MAX8537/MAX8538/MAX8539
Dual-Synchronous Buck Controllers for Point-of-
Load, Tracking, and DDR Memory Power Supplies
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Case 2: Crossover frequency is greater than the
output-capacitor ESR zero (f
C
> f
Z_ESR
).
The modulator gain at f
C
is:
G
MOD(FC)
= G
MOD(DC)
x (f
P_LC
)
2
/ (f
Z_ESR
x f
C
)
Since the output-capacitor ESR-zero frequency is high-
er than the LC double-pole frequency but lower than
the closed-loop crossover frequency, where the modu-
lator already has -1 slope, the error-amplifier gain must
have zero slope at f
C
so the loop crosses over at the
desired -1 slope.
The error-amplifier circuit configuration is the same as
case 1 above; however, the closed-loop crossover fre-
quency is now between f
P2
and f
P3
as illustrated in
Figure 7.
The equations that define the error amplifier’s zeros
(f
Z1_EA
, f
Z2_EA
) and poles (f
P2_EA
, f
P3_EA
) are the same
as case 1; however, f
P2_EA
is now lower than the
closed-loop crossover frequency. Therefore, the error-
amplifier gain between f
Z1_EA
and f
Z2_EA
is now calcu-
lated as:
G
EA
(f
Z1_EA
- f
Z2_EA
) = G
EA(FC)
x f
Z2_EA
/ f
P2_EA
=
f
Z2_EA
/ (f
P2_EA
x G
MOD(FC)
)
This gain is set by the ratio of R4/R1, where R1 is calcu-
lated in the Output Voltage Setting section. Thus:
R4 = R1 x f
Z2_EA
/ (f
P2_EA
x G
MOD(FC)
)
where f
Z2_EA
= f
P_LC
and f
P2_EA
= f
Z_ESR
.
Similar to case 1, C2 can be calculated as:
C2 = 2 / (π x R4 x f
P_LC
)
Set the error-amplifier third pole, f
P3_EA
, at half the
switching frequency, and let R
I
= (R1 x R3) / (R1 + R3).
The gain of the error amplifier between f
P2_EA
and
f
P3_EA
is set by the ratio of R4/R
I
and is equal to
G
EA(FC)
= 1 / G
MOD(FC)
. Then:
R
I
= R4 x G
MOD(FC)
Similar to case 1, R3, C1, and C3 can be calculated as:
R3 = R1 x Ri / (R1 - R
I
)
C1 = 1 / (2π x R3 x f
Z_ESR
)
C3 = C2 / ((2π x C2 x R4 x f
P3_EA
) - 1)
Applications Information
PC Board Layout Guidelines
Careful PC board layout is critical to achieve low
switching losses and clean, stable operation. The
switching-power stage requires particular attention.
Follow these guidelines for good PC board layout:
1) Place the decoupling capacitors as close to the IC
pins as possible.
N.C.
