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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, R

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 V

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:

0.95

f

Cosc Rosc

For operation above 1 MHz,

0.95

f

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.

Fig. 4. Oscillator frequency component selection for IXDP630.

Fig. 3: IXDP630 internal Schmitt Trigger

inverter oscillator (R

external)

C

C

C

C

C

C

C

C

0.1

1

10

100

1000

10 000

Oscillator - kHz