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LTC1646

1646fa

APPLICATIO S I FOR ATIO

Figure 14. Recommended Layout for

Transient Protection Components

VIAS TO

GND PLANE

3VIN

GND

LTC1646*

*ADDITIONAL DETAILS OMITTED FOR CLARITY

DRAWING IS NOT TO SCALE!

1646 F14

5VIN

TZ1

TZ2

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 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 1

Ω to 10Ω. Note that in all

Figure 13. Place Transient Protection Device Close to the LTC1646

0.01

µF

0.1

µF

0.1

µF

3VIN

VIN2

3.3V

VIN1

3VSENSE

3VOUT

5VIN

5VOUT

5VSENSE

GATE

Ω

5VOUT

AT 5A

3VOUT

AT 7.6A

0.005

Ω

IRF7413

0.007

Ω

LTC1646**

1646 F13

GND

Z1, Z2: BZX84C6V2

**ADDITIONAL DETAILS OMITTED FOR CLARITY

R6 2.7

Ω

LONG 5V

R7 1.8

Ω

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.

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