Applications Information

PC Board Layout Considerations

Careful PC board layout is critical to achieve clean and

stable operation. The switching power stage requires

particular attention. Follow these guidelines for good

PC board layout:

1) Place decoupling capacitors as close to the IC as

possible. Keep power ground plane (connected to

PGND) and signal ground plane (connected to

GND) separate.

2) Connect input and output capacitors to the power

ground plane; connect all other capacitors to the

signal ground plane.

3) Keep the high-current paths as short and wide as

possible. Keep the path of switching current short

and minimize the loop area formed by the high-side

MOSFET, the low-side MOSFET, and the input

capacitors. Avoid vias in the switching paths.

4) If possible, connect IN, LX, and PGND separately to

a large copper area to help cool the IC to further

improve efficiency and long-term reliability.

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 sen-

sitive analog areas (FB, COMP).

Chip Information

TRANSISTOR COUNT: 3352

PROCESS: BiCMOS

3A, 1MHz, 1% Accurate, Internal Switch

Step-Down Regulator with Power-OK

______________________________________________________________________________________

13

MAX8505

2.6V TO 5.5V

POWER-OK

1.2V

BST

IN

CTL

POK

GND

REF

COMP

FB

PGND

LX

C2

2 x 22

µF

(10V CERAMIC)

47

µF

(6.3V CERAMIC)

L1

1

µH

(FDV3H-IRON)

R7

10

Ω

R2

11.3k

Ω

R3

22.6k

Ω

R1

51k

Ω

C7

0.1

µF

D1

(CENTRAL CMOSH-3)

R6

20k

Ω

C5

0.1

µF

C6

0.01

µF

C8

220pF

C9

3300pF

Typical Application Circuit