MAX8643 Maxim Integrated Products, MAX8643 Datasheet - Page 13

• Manufacturer Part Number: MAX8643
• Description: 2MHz Step-Down Regulator
• Manufacturer: Maxim Integrated Products

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Figure 4. Type III Compensation Illustration
The above equations are based on the assumptions
that C1>>C2, and R3>>R2, which are true in most
applications. Placements of these poles and zeros are
determined by the frequencies of the double pole and
ESR zero of the power transfer function. It is also a
function of the desired closed-loop bandwidth. The fol-
lowing section outlines the step-by-step design proce-
dure to calculate the required compensation
components for the MAX8643. When the output voltage
of the MAX8643 is programmed to a preset voltage, R3
is internal to the IC and R4 does not exist (Figure 3b).
When externally programming the MAX8643 (Figure
3a), the output voltage is determined by:
GAIN (dB)
POWER-STAGE
f
P
f
f
P
Z EA
2
R
COMPENSATION
TRANSFER
FUNCTION
TRANSFER
FUNCTION
3
1
_
______________________________________________________________________________________
4
_
_
EA
FIRST AND SECOND ZEROS
EA
=
(
=
V
=
=
OUT
0 6
2
DOUBLE POLE
2
2
.
π
π
π
×
x R x C
x R x C
x R x C
−
R
0 6
1
3
1
1
2
1
1
.
)
OPEN-LOOP
SECOND
1
2
POLE
3
GAIN
THIRD
POLE
3A, 2MHz Step-Down Regulator
with Integrated Switches
The zero-cross frequency of the closed-loop, f
be between 10% and 20% of the switching frequency,
f
sient response. Once f
from the following equation:
Due to the underdamped nature of the output LC dou-
ble pole, set the two zero frequencies of the type III
compensation less than the LC double-pole frequency
to provide adequate phase boost. Set the two zero fre-
quencies to 80% of the LC double-pole frequency.
Hence:
Setting the second compensation pole, f
f
Set the third compensation pole at 1/2 of the switching
frequency to gain some phase margin. Calculate C2 as
follows:
The above equations provide accurate compensation
when the zero-cross frequency is significantly higher
than the double-pole frequency. When the zero-cross
frequency is near the double-pole frequency, the actual
zero-cross frequency is higher than the calculated fre-
quency. In this case, lowering the value of R1 reduces
the zero-cross frequency. Also, set the third pole of the
type III compensation close to the switching frequency
if the zero-cross frequency is above 200kHz to boost
the phase margin. The recommended range for R3 is
2kΩ to 10kΩ. Note that the loop compensation remains
unchanged if only R4’s resistance is altered to set dif-
ferent outputs.
S
Z_ESR
. A higher zero-cross frequency results in faster tran-
yields:
C
R
3
1
=
=
C
0 8
1
0 8
.
=
.
C
2
1
x R
1
x C
2
x
R
π
=
3
1
2
x R x
π
=
x
x
C
C
x R x f
3
2 5
L x C
L x C
O
.
is chosen, C1 is calculated
C
1
V
(
x ESR
1
1
3
IN
+
O
O
R
R
R
S
R
O
L
L
L
x R
x R
×
)
+
+
(
(
2
×
R
R
O
O
f
C
O
O
+
+
ESR
ESR
)
)
P2_EA
C
, should
, at
13