Rainbow Electronics MAX15022 - Manual (Page 20)

MAX15022
Dual, 4A/2A, 4MHz, Step-Down DC-DC
Regulator with Dual LDO Controllers
20 ______________________________________________________________________________________
Solving for R1:
where V FB is the 0.6V (typ) FB_ input-voltage set-point,
L is the value of the regulator inductor, ESR is the
series resistance of the output capacitor, and V
OUT_ is
the desired output voltage.
1) C
F
is determined from the compensator’s leading
zero, f
Z1
, and R
F
as follows:
2) C CF is determined from the compensator’s high-fre-
quency pole, f P1 , and R F as follows:
3) Calculate R 2 using the following equation:
where V FB = 0.6V (typ) and V OUT_ is the output voltage
of the regulator.
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 (MLCCs) are used, the ESR zero fre-
quency (f
ESR ) is usually much higher than the desired
crossover frequency (f CO ). In this case, a Type III com-
pensation network is recommended (see Figure 6a).
As shown in Figure 6b, 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:
Two midband zeros (f Z1 and f Z2 ) are designed to com-
pensate for the pair of complex poles introduced by the
LC filter.
f P1 introduces a pole at zero frequency (integrator) for
nulling DC output-voltage errors.
f
P1
= at the origin (0Hz)
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.
f P3 attenuates the high-frequency output ripple.
Since C CF << C F then:
f
1
2 R C
P3
F CF
=
Ч Ч π
f
1
2 R C C
1
2 R
C C
C C
P3
F F CF
F
F CF
F CF
=
Ч Ч
( )
=
Ч Ч
Ч
+
π
π
f
1
2 R C
P2
I I
=
Ч Ч π
f
1
2 R C
f
1
2 C (R R )
Z1
F F
Z2
I 1 I
=
Ч Ч
=
Ч Ч +
π
π
R [k ] R [k ]
V [V]
V [V] V [V]
2 1
FB
OUT_ FB
Ω Ω = ×
−
C F]
1
2 R [k ] f [kHz]
CF
F P1
[ μ
π
=
Ч Ч Ω
C [ F]
1
2 R [k ] f [kHz]
F
F Z1
μ
π
=
Ч Ч Ω
R [k ]
R [k ] 4 ESR[m ] V [V]
2 f [kHz] L[ H] V [V]
1
F FB
CO OUT_
Ω
Ω Ω
=
Ч Ч Ч
Ч Ч Ч π μ
R 1
V REF
R
F
FB_
COMP_
V OUT_
R 2
C F
C
CF
R I
C
I
Figure 6a. Type III Compensation Network
GAIN
(dB)
1ST ASYMPTOTE
( ωR
I C F ) -1
3RD ASYMPTOTE
(
ωR
F C I ) -1
5TH ASYMPTOTE
(R
I C CF ) -1
ω (rad/sec) 1ST POLE
(AT ORIGIN)
2ND POLE
(R I C I ) -1
2ND ZERO
(R 1 C I ) -1
3RD POLE
(R F C CF ) -1
1ST ZERO
(R F C F ) -1
4TH ASYMPTOTE
R F
R I
( )
2ND ASYMPTOTE
R F
R 1
( ) -1
Figure 6b. Type III Compensation Network Response