SN65HVS882: Input field range of 36-72VDC

Nir Bohadana

Part Number: SN65HVS882
Other Parts Discussed in Thread: ISO1212, ISO1211,

I'm trying to integrate the part in my Digital Input module but our digital input fields values are within range of 36-72V, nominal is 60V fo 'ON' value.

what is the optimal value for Rin you suggest to meet the part requirement, if possible.

Thanks

Nir

over 5 years ago

0 Fan.Wang over 5 years ago

TI__Intellectual 2900 points

Hi Nir,

Thanks for considering our SN65HV882. Are both 36V DC and 72V DC considered logic high? If yes, please see my rough estimation below.

Assume the current limit is set to I_limit.

For 36V considered as logic high, it requires

36V - I_limit * Rin > internal threshold voltage = 5.5V (max)

=> I_limit * Rin < 30.5V

For 72V does not over stress the pin, it requires

72V - I_limit * Rin < 34 V (max)

=> I_limit * Rin > 38V

Two requirements contradict each other.

My follow-up question is that why the input voltage range is as wide as 36V to 72V.

If 36V and 72V two endpoints are transient behavior instead of constant voltage, SN65HV882 can filter out signal debouncing. If they are indeed constant voltage, We need figure out a way to reduce the signal range.

0 Anant Kamath over 5 years ago in reply to Fan.Wang

TI__Intellectual 2160 points

Fan, Nir,

TI's new devices, the ISO1212 and ISO1211, Isolated 24-V to 60-V Digital Input Receivers for Digital Input Modules, can be considered for this application.

www.ti.com/.../ISO1212

With RSENSE=560 Ohm, and RTHR=10k, a max input voltage of 80V can be supported. With this setting, the voltage transition threshold is also set at ~30V, which is suitable for the ON voltage range of 36V to 60V. (Max input voltage as well as voltage transition threshold will scale up and down with RTHR).

Rgds,
Anant

0 Nir Bohadana over 5 years ago in reply to Fan.Wang

Prodigy 95 points

Hi,
You are correct, i don't need the range to be this wide.
The nominal voltage is 60V so for input range of 50-65V:
8.6K <= Rin <=12.36K and I_limit = 3.6mA.
I choose Rin=10K@0.5W it should be OK for a range : 41.5<V_iph<70
Is my calculation OK ?

thanks
Nir

0 Fan.Wang over 5 years ago in reply to Nir Bohadana

TI__Intellectual 2900 points

Hi Nir,

Your calcuation is correct. Do you think TI's latest ISO1212 or ISO1211 is also good fit? I feel ISO solution will be better because both were built on the latest technology.

0 Nir Bohadana over 5 years ago in reply to Fan.Wang

Prodigy 95 points

Hi,

Yes they both a very good choice.

Now i need to choose between the ISO1212 and the SN65HVS882.

The SN65HVS882 has an advantage of cascading I/O , reducing the need for extra discrete components,

saving precious PCB space.

Also, reducing uC I/O when you need 32 Digital Inputs, all you need is a 4 I/O for SPI interface.

Thanks.

SN65HVS882

0 Anant Kamath over 5 years ago in reply to Nir Bohadana

TI__Intellectual 2160 points

Hi Nir,

Thanks for your comments. I agree with you, there are benefits to both approaches.

ISO1212 has an in-built multiplexer, with the output ENable functionality. See Fig 23 of the datasheet. You can interface 32 channels with 10-12 uC pins. But this is, of course still higher than 4.

Please note the following with the ISO1212: The device is capable of handling 60V, so you have the flexibility to set the high threshold at 30V - middle of the range - and still have margin to your 72V max voltage. The additional component needed per channel is a 0402 RSENSE resistor to set the current limit. This approach will help you spread the heat on the board, and avoid hot-spots, which could be a concern with 70V operation. No field side power needed. I believe this will save you an LDO on field side, and protection circuitry associated with field side power. Inbuilt isolation will save space on the board as well. The device also has -60V reverse protection, with no current for negative voltages.

Rgds,

Anant

0 Nir Bohadana over 5 years ago in reply to Anant Kamath

Prodigy 95 points

Hi Anant,
I agree with you.
I believe that the ISO1212 is the best choice for me.

Thanks again

0 Nir Bohadana over 5 years ago in reply to Anant Kamath

Prodigy 95 points

Hi,

Consider using ISO1212.

What is the recommended value for Cin (IEC 61131-2 Type 1) ?

And if i choose to use a TVS, instead of a capacitor, for a range of 30-80V is SMF60A would be OK ?

RSENSE=560 Ohm and RTHR=10k .

Thanks in advanced,
Nir

0 Anant Kamath over 5 years ago in reply to Nir Bohadana

TI__Intellectual 2160 points

Nir,

What is the pulse width you want to operate at?

If you use Cin=2nF, you will get a time constant of 20us with 10k, so pulse widths of 100us should be possible.With this you do not need to use a TVS to pass 1kV surge, and 6 kV contact ESD.

Please let me know your pulse width requirement and EMC requirement.

Rgds,

Anant

0 Anant Kamath over 5 years ago in reply to Nir Bohadana

TI__Intellectual 2160 points

Nir,

Just for completeness, I would like to add: Please refer to suggested layout in the datasheet for optimum placement of RTHR, RSENSE and CIN.

Rgds,

Anant

0 Nir Bohadana over 5 years ago in reply to Anant Kamath

Prodigy 95 points

Hi,

The minimum pulse width is 500ms .

Thanks,

Nir

0 Anant Kamath over 5 years ago in reply to Nir Bohadana

TI__Intellectual 2160 points

Nir,

In that case, you have a lot of room for input filtering. I suggest you use Cin=10nF. No TVS will be needed. Please follow the layout guidelines provided in the datasheet.

Rgds,

Anant

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SN65HVS882: Input field range of 36-72VDC

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