IXDN414PI / N414CI / N414CM / N414YI / N414YM
IXDI414PI / I414CI / I414CM / I414YI / I414YM
Supply Bypassing, Grounding Practices and Output Lead inductance
When designing a circuit to drive a high speed
MOSFETutilizingtheIXDN414/IXDI414,itisvery
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.
GROUNDING
Inorderforthedesigntoturntheloadoffproperly,
the IXDN414 must be able to drain this 5A of
currentintoanadequategroundingsystem. There
are three paths for returning current that need to
beconsidered: Path#1isbetweentheIXDN414
anditsload. Path#2isbetweentheIXDN414and
itspowersupply. Path#3isbetweentheIXDN414
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. Inaddition,everyeffortshouldbemade
to keep these three ground paths distinctly
separate. Otherwise,thereturninggroundcurrent
from the load may develop a voltage that would
have a detrimental effect on the logic line driving
theIXDN414.
Say, for example, we are using the IXDN414 to
charge a 5000pF capacitive load from 0 to 25
voltsin25ns.
Using the formula: I= ∆V C / ∆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
somewherearound8A).
OUTPUT LEAD INDUCTANCE
SUPPLY BYPASSING
Of equal importance to Supply Bypassing and
GroundingareissuesrelatedtotheOutputLead
Inductance. Everyeffortshouldbemadetokeep
theleadsbetweenthedriverandit’sloadasshort
andwideaspossible. Ifthedrivermustbeplaced
fartherthan2”(5mm)fromtheload,thentheoutput
leadsshouldbetreatedastransmissionlines. 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
terminaloftheload.
Inorderforourdesigntoturntheloadonproperly,
the IXDN414 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
driverwithacapacitancevaluethatisamagnitude
larger than the load capacitance. Usually, this
wouldbeachievedbyplacingtwodifferenttypes
of bypassing capacitors, with complementary
impedancecurves,veryclosetothedriveritself.
(These capacitors should be carefully selected,
lowinductance,lowresistance,high-pulsecurrent-
servicecapacitors). Leadlengthsmayradiateat
highfrequencyduetoinductance,socareshould
betakentokeepthelengthsoftheleadsbetween
these bypass capacitors and the IXDN414 to an
absoluteminimum.
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