MAX8643 Maxim Integrated Products, MAX8643 Datasheet - Page 12

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

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The input capacitor reduces the current peaks drawn
from the input power supply and reduces switching
noise in the IC. The total input capacitance must be
equal to or greater than the value given by the following
equation to keep the input ripple voltage within specs
and minimize the high-frequency ripple current being
fed back to the input source:
where V
voltage across the input capacitors and is recommend-
ed to be less than 2% of the minimum input voltage. D
is the duty cycle (V
period (1/f
The impedance of the input capacitor at the switching
frequency should be less than that of the input source so
high-frequency switching currents do not pass through
the input source but are instead shunted through the
input capacitor. High source impedance requires high
input capacitance. The input capacitor must meet the
ripple current requirement imposed by the switching cur-
rents. The RMS input ripple current is given by:
where I
The power transfer function consists of one double pole
and one zero. The double pole is introduced by the out-
put filtering inductor, L, and the output filtering capacitor,
C
the zero. The double pole and zero frequencies are
given as follows:
where R
DCR and the internal switch resistance, R
value for R
tance, which is equal to the rated output voltage divided
by the rated output current. ESR is the total equivalent
series resistance of the output filtering capacitor. If there
is more than one output capacitor of the same type in
3A, 2MHz Step-Down Regulator
with Integrated Switches
12
O
. The ESR of the output filtering capacitor determines
___________________________________________________
f
P LC
RIPPLE
1
_
IN-RIPPLE
L
I
RIPPLE
S
is equal to the sum of the output inductor’s
DSON
).
=
C
f
f
is the input RMS ripple current.
P
Z ESR
IN MIN
2
_
=
_
_
is 37mΩ. R
LC
is the maximum allowed input ripple
I
LOAD
OUT
=
=
Input-Capacitor Selection
=
2
2π
π
D x t x I
/ V
×
V
IN RIPPLE
x L x C
x ESR x C
Compensation Design
IN
−
V
S
O
OUT
), and t
1
is the output load resis-
OUT
×
O
(
1
V
V
x
S
IN
IN
⎛
⎜
⎝
O
is the switching
R
−
R
DSON
O
V
O
OUT
+
+
ESR
R
. A typical
L
)
⎞
⎟
⎠
parallel, the value of the ESR in the above equation is
equal to that of the ESR of a single output capacitor
divided by the total number of output capacitors.
The high switching frequency range of the MAX8643
allows the use of ceramic output capacitors. Since the
ESR of ceramic capacitors is typically very low, the fre-
quency of the associated transfer function zero is higher
than the unity-gain crossover frequency, f
cannot be used to compensate for the double pole creat-
ed by the output filtering inductor and capacitor. The dou-
ble pole produces a gain drop of 40dB/decade and a
phase shift of 180°/decade. The error amplifier must com-
pensate for this gain drop and phase shift to achieve a
stable high-bandwidth closed-loop system. Therefore,
use type III compensation as shown in Figure 3 and
Figure 4. Type III compensation possesses three poles
and two zeros with the first pole, f
frequency (DC). Locations of other poles and zeros of the
type III compensation are given by:
Figure 3. Type III Compensation Network
VOLTAGE
SELECT
CTL1
CTL2
CTL1
CTL2
MAX8643
MAX8643
8kΩ
R3
f
Z EA
1
a) EXTERNAL RESISTOR-DIVIDER
COMP
COMP
_
b) INTERNAL PRESET VOLTAGE
OUT
OUT
LX
LX
FB
FB
=
2
π
L
L
x R x C
R1
R1
1
C2
C2
P1_EA
1
C
C
OUT
OUT
C1
C1
1
, located at zero
C
, and the zero
R3
R4
V
V
OUT
OUT
R2
R2
C3
C3