Inverting Output

a negative voltage supply (Figure 4). The component

selections are the same as for a positive voltage con-

verter. The absolute maximum ratings limit the output

voltage range to -1.25V to +5.5V and the maximum

input voltage range to 14V -

PC Board Layout

High switching frequencies and large peak currents

make PC board layout a very important part of design.

Poor design can result in excessive EMI on the feed-

back paths and voltage gradients in the ground plane,

both of which result in instability or regulation errors.

Power components such as the MAX1684/MAX1685

inductor, input filter capacitor, and output filter capacitor

should be placed as close together as possible, and

their traces kept short, direct, and wide, Connect their

ground nodes in a star-ground configuration. Keep the

extra copper on the board and integrate into ground as

a pseudo-ground plane.

When using external feedback, the feedback network

should be close to FB, within 0.2 inch (5mm), and the

output voltage feedback should be tapped as close to

the output capacitor as possible. Keep noisy traces,

such as those from LX, away from the voltage feedback

network. Separate the noisy traces by grounded cop-

per. Place the small bypass capacitors within 0.2 inch

(5mm) of their respective inputs. The MAX1684 evalua-

tion kit manual illustrates an example PC board layout,

routing, and pseudo-ground plane.

Connect AIN to IN with a short (0.2 inch) metal trace or a

1

Ω resistor and bypass AIN to PGND with a 0.1µF capac-

itor. This acts as a lowpass filter to reduce noise at AIN.

Low-Noise, 14V Input, 1A, PWM

Step-Down Converters

______________________________________________________________________________________

13

Pin Configuration

16

15

14

13

12

11

10

9

1

2

3

4

5

6

7

8

CVH

PGND

SHDN

LX

LX

BOOT

STBY

ILIM/SS

SYNC/PWM

TOP VIEW

MAX1684

MAX1685

QSOP

AIN

IN

REF

CVL

AGND

FB

CC

Chip Information

TRANSISTOR COUNT: 2061