Datasheet

LTC1646
17
1646fa
APPLICATIO S I FOR ATIO
WUUU
Figure 14. Recommended Layout for
Transient Protection Components
1 2 3 4 5 6 7 8
16 15 14 13 12 11 10
9
VIAS TO
GND PLANE
3V
IN
GND
LTC1646*
*ADDITIONAL DETAILS OMITTED FOR CLARITY
DRAWING IS NOT TO SCALE!
1646 F14
5V
IN
TZ1
TZ2
C2
C3
Since there is no bulk capacitance to damp the parasitic
trace inductance, supply voltage transients excite para-
sitic resonant circuits formed by the power MOSFET
capacitance and the combined parasitic inductance from
the wiring harness, the backplane and the circuit board
traces. These ringing transients appear as a fast edge on
the 3.3V or 5V supply, exhibiting a peak overshoot to 2.5
times the steady-state value followed by a damped sinu-
soidal response whose duration and period is dependent
on the resonant circuit parameters. Since the absolute
maximum supply voltage of the LTC1646 is 10V, transient
protection against 3.3V and 5V supply voltage 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.
Snubbers are RC networks whose time constants are
large enough to safely damp the inductance of the board’s
parasitic resonant circuits. As a starting point, the shunt
capacitors in these networks are chosen to be 10× to 100×
the power MOSFET’s C
OSS
under bias. The value of the
series resistor (R6 and R7 in Figure 13) is then chosen to
be large enough to damp the resulting series R-L-C circuit
and typically ranges from 1Ω to 10Ω. Note that in all
Figure 13. Place Transient Protection Device Close to the LTC1646
C1
0.01µF
C2
0.1µF
C3
0.1µF
3V
IN
V
IN2
3.3V
V
IN1
5V
3V
SENSE
8
3V
OUT
7
5V
IN
12
5V
OUT
5
5V
SENSE
119
GATE
10
R3
10Ω
R4
10Ω
5V
OUT
AT 5A
3V
OUT
AT 7.6A
R5
1k
R1
0.005Ω
Q2
IRF7413
Q1
IRF7413
R2
0.007Ω
Z1
Z2
LTC1646**
1646 F13
GND
6
Z1, Z2: BZX84C6V2
**ADDITIONAL DETAILS OMITTED FOR CLARITY
R6 2.7Ω
LONG 5V
R7 1.8Ω
LONG 3.3V
LTC1646 circuit schematics, Zener diodes and snubber
networks have been added to each 3.3V and 5V supply rail
and should be used always. These protection networks
should be mounted very close to the LTC1646’s supply
voltage using short lead lengths to minimize lead induc-
tance. This is shown schematically in Figure 13 and a
recommended layout of the transient protection devices
around the LTC1646 is shown in Figure 14.