PC Board Layout Guidelines

Careful PC board layout is critical to achieving low

switching losses and clean, stable operation. The

switching power stage requires particular attention. If

possible, mount all the power components on the top

side of the board with their ground terminals flush

against one another. Follow these guidelines for good

PC board layout:

1) Keep the high-current paths short, especially at the

ground terminals. This practice is essential for sta-

ble, jitter-free operation.

2) Connect the power and analog grounds close to

the IC.

3) The IC needs two bypassing ceramic capacitors for

input and supply. C1 isolates the IC from current

pulses at N1, and should be placed such that the

path between C1 and N1 is not shared with the IC.

C7 bypasses the IC and should be placed adjacent

to the IC.

4) Keep the power traces and load connections short.

This practice is essential for high efficiency. Using

thick copper PC boards (2oz vs. 1oz) can enhance

full-load efficiency by 1% or more. Correctly routing

PC board traces is a difficult task that must be

approached in terms of fractions of centimeters,

where a few milliwatts of excess trace resistance

cause a measurable efficiency penalty.

5) LX and GND connections to N2 for current sensing

must be made using Kelvin sense connections to

guarantee the current-limit accuracy. With 8-pin SO

MOSFETs, this is best done by routing power to the

MOSFETs from the outside using the top copper

layer, while connecting LX and GND inside (under-

neath) the 8-pin SO package.

6) When tradeoffs in trace lengths must be made, it is

preferable to allow the inductor charging current

path to be longer than the discharge path. For

example, it is better to allow some extra distance

between the inductor and the low-side MOSFET or

between the inductor and the output filter capacitor.

7) Ensure that the connection between the inductor

and C3 is short and direct.

8) Route switching nodes (BST, LX, DH, and DL) away

from sensitive analog areas (COMP, FB).

9) Ensure that the C1 ceramic bypass capacitor is

immediately adjacent to the pins and as close to

GND pins of MAX1966/MAX1967 must terminate at

the two ends of C1 before connecting to the power

switches and C2.

Layout Procedure

1) Place the power components first, with ground ter-

minals adjacent (N2 source, C2, C3). If possible,

make all these connections on the top layer with

wide, copper-filled areas.

2) Mount the MAX1966/MAX1967 adjacent to MOSFET

N2, preferably on the backside opposite N2 in

order to keep LX, GND, and the DL gate-drive lines

short and wide. The DL gate trace must be short

and wide measuring 50mils to 100mils wide if the

MOSFET is 1in from the MAX1966/MAX1967.

terminate at the two ends of C1 before connecting

to the power switches and C2. C1’s ground con-

nection must be as close to the IC’s GND pin as

possible.

4) On MAX1966, C7 must be connected to the VIN

and GND pins with mimimum distance. On the

MAX1967, C7 must be connected to VL and GND

pins with minimum distance.

5) Group the gate-drive components (BST diode and

C5) together near the controller IC.

6) Make the MAX1966/MAX1967 ground connections

to three separate ground planes: the output ground

plane, where all the high-power components con-

nect; the power ground plane, where the output

bypass capacitor C3 connects; and the analog

ground plane, where sensitive analog components

connect. The analog ground plane and power

ground plane must meet only at a single point

directly beneath the IC. These two planes are then

connected to the high-power output ground with a

short connection for the C3 capacitor to the source

of the low-side MOSFET, N2 (the middle of the star

ground). This point must also be very close to the

output capacitor ground terminal.

Refer to the MAX1966/MAX1967 EV kit manual for a PC

board layout example.

Low-Cost Voltage Mode PWM

Step-Down Controller

______________________________________________________________________________________

13

TOP VIEW

1

2

3

4

5

10

9

8

7

6

BST

DH

LX

GND

VIN

VCC

FB

COMP/EN

MAX1967

µMAX

DL

VL

Pin Configurations (continued)