Datasheet

LTC1644
21
1644f
APPLICATIO S I FOR ATIO
WUU
U
that in all LTC1644 circuit schematics, zener diodes and
snubber networks have been added to the 12V
IN
and V
EEIN
(–12V) supply rail and should be used always. Since the
absolute maximum supply voltage of the LTC1644 is
13.2V, snubber networks are not necessary on the 3V
IN
or
the 5V
IN
supply lines. Zener diodes, however, are recom-
mended as these devices provide large-scale transient
protection for the LTC1644 against PCI backplane fault
occurrences. All protection networks should be mounted
very close to the LTC1644’s supply voltage using short
lead lengths to minimize lead inductance. This is shown
schematically in Figures 15 and 16 and a recommended
layout of the transient protection devices around the
LTC1644 is shown in Figure 17.
sinusoidal response whose duration and period is depen-
dent on the resonant circuit parameters. Since the abso-
lute maximum supply voltage of the LTC1644 is 13.2V,
transient protection against 12V
IN
and V
EEIN
supply volt-
age spikes and ringing is highly recommended.
In these applications, there are two methods for eliminat-
ing these supply voltage transients: using Zener diodes to
clip the transient to a safe level and snubber networks.
Snubber networks are series RC networks whose time
constants are experimentally determined based on the
board’s parasitic resonant circuits. As a starting point, the
capacitors in these networks are chosen to be 10× to 100×
the power MOSFET’s C
OSS
under bias. The series resistor
is a value determined experimentally and ranges from 1Ω
to 50Ω, depending on the parasitic resonant circuit. Note
Figure 15. Place Transient Protection Devices Close to LTC1644’s 5V
IN
and 3V
IN
Pins
C1
0.047µF
3V
IN
3V
IN
3.3V
5V
IN
5V
3V
SENSE
17
3V
OUT
18
5V
IN
13
5V
OUT
3
5V
SENSE
1416
GATE
15
R3
10Ω
R4
10Ω
5V
OUT
5V
3V
OUT
3.3V
R5
1k
R1
0.005Ω
Q1
IRF7413
Q2
IRF7413
R2
0.007Ω
Z3 Z4
LTC1644*
1644 F15
GND
8
Z3, Z4: 1PMT5.0AT3
*ADDITIONAL DETAILS OMITTED FOR CLARITY
Figure 16. Place Transient Protection Devices
Close to LTC1644’s 12V
IN
and V
EEIN
Pins
Figure 17. Recommended Layout for Transient Protection Components
12V
IN
1
V
EEIN
12V
IN
–12V
IN
2
Z1 Z2
R16
1Ω
C5
0.01µF
LTC1644*
1644 F16
GND
8
Z1, Z2: SMAJ12CA
*ADDITIONAL DETAILS OMITTED FOR CLARITY
R15
1Ω
C4
0.01µF
1 2 3 4 5 6 7 8 9
10
20 19 18 17 16 15 14 13 12 11
VIAS TO
GND PLANE
C4
R13
GND
5V
IN
LTC1644*
*ADDITIONAL DETAILS OMITTED FOR CLARITY
DRAWING IS NOT TO SCALE!
1644 F17
3V
IN
V
EEIN
12V
IN
Z1
Z2
Z3 Z4
C5
R14