MAX1953/MAX1954/MAX1957
Low-Cost, High-Frequency, Current-Mode PWM
Buck Controller
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so the loop-gain equation at the crossover frequency is:
For the case where f
zESR
is greater than f
c
:
then R
C
is calculated as:
where g
mEA
= 110µS.
The error amplifier compensation zero formed by R
C
and C
C
should be set at the modulator pole f
pMOD
.
C
C
is calculated by:
As the load current decreases, the modulator pole also
decreases. However, the modulator gain increases
accordingly, and the crossover frequency remains the
same. For the case where f
zESR
is less than f
C
, add
another compensation capacitor C
f
from COMP to GND
to cancel the ESR zero at f
zESR
. C
f
is calculated by:
Figure 6 illustrates a numerical example that calculates
R
C
and C
C
values for the typical application circuit of
Figure 1 (MAX1953).
Applications Information
See Table 2 for suggested manufacturers of the com-
ponents used with the MAX1953/MAX1954/MAX1957.
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 decoupling capacitors as close to IC pins as
possible. Keep separate power ground plane (con-
nected to pin 7) and signal ground plane (connect-
ed to pin 4).
2) Input and output capacitors are connected to the
power ground plane; all other capacitors are con-
nected to the signal ground plane.
3) Keep the high current paths as short as possible.
4) Connect the drain leads of the power MOSFET to a
large copper area to help cool the device. Refer to
the power MOSFET data sheet for recommended
copper area.
5) Ensure all feedback connections are short and
direct. Place the feedback resistors as close to the
IC as possible.
6) Route high-speed switching nodes away from sensi-
tive analog areas (FB, COMP).
7) Place the high-side MOSFET as close as possible to
the controller and connect IN (MAX1953/MAX1957)
or HSD (MAX1954) and LX to the MOSFET.
8) Use very short, wide traces (50mils to 100mils wide
if the MOSFET is 1in from the device).
Chip Information
TRANSISTOR COUNT: 2930
PROCESS: BiCMOS
