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IXDF502 / IXDI502 / IXDN502

When designing a circuit to drive a high speed MOSFET utilizing

the IXD_502, 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_502 to charge a 1500pF

capacitive load from 0 to 25 volts in 25ns.

Using the formula: I = C ∆V/∆t, where ∆V=25V C=1500pF &

∆t=25ns, we can determine that to charge 1500pF to 25 volts in

25ns will take a constant current of 1.5A. (In reality, the charging

current won’t be constant, and will peak somewhere around 2A).

SUPPLY BYPASSING

In order for our design to turn the load on properly, the IXD_502

must be able to draw this 1.5A 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_502 to an

absolute minimum.

Supply Bypassing, Grounding Practices And

Output Lead inductance

GROUNDING

In order for the design to turn the load off properly, the IXD_502

must be able to drain this 1.5A 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_502 and its

load. Path #2 is between the IXD_502 and its power supply. Path

#3 is between the IXD_502 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_502.

OUTPUT LEAD INDUCTANCE

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.