XTR111 IS pin voltage

Hi Atsushi,

There shouldn't be any issues operating the XTR111 without utilizing the VREG circuit.  Please be sure to follow the correct VREG termination suggestions listed on Page 18 of the product datasheet.  The resistor between REGF and REGS is not required and should be removed.  An output capacitor is required on VREG whether it is used or not.

It's really hard to answer any of the rest of your questions without seeing the customer schematic.  Your description leaves a little bit to the imagination and I'd rather not guess on their connections.   

Could you please attach a customer schematic with the voltages you've mentioned labelled on the correct pins? Based on that we should be able to provide feedback on the appropriateness of the device operation.

Dear Collin-san,

Thank you for your help.

I attached the file of the circuit.

0143.Circuit.pdf

Could you please advise it and reply questions?

Best regards,

Atsushi Yamauchi

Hello Yamauchi-san,

Thank you for the customer schematic.  They need to choose one of the options listed on page 18 of the XTR111 datasheet that explain how to properly terminate the REG output when it is not in use.  Other than that, the rest of the schematic looks okay.

I performed some tests in the lab this morning to measure the voltages on IS and VG when OD is both HIGH and LOW. 

With OD Low and IOUT = 20mA:
IS    = 10.437 V
VG  = 6.326 V

With OD HIGH and IOUT = 0mA:
IS    = 11.377 V
VG  = 11.503 V

So it would appear that it is possible for the VG voltage to be slightly higher than the IS voltage when the OD pin is HIGH.

Please confirm the voltages that the customer is looking at and we'll continue from there.

Dear Collin-san,

Thank you for your help.

I confirmed the customer's result.

They changed REGS/REGF connection (REGS:High, REGF:OPEN) and checked the IS and VG voltages.

The result was below;

With OD HIGH and IOUT = 0mA:
IS = 6.30 V
VG = 11.66 V

This result is different from your result.

Is this result correct? Or what did the problem occur?

If there is the problem, what do you think the cause of this problem (these voltages)?

Best regards,

Atsyshi Yamauchi

Hello,

Unfortunately this result is not correct.  If the supply voltage is at 12V and the IS pin is at 6.3V it means that current is flowing out of the IS pin and causing a voltage drop across the internal current sensing circuitry.  Could you put an ammeter in series with the IS pin to measure the current flow? 

Dear Collin-san,

I will request the measurement of the current flow to the customer.

I have a question.

Why should we measure it?

Do you think the cause of the problem (ex. the selection of FET, the failure of the parts and so on)?

Best regards,

Atsushi Yamauchi

Hello Yamauchi-san,

There is a current sensing element between the supply and the IS pin.  As current flow increases, the voltage drop across this circuitry increases as shown in FIgure 17.  Therefore because the customer reported 6.3V on the IS pin, this indicates that a large amount of current is flowing out of the IS pin to somewhere in their system.  Measuring the current will prove whether or not this is occuring. 

Dear Collin-san,

The customer checked the current flow of IS pin. The current was 0.01mA.

And they confirm when Vin is applied in the case OD is "High" , IS voltage drops.

What is the reason of IS voltage drop?

Best regards,

Atsushi Yamauchi

Hello Atsushi,

We will have to investigate this further on the bench.  At this time we're not sure what's causing the voltage drop.

Is the voltage drop causing any issues or device damage?

So we can replicate the customer's testing, could you please provide the input voltage as well as the exact load (including cables) that is connected to the IOUT terminal? 

Dear Collin-san,

I attached the file of circuit. Input voltage connected IOUT terminal is 24V.

5140.circuit2.pdf

The issue and the damage don't occur.

The customer apprehend over the breakdown voltage for FET in the operation.

Best regards,

Atsushi Yamauchi

Hello Atsushi,

I'm sorry to ask more questions, but I'm still not sure what the loading on the IOUT terminal is? 

What are the values of the resistance and capacitance on the IOUT Load?  How long are their cables and what type of cabling is used from IOUT to the load?