Datasheet

MAX8855/MAX8855A
Dual, 5A, 2MHz Step-Down Regulators
______________________________________________________________________________________ 15
These equations are based on the assumptions that C9
>> C10, and R4 >> R8, which are true in most applica-
tions. Placement of these poles and zeros is deter-
mined by the frequencies of the double pole and ESR
zero of the power stage transfer function. It is also a
function of the desired closed-loop bandwidth. Figure 5
shows the pole zero cancellations in the type III com-
pensation design.
The following section outlines the step-by-step design
procedure to calculate the required compensation com-
ponents. Begin by setting the desired output voltage as
described in the Setting the Output Voltage section.
The crossover frequency f
C
(or closed-loop, unity-gain
bandwidth of the regulator) should be between 10%
and 20% of the switching frequency, f
S
. A higher
crossover frequency results in a faster transient
response. Too high of a crossover frequency can result
in instability. Once f
C
is chosen, calculate C9 (in farads)
from the following equation:
where V
IN
is the input voltage in volts, f
C
is the
crossover frequency in Hertz, R4 is the upper feedback
resistor (in ohms), R
L
is the sum of the inductor resis-
tance and the internal switch on-resistance, and R
O
is
the output load resistance (V
OUT
/I
OUT
).
Due to the underdamped nature of the output LC double
pole, set the two zero frequencies of the type III com-
pensation 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:
Set the third compensation pole, f
P3_EA
, at f
Z_ESR
,
which yields:
R
C ESR
C
O
8
11
=
×
C
R
L C R ESR
RR
OO
LO
11
1
08 4
=
×
×
×× +
()
+.
R
C
L C R ESR
RR
OO
LO
7
1
08 9
=
×
×
×× +
()
+.
C
V
fR
R
R
IN
C
L
O
9
25
241
=
×
×× ×+
⎛
⎝
⎜
⎞
⎠
⎟
.
π
f
RC
PEA3
1
2811
_
=
××π
f
RC
PEA2
1
2710
_
=
××π
f
RC
ZEA2
1
2411
_
=
××π
Figure 5. Pole Zero Cancellations in Compensation Design
OPEN-LOOP GAIN
DOUBLE POLES
THIRD POLE
SECOND POLE
FIRST AND SECOND ZEROS
POWER-STAGE TRANSFER FUNCTION
COMPENSATION TRANSFER FUNCTION
FREQUENCY
GAIN