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IXDI509 / IXDN509

When designing a circuit to drive a high speed MOSFET

utilizing the IXD_509, it is very important to observe certain

design criteria in order to optimize performance of the driver.

Particular attention needs to be paid to Supply Bypassing,

Grounding, and minimizing the Output Lead Inductance.

Say, for example, we are using the IXD_509 to charge a 5000pF

capacitive load from 0 to 25 volts in 25ns.

Using the formula: I

C

=C(

∆V/∆t), where ∆V=25V C=5000pF &

∆t=25ns, we can determine that to charge 5000pF to 25 volts

in 25ns will take a constant current of 5A. (In reality, the charging

current won’t be constant, and will peak somewhere around

8A).

SUPPLYBYPASSING

In order for our design to turn the load on properly, the IXD_509

must be able to draw this 5A of current from the power supply

in the 25ns. This means that there must be very low impedance

between the driver and the power supply. The most common

method of achieving this low impedance is to bypass the power

supply at the driver with a capacitance value that is an order of

magnitude larger than the load capacitance. Usually, this

would be achieved by placing two different types of bypassing

capacitors, with complementary impedance curves, very close

to the driver itself. (These capacitors should be carefully

selected and should have low inductance, low resistance and

high-pulse current-service ratings). Lead lengths may radiate

at high frequency due to inductance, so care should be taken

to keep the lengths of the leads between these bypass

capacitors and the IXD_509 to an absolute minimum.

GROUNDING

In order for the design to turn the load off properly, the IXD_509

must be able to drain this 5A of current into an adequate

grounding system. There are three paths for returning current

that need to be considered: Path #1 is between the IXD_509

and its load. Path #2 is between the IXD_509 and its power

supply. Path #3 is between the IXD_509 and whatever logic is

driving it. All three of these paths should be as low in resistance

and inductance as possible, and thus as short as practical. In

addition, every effort should be made to keep these three

ground paths distinctly separate. Otherwise, the returning

ground current from the load may develop a voltage that would

have a detrimental effect on the logic line driving the IXD_509.

OUTPUTLEADINDUCTANCE

Of equal importance to Supply Bypassing and Grounding are

issues related to the Output Lead Inductance. Every effort

should be made to keep the leads between the driver and its

load as short and wide as possible. If the driver must be placed

farther than 0.2” (5mm) from the load, then the output leads

should be treated as transmission lines. In this case, a twisted-

pair should be considered, and the return line of each twisted

pair should be placed as close as possible to the ground pin

of the driver, and connected directly to the ground terminal of the

load.

Supply Bypassing, Grounding Practices And Output Lead inductance