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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).
SUPPLY BYPASSING
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.
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.
Supply Bypassing, Grounding Practices And Output Lead inductance
