LTC1646
W U U
APPLICATIO S I FOR ATIO
U
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.
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.
5V
IN
VIAS TO
GND PLANE
TZ1
C2
C3
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
capacitorsinthesenetworksarechosentobe10× to100×
the power MOSFET’s COSS 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
LTC1646*
TZ2
GND
3V
IN
*ADDITIONAL DETAILS OMITTED FOR CLARITY
DRAWING IS NOT TO SCALE!
1646 F14
Figure 14. Recommended Layout for
Transient Protection Components
R2
0.007Ω
Q1
IRF7413
V
5V
IN1
OUT
5V
AT 5A
R6 2.7Ω
R1
0.005Ω
Q2
LONG 5V
IRF7413
V
3V
OUT
AT 7.6A
IN2
3.3V
R7 1.8Ω
LONG 3.3V
C1
0.01µF
R3
10Ω
R4
10Ω
R5
1k
8
9
10
7
12
11
5
3V
IN
3V
SENSE
GATE 3V
5V
IN
5V
SENSE
5V
OUT
OUT
Z1
LTC1646**
GND
Z2
C2
C3
0.1µF
0.1µF
1646 F13
6
Z1, Z2: BZX84C6V2
**ADDITIONAL DETAILS OMITTED FOR CLARITY
Figure 13. Place Transient Protection Device Close to the LTC1646
1646fa
17
Linear [ Linear ]
