ISO7240 Isolator lock up

Yoni

Other Parts Discussed in Thread: ISO7240C, ISO7240M

Hello,

I used ISO7240 in my design to isolate external GPIO. Several of external GPIO are regular rocket switch, there is no debouncer circuit between isolator and between the GPIO.

The rocket switch operated manually, in some occasions i can catch strange behavior of isolator like as output lockup. On switching to logic high in the input, the output remains on logic low. I believe this happened due to glitches on input. I tried low pass filter and found that catching lock up became much harder then before.

I would like to know if my assumption regarding lockup is correct and if this is normal IC behavior?

Thanks

Yoni

over 14 years ago

0 Thomas Kugelstadt over 14 years ago

TI__Mastermind 28265 points

If you use the ISO7240 A version, than lock-up is possible due to some glitch problems stated in the data sheet.

please use the ISO7240C version without glitch problems.

regards,

Thomas

0 Yoni over 14 years ago in reply to Thomas Kugelstadt

Prodigy 40 points

Thank you, i didn't found any documentation on this issue, could you please sent me some if you have. Regards.

0 Thomas Kugelstadt over 14 years ago in reply to Yoni

TI__Mastermind 28265 points

see page 13 in the ISO724xA data sheet under "Product Notification".

regards.

0 George Peterson over 14 years ago in reply to Thomas Kugelstadt

TI__Expert 3170 points

This forum post has been helpful. We would like to confirm one additional point another customer has been asking:

I just want to make sure it is clear that I’m not asking if the exact same failure condition is present on the C version but if a similar failure condition is present. Since the C version minimum bit width is 40nS instead of 1uS I wonder if a negative going pulse less then the minimum bit width will cause the same issue.

0 Thomas Kugelstadt over 14 years ago in reply to George Peterson

TI__Mastermind 28265 points

Hi George,

our design team has confirmed that the filter within the C version works correctly. Positive and negative pulses are filtered correctly, or if longer than the specified time, appear at the output. However, the device will not lock-up.

regards,

Thomas

0 Claudio Ninni69743 over 14 years ago in reply to Thomas Kugelstadt

Prodigy 10 points

Hi Thomas,

the lock-up problem is possible also on the ISO7240 M version?

Or, eventually, a capacitor (about 1nF) connected on one of the output of the ISO7240M could cause the lock-up problem on that output?

P.S. Using the high speed M version with 1nF connected to the output could seem strange; but I use the M version only because, when I bought this isolator, C version was not available. So I use the 1nF capacitor in order to filter noise (that on the C version would be already internally filtered).

Thanks, best regards

Claudio Ninni

0 Thomas Kugelstadt over 14 years ago in reply to Claudio Ninni69743

TI__Mastermind 28265 points

Hi Claudio,

when using external filtering please start with a 100 ohms and 100pF R-C filter at the inputs

remember, every capacitor when discharged represents a short. Because in data applications capacitors get charged and discharged all the time, you will need a resistor in series to prevent the outputs from micro controllers and isolators driving into a short.

If you need to increase the capcitor value later, it's okay, but you need a series resistor. I suggest 100 ohms.

best regads,

Thomas,

0 Dr. Y.B. Patrick Kwan over 14 years ago in reply to Thomas Kugelstadt

Prodigy 30 points

May I ask what is the frequency for the internal oscillator of the ISO7240M (see Fig. 1 of Design Guide SLLA284).

Many thanks in advance,

Patrick

0 Thomas Kugelstadt over 14 years ago in reply to Dr. Y.B. Patrick Kwan

TI__Mastermind 28265 points

Hi Doc,

the internal OSC is 2 MHz nominal.

regards, Thomas

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ISO7240 Isolator lock up