLE 1ST ZERO
(AT ORIGIN)
R_FC_F
2ND POLE ^{ω (rad/sec)}
R_FC_CF
Figure 6b. Type II Compensation Network Response
When the f_CO is greater than f_ESR, a Type II compensa-
tion network provides the necessary closed-loop
response. The Type II compensation network provides
a midband compensating zero and high-frequency
pole (see Figures 6a and 6b).
R_FC_F provides the midband zero f_MID,ZERO, and
R_FC_CF provides the high-frequency pole f_HIGH,POLE.
Use the following procedure to calculate the compen-
sation network components.
1) Calculate the f_ZERO,ESR and LC double pole, f_LC:
f_ESR
=
1
2π ×ESR× COUT
f_LC
=
2π×
1
L × COUT
2) Calculate the unity-gain crossover frequency as:
f_CO
≤ ^fSW
10
3) Determine R_F from the following:
R_F
=
V_RAMP (2π × f_CO × L) V_OUT
V_OUT × V_IN × g_m × ESR
Note: R_F is derived by setting the total loop gain at
crossover frequency to unity, e.g., G_EA(f_CO) x G_m(f_CO)
= 1V/V. The transconductance error amplifier gain is
G_EA(f_CO) = G _m x R_F while the modulator gain is:
( ) ^G_MOD ^f_CO
=
VIN
VRAMP
×
ESR
2π×fCO ×L
×
VFB
VOUT
The total loop gain can be expressed logarithmically as
follows:
20log₁₀ ⎡⎣G_mR_F ⎤⎦ +
20log₁₀
⎡
⎢
⎣⎢
(2π
×
ESR× V_IN × V_FB
f_CO ×L)× V_OUT ×
V_RAMP
⎤
⎥
⎦⎥
=
0dB
where V_RAMP is the peak-to-peak ramp amplitude
equal to 2V.
4) Place a zero at or below the LC double pole, f_LC:
C_F
=
1
2π×RF ×fLC
5) Place a high-frequency pole at or below f_P = 0.5 x
f_SW:
C_CF
=
1
π×RF ×fSW
6) Choose an appropriately sized R₁ (connected from
OUT_ to FB_, start with a 10kΩ). Once R₁ is select-
ed, calculate R₂ using the following equation:
R₂
=
R1 ×
VFB
VOUT −VFB
where V_FB = 0.6V.
Maxim Integrated
21