PCB Layout Guidelines

Careful PCB layout is important for proper operation. Use

the following guidelines for good PCB layout:

1) Minimize the area of the high current-switching loop

of the rectifier diode, external MOSFET, sense resis-

tor, and output capacitor to avoid excessive switching

noise. Use wide and short traces for the gate-drive

loop from the EXT pin, to the MOSFET gate, and

through the current-sense resistor, then returning to

the IC GND pin.

2) Connect high-current input and output components

with short and wide connections. The high-current

input loop goes from the positive terminal of the input

capacitor to the inductor, to the external MOSFET,

then to the current-sense resistor, and to the input

capacitor’s negative terminal. The high-current out-

put loop is from the positive terminal of the input

capacitor to the inductor, to the rectifier diode, to

the positive terminal of the output capacitors,

reconnecting between the output capacitor and

input capacitor ground terminals. Avoid using vias

in the high-current paths. If vias are unavoidable,

use multiple vias in parallel to reduce resistance

and inductance.

3) Create a ground island (PGND) consisting of the

input and output capacitor ground and negative ter-

minal of the current-sense resistor. Connect all

these together with short, wide traces or a small

ground plane. Maximizing the width of the power

ground traces improves efficiency and reduces out-

put-voltage ripple and noise spikes. Create an ana-

log ground island (AGND) consisting of the

overvoltage detection-divider ground connection,

the ISET and FSET resistor connections, CCV and

CPLL capacitor connections, and the device’s

exposed backside pad. Connect the AGND and

PGND islands by connecting the GND pins directly

to the exposed backside pad. Make no other con-

nections between these separate ground planes.

4) Place the overvoltage detection-divider resistors as

close to the OV pin as possible. The divider’s cen-

ter trace should be kept short. Placing the resistors

far away causes the sensing trace to become

antennas that can pick up switching noise. Avoid

running the sensing traces near LX.

5) Place the IN pin bypass capacitor as close to the

device as possible. The ground connection of the

IN bypass capacitor should be connected directly

to GND pins with a wide trace.

6) Minimize the size of the LX node while keeping it

wide and short. Keep the LX node away from the

feedback node and ground. If possible, avoid run-

ning the LX node from one side of the PCB to the

other. Use DC traces as shields, if necessary.

7) Refer to the MAX8790 evaluation kit for an example

of proper board layout.

Six-String White LED Driver with Active

Current Balancing for LCD Panel Applications

______________________________________________________________________________________

21

FB1

N1

1M

Ω

SHDN

EXT

FB2

FB3

FB4

FB5

FB6

L1

D1

MAX8790

Figure 6. Startup Circuit with Large Capacitors on LED Strings