XTR116: PWM to 4-20mA

Hi Danilo,

what is the loop voltage connected to the XTR116? And what load resistor is connected to the output of XTR116?

VREG usually breaks down when too much current is drawn from this pin. But even with a normal current the voltage at the VREG pin will not be exactly 5.0V. See the "VREG voltage versus IREG current" plot on page 4 of datasheet. You would need to use the VREF pin for a more accurate supply voltage. But keep in mind that current you can draw from these both pins in total is limited to 3.7mA maximum.

Since you run a PWM signal through the ISO7321, you may need some supply voltage decoupling from the VCC2 pin to the GND2 pin. Figure 5 of datasheet explains how to accomplish this.

Kai

Hi Danilo, 

Danilo A. said:

1. The first one is that on VReg he only gets 4.7V.

Please see Kai's comments. I think that you may overload the Vreg pin more than >3mA at 5Vdc. The sum of currents draw from pin6 and pin1 < 3.7mA.  

Danilo A. said:

2. The second is that he can't get less than 7.7mA on the output when he put 0V on the output of the ISO7320.

Since the above configuration exceeds 4mA current requirement, the minimum current will be go down to 4mA at Vpwm_avg = 0Vdc, please see the simulation. 

In the sim example, Vref = 4.096Vdc and I_in = Vref/Ros = 4.096V/102.4kΩ = 40uA when Vpwm_avg = 0Vdc. The current gain of XTR116 is 100A/A, which Iout = 40uA*100 = 4mA. Iret node needs to be floating per XTR116 configuration. 

If you fix the above issues, you should resolve the 2nd issues. BTW, the minimum Vloop voltage for XTR116 is 7.5Vdc. 

XTR116 PWM 01172023.TSC

If you have additional questions, please let us know. 

Best,

Raymond

Hi Kai, 

Please see the response of our product specialist.

To answer the questions, I have tried to use a 12 and 24 V power supply connected with a 220Ohm resistor and reading the current going through the resistor. I only use one channel on the ISO7321 which should maximum draw a current of 1.7mA so I don’t quite understand why I can’t get the output current lower than 7.7mA.

I looked at the plots and the figures and hopefully this will help me get a more stable voltage supply.

Hi Raymond,

I have forwarded your comment t our customer and waiting for his feedback.

Regards,

Danilo

Hi Danilo, 

Please note that GND2 in customer's schematic needs to be floating. GND2 node is a different from GND connection from Vloop supply on the XTR116's Vs input.

Please let us know when the customer has additional questions. 

Best,

Raymond