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IXDP631 Datasheet PDF : 7 Pages
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IXDP630
IXDP631
Selecting Components for a Specific
Requirement
Deadtime in the IXDP630/631 is exactly
8 clock periods: DT= 8/fclk. Once the
worst case (minimum) deadtime has
been determined (from Power switching
component manufacturer data sheets,
drive circuit analysis, breadboard
measurements, etc.) the clock
frequency is calculated: fclk(max) =
8/ DT(min).
This is the highest allowable clock
frequency, including the effects of initial
accuracy, tolerance, temperature coeffi-
cient, etc. When choosing oscillator
components, special attention to
resistor and capacitor construction is
mandatory.
Oscillator Design
There are two versions of the deadtime
generator. They have distinctly different
internal oscillator designs to serve
different application. In either case,
however, the internal oscillator can be
disabled by simply leaving its external
components off. An HCMOS compatible
clock up to 24 MHz can be fed directly
into the RCIN or XTLIN pin.
IXDP630 RC Oscillator Design
The IXDP630 uses a Schmitt trigger
inverter oscillator (Fig. 3). Two external
components, ROSC and COSC, determine
the clock frequency and consequently
the deadtime. This design allows a
significant cost reduction over a
standard crystal oscillator, but entails a
trade-off in frequency accuracy. The
initial accuracy and drift are a function
of the external component tolerance
and temperature coefficients, supply
voltage, and IXDP630 internal para-
meters. At frequencies under 1 MHz,
assuming the external components
were perfect, the IXDP630 would
introduce an initial accuracy error of
5 %, and a temperature dependence of
-400 ppm. The shift in frequency over
the VCC range 4.5 V to 5.5 V is typically
less than 5 %.
At higher frequencies and with resistor
values below 1 kΩ, the IXDP630
internal parameters become more
influential factors. This results in
greater frequency variation from one
device to another, as well as with
temperature and supply voltage
variations. If high accuracy is a
requirement, the IXDP631 with a crystal
oscillator would be the better choice.
Oscillator frequency vs. Rosc and Cosc
is shown in Fig. 4. For an analytical
method of setting the oscillator, the
design equation is for operation below
1 MHz approximately:
fOSC ≈
0.95
Cosc Rosc
For operation above 1 MHz,
0.95
fOSC ≈
Cosc (Rosc+30) + 3 10-8
IXDP631 Precision Crystal Oscillator
Design
The IXDP631 uses a more common
standard internal crystal oscillator
design. For proper operation the crys-
tal must be of the parallel resonant
type, resonating at the crystal's funda-
mental frequency. Fig. 5 illustrates the
recommended oscillator configuration.
Note the external components required.
The capacitors are needed to achieve
the calibrated crystal frequency (their
value is determined by the crystal
manufacturer), and the resistor is
necessary to assure that the circuit
starts in every case. While the circuit
will usually operate without these extra
parts, this is not recommended.
The crystal oscillator in the IXDP631 is
significantly more accurate than the RC
oscillator in the IXDP630. The total
tolerance (including effects of initial
accuracy, temperature, supply voltage,
drift, etc.) is better than ±100 ppm. This
improves the accuracy and repeatability
of the desired deadtime, but at the
added expense of a crystal.
Which version is appropriate for your
application? That depends on how you
are willing to trade off component cost
for deadtime accuracy.
COSC = 470 pF
COSC = 270 pF
COSC = 100 pF
COSC = 47 pF
COSC = 1 nF
COSC = 2.2 nF
COSC = 4.7 nF
COSC = 10 nF
Fig. 3: IXDP630 internal Schmitt Trigger
inverter oscillator (ROSC, COSC are
external)
0.1
1
10
100
100 0
Oscillator - kHz
Fig. 4. Oscillator frequency component selection for IXDP630.
10 000
I - 18
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