Datasheet

4) Next, calculate the pole set by each linear regulator’s
feedback resistance and the capacitance (C
FBL_
)
between FBL_ and AGND (approximately 5pF includ-
ing stray capacitance):
POLE(FBL1)
FBL1
POLE(FBL2)
FBL2
POLE(FBL3)
FBL3
POLE(FBL4)
FBL4
POLE(FBL5)
FBL5
1
f
2µC (R9 || R10)
1
f
2µC (R18 || R19)
1
f
2µC (R25 || R26)
1
f and
2µC (R22 || R23)
1
f
2µC (R28 || R29)
=
=
=
=
=
5) Next, calculate the zero caused by the output capaci-
tor’s ESR:
ESR_ZERO
LR ESR
1
f
2µC R
=
where R
ESR
is the equivalent series resistance of
C
LR
.
6) To ensure stability, choose C
LR
large enough so that
the crossover occurs well before the poles and zero
calculated in steps 2) to 5). The poles in steps 3) and
4) generally occur at several megahertz and using
ceramic capacitors ensures the ESR zero occurs at
several megahertz as well. Placing the crossover
below 500kHz is sufficient to avoid the amplifier-delay
pole and generally works well, unless unusual com-
ponent choices or extra capacitances move the other
poles or zero below 1MHz.
PC Board Layout and Grounding
Careful PC board layout is important for proper operation.
Use the following guidelines for good PC board layout:
1) Place the high-power components of the step-down
regulator (input capacitors, MOSFETs, inductor, and
output capacitors) first, with any grounded connec-
tions adjacent. Connect these components with short,
wide traces. Avoid using vias in the high-current paths.
If vias are unavoidable, use many vias in parallel to
reduce resistance and inductance.
2) Create islands for the analog ground (AGND), power
ground (PGND), and individual linear regulator
grounds. Connect all these ground areas (islands)
together at only one location, which is a via connected
to the backside pad of the device. All voltage-feedback
dividers should be connected to the analog ground
island. The step-down regulator’s input and output
capacitors, and the charge pump components should
be a wide power ground plane. The power ground
plane should be connected to the power ground pin
(PGND) with a wide trace. Maximizing the width of the
power ground traces improves efficiency, and reduces
output voltage ripple and noise spikes. All other ground
connections, such as the VL and IN pin bypass capaci-
tor and the linear regulator output capacitors, should
be star-connected to the backside of the device with
wide traces. Make no other connections between
these separate ground planes.
3) Place the IN pin and VL pin bypass capacitors within
5mm from the IC and connect them to their respective
pins with short, direct connections.
4) Since both MOSFETs are used for current sensing,
care must be taken to ensure that noise and DC errors
do not corrupt the sense signals. Place both MOSFETs
close to the IC. Connect PGND to the source of the
low-side MOSFET with a short, wide trace. Connect
DL to the gate of the low-side MOSFET with a short,
wide trace. Ensure that the traces from DL to low-side
MOSFET to PGND total no more than 50 squares.
Connect LX close to the connection point between
the low-side and highside MOSFETs with a short,
wide trace. Connect DH to the gate of the high-side
MOSFET with a short, wide trace. Ensure that the
traces from DH to high-side MOSFET to LX total no
more than 50 squares (50 squares corresponds to 20
mils wide if the total trace is 1in long).
5) Place all feedback voltage-divider resistors as close
to their respective feedback pins as possible. The
divider’s center trace should be kept short. Placing the
resistors far away causes their FB traces to become
antennas that can pick up switching noise. Care
should be taken to avoid running any feedback trace
near LX or the switching nodes in the charge pumps.
6) Minimize the length and maximize the width of the
traces between the output capacitors and the load for
best transient responses.
7) Minimize the size of the LX node while keeping it wide
and short. Keep the LX node away from feedback
nodes and analog ground. Use DC traces as shield if
necessary.
MAX1530/MAX1531 Multiple-Output Power-Supply
Controllers for LCD Monitors
www.maximintegrated.com
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