Rainbow Electronics MAX1685 - Manual (Page 13)

Inverting Output
Interchanging the ground and V OUT connections yields
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 -
 V
OUT
.
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.
MAX1684/MAX1685
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