IXDD415SI
APPLICATIONS INFORMATION
High Frequency Gate Drive Circuit
returning current that need to be considered: Path #1 is
betweentheIXDD415anditsload. Path#2isbetweenthe
IXDD415 and its power supply. Path #3 is between the
IXDD415andwhateverlogicisdrivingit. Allthreeofthese
paths should be as low in resistance and inductance as
possible, and thus as short as practical.
The circuit diagram in figure 14 is a circuit diagram for a
veryhighswitchingspeed, highfrequencygatedriver
circuit using the IXDD415SI. This is the circuit used in
theEVDD415EvaluationBoard,andiscapableofdriving
a MOSFET at up to the maximum operating limits of the
IXDD415. Thecircuit'sveryhighswitchingspeedand
high frequency operation dictates the close attention to
several important issues with respect to circuit design.
The three key elements are circuit loop inductance, Vcc
bypassingandgrounding.
Output Lead Inductance
Ofequalimportancetosupplybypassingandgroundingare
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, and treated as
coplanar transmission lines.
Circuit Loop Inductance
Referring to Figure 14, the Vcc to Vcc ground current
path defines the loop which will generate the inductive
term. This loop must be kept as short as possible. The
output leads (pins 24, 23, 22, 21, 20, and 19) must be
no further than 0.375 inches (9.5mm) from the gate of
the MOSFET. Furthermore the output ground leads (pins
25, 26, 27 and 28 on one end of the IC and pins 15, 16,
17, and 18 on the other end of the IC) must provide a
balancedsymmetriccoplanargroundreturnforoptimum
Inconfigurationswheretheoptimumconfigurationofcircuit
layout and bypassing cannot be used, a series resistance
ofafewOhmsinthegateleadmaybenecessarytoprevent
ringing.
Heat Sinking
For high power operation, the bottom side metalized heat
sink pad should be epoxied to the circuit board ground
plane, or attached to an appropriate heat sink, using
thermallyconductiveepoxy.Theheatsinktabisconnected
operation.
VccBypassing
toground.
In order for the circuit to turn the MOSFET on properly,
the IXDD415 must be able to draw up to 15A of current
per output channel from the Vcc power supply in 2-6ns
(depending upon the input capacitance of the MOSFET
being driven). This means that there must be very low
impedancebetweenthedriverandthepowersupply.
The most common method of achieving this low
impedance is to bypass the power supply at the driver
with a capacitance value that is at least two orders of
magnitude larger than the load capacitance. Usually,
this is achieved by placing two or three different types of
bypassing capacitors, with complementary impedance
curves, very close to the driver itself. (These capacitors
should be carefully selected, low inductance, low
resistance,high-pulsecurrent-servicecapacitors). Care
should be taken to keep the lengths of the leads
between these bypass capacitors and the IXDD415 to an
absoluteminimum.
Figure 15: IXDD415SI Bottom Side
Heat Sinking Metalization
Thebypassingshouldbecomprisedofseveralvaluesof
chip capacitors symmetrically placed on ether side of
the IC. Recommended values are .01uF, .47uF chips
and at least two 4.7uF tantalums.
Grounding
In order for the design to turn the load off properly, the
IXDD415 must be able to drain this 15A of current into an
adequate grounding system. There are three paths for
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