LTC4449 datasheet(12/14 Pages) AD

12 / 14 page

LTC7067

Rev. 0

For more information www.analog.com

APPLICATIONS INFORMATION

Unlike a pure capacitive load, a power MOSFET’s gate

capacitance seen by the driver output varies with its VGS

voltage level during switching. A MOSFET’s capacitive

load power dissipation can be calculated using its gate

charge, QG. The QG value corresponding to the MOSFET’s

VGS value (VCC in this case) can be readily obtained

from the manufacturer’s QG vs VGS curves. For identical

MOSFETs on G2 and G1:

PQG ≈ 2(QG)(fIN)(VCC)

BYPASSING AND GROUNDING

The LTC7067 requires proper bypassing on the VCC,

VG1VCC-G1RTN and VG2VCC-G2RTN supplies due to its high

speed Switching (nanoseconds) and large AC currents

(amperes). Careless component placement and PCB trace

routingmaycauseexcessiveringingandunder/overshoot.

To obtain the optimum performance form the LTC7067:

• Mount the bypass capacitors as close as possible

between the VCC and SGND pins, the G2VCC and

G2RTN pins, and the G1VCC and G1RTN pins. The leads

should be shortened as much as possible to reduce

lead inductance.

• Use a low inductance, low impedance ground place

to reduce any ground drop and stray capacitance.

Remember that the LTC7067 switches greater than

5A peak currents and any significant ground drop will

degrade signal integrity.

• Plan the power/ground routing carefully. Know where

the large load switching current is coming from and

going to. Maintain separate ground return paths for

the input pin and the output power stage.

• Kelvin connect the G1 pin to the G1 MOSFET gate and

G1RTN pin to the G1 MOSFET source. Kelvin connect

the G2 pin to the G2 MOSFET gate and G2RTN to the

G2 MOSFET source. Keep the copper trace between the

driver output pin and load short and wide.

• Be sure to solder the Exposed Pad on the back side of

the LTC7067 packages to the board. Failure to make

good thermal contact between the exposed back side

and the copper board will result in thermal resistances

far greater than specified for the packages.

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