IXDR502 / IXDS502
Fig. 29
Fig. 28
Low State Output Resistance vs. Supply Voltage
High State Output Resistance vs. Supply Voltage
4.5
6
5
4
3
2
1
0
4
3.5
3
2.5
2
1.5
1
0.5
0
0
5
10
15
20
25
30
35
40
0
5
10
15
20
25
30
35
40
Supply Voltage (V)
Supply Voltage (V)
Supply Bypassing, Grounding Practices And Output Lead Inductance
GROUNDING
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.
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
groundpathsdistinctlyseparate.Otherwise,thereturningground
current from the load may develop a voltage that would have a
detrimental effect on the logic line driving the IXD_502.
Say, forexample, weareusingtheIXD_502tochargea1500pF
capacitive load from 0 to 25 volts in 25ns.
Using the formula: I= ∆V C / ∆t, where ∆V=25V C=1500pF &
∆t=25ns, we can determine that to charge 1500pF to 25 volts in
25nswilltakeaconstantcurrentof1.5A. (Inreality, thecharging
current won’t be constant, and will peak somewhere around
2A).
OUTPUT LEAD INDUCTANCE
SUPPLY BYPASSING
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
fartherthan2”(5mm)fromtheload, thentheoutputleadsshould
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.
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
magnitudelargerthantheloadcapacitance. Usually, thiswould
be achieved by placing two different types of bypassing
capacitors, with complementary impedance curves, very close
tothedriveritself.(Thesecapacitorsshouldbecarefullyselected
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.
10
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