ISO1228: Impossible to read proximity switches

Maria Vittoria Pelosi

Part Number: ISO1228

Tool/software:

Hello,

I am using ISO1228 receiver and a SOM KRIA K26 to read inputs external to my board. Inputs are related to proximity sensors and they are active low. Reference voltage for field side is 15V, while POWER LOGIC in the attached schematic is 1.8V. As can be seen from schematic, we are using two ISO1228 components with identical configuration, but U21 is working properly, while we are not able to read input signals from U22 correctly. Is there any mistake in the schematics or anything we can modify?

Thank you in advance for your attention,

Maria Vittoria

6 months ago

0 Andrew J 6 months ago

TI__Mastermind 32185 points

Hello Maria,

Thank you for reaching out. The recommended resistor value for source mode is shown below.

For correct device operation across conditions, the RILIM and RPAR should match either of the above sections. Currently the schematic uses an RTHR and RPAR that are outside of the recommended ranges for an RILIM = 1k. I suspect that the current set up is okay for U21 but not for U22.

Switch RILIM to 0-ohm to also for a wider range of RTHR and then design RTHR to be greater than 180-ohms.

Best,
Andrew

0 Maria Vittoria Pelosi 6 months ago in reply to Andrew J

Prodigy 30 points

Hello Andrew,

Thank you for your answer. I changed the resistors values. Now we have R343=0R and R354, R357, R359, R365 = 1K, but U22 is still not working properly. Is there anything else wrong on the schematics?

Thank you again,

Maria Vittoria

0 Andrew J 6 months ago in reply to Maria Vittoria Pelosi

TI__Mastermind 32185 points

Hello Maria,

I will continue to review. However, can you make R370=0R as well?

Also, AVCC should have a 1nF capacitor less than 2mm away from the supply pin (C_FIL in the data sheet).

0 Maria Vittoria Pelosi 6 months ago in reply to Andrew J

Prodigy 30 points

Hello Andrew,

we made R370=0R as well but U22 is still not working. I also confirm that there is C_FIL=1nF close to each component.

We measure OUT_EN pin (27) and we get 1.8V on both components. As regards nFAULT pin (26), we get 1.8V on U21 while we measure 0V on U22.

Thank you,

Maria Vittoria

0 Andrew J 6 months ago in reply to Maria Vittoria Pelosi

TI__Mastermind 32185 points

Hello Maria,

I relized after another review that since the AVCC = 15V the RTHRs will need to be changed. I recalculated RTHR using ISO1228-CALC Calculation tool | TI.com. Assuming AVCC =15V and ON_STATE_MIN = 8V, RTHR should be ~500-ohms. R354, R357, R359, R365 = 500-ohms.

Best,
Andrew

0 Maria Vittoria Pelosi 6 months ago in reply to Andrew J

Prodigy 30 points

Hello Andrew,

even using RTHR=500 Ohm, we stil get 0V on nFAULT pin. We measure 0V on inputs on field side and also 0V on output pins on U22. On U21 we correctly measure 1.8V on output pins.

Thank you,

Maria Vittoria

0 Andrew J 6 months ago in reply to Maria Vittoria Pelosi

TI__Mastermind 32185 points

Hello Maria,

Maria Vittoria Pelosi said:
We measure 0V on inputs on field side and also 0V on output pins on U22.

This makes sense to me because, when the input on the field side is 0V the output on U22's logic side should also be 0V. Only when the field-side input voltage is at 15V will the outputs move to 1.8V (logic 'high').

Isolation

Isolation forum

ISO1228: Impossible to read proximity switches