TPSM846C23EVM-806: Extending the remote sensing points

Hi, 

Would that be fine if I replace the R16 and R18 with 47 ohms resistance rather than 10 ohms?

Hi Fatih,

First off, I think 47ohms resistance for R16 and R18 is reasonable and shouldn't impact the performance of the sense lines. As for the diode, I would double check to make sure it is capable of handling 3A current through it. Also in the drawing of using the TPSM846C23EVM schematic snippet, your diode should be reversed such that the anode of the diode is connected to VS- and cathode of the diode is connected to PGND.

As for the sense connection with the diode, I can't think of anything wrong off the top of my head. However I've never done this type of connection before. I'd advise you to test both types of connections: 1. VS- connected at the anode of the diode, 2. VS- connected at the cathode of the diode/ PGND. If I get anymore information on this connection I'll be sure to comment and let you know.

Regards,

Jimmy

Hi Jimmy, 

First off, thanks for your prompt reply.

Let me explain my goal in detail. My goal is to apply a voltage across the load resistance changing from 0V to 1.1 V as I showed in my block diagram. But, as you also know that TPSM846C23 can have minimum 0.35V output. I used diode so that the forward voltage drop can be used an an negative offset and thus I can adjust the voltage across the load resistance starting from 0V. 

But, the voltage across the diode changes based on the current and temperature. Thus, I simply wanted to connect the VS pins not right at the output pins rather right across the load resistance. In this way, the voltage control loop inside TPSM846C23 will take the voltage drop across the diode and adjust its output until the voltage across the load resistance is equal to VDC_COMMAND. Here is an example, lets say forward voltage drop is 0.45V and processor sent VDC_COMMAND as 512 * 0.7V hoping that 0.7V will be observed right across the load resistance. My understanding in such case is that TPSM846C23 will adjust its output to  0.45 + 0.7V = 1.15V since whatever commanded to TPSM846C23  should be equal to the remotely sensed voltage, which is the voltage across the load resistance. Please let me know if there is something wrong in my understanding. I am confused how to connect the VS pins? If I remove the  0 ohm resistances and replace them with 47ohms VS pins will be still connected to the VOUT and PGND. I was thinking if I want to extend VS pins to another sense point I should not have any connection with VOUT. Lets say I replaced the 0 ohm resistances with 47 ohms from where exactly I will extend wires to the across the load resistance. That would be great if you show it to me on the schematic. Thanks a lot... 

Here is what I have tested today. As I guesses the output voltage was adjusted by the device (around 1.12V) until the voltage across the remote sensing points are equal to the commanded voltage (0.6 V by default).

It looks it works but I have another concern. Here, VDC_COMMAND was 0.6V but I may like to command as low as possible (Lets say 0.01v). Not sure is that is possible because the datasheet show the valid range for the VDC_COMMAND but does not mentions what happens if less than .035 V is commanded. In other words, can I set the VOUT_MIN to 0.01V  so that when I set the VDC_COMMAND to 0.01V it will be taken as valid data? 

Hi Jimmy,

The output voltage of the TPSM846C23 will never go below 0.35 V even it I write 0.01V to the VDC_COMMAND register since there is an voltage droop across the diode. It is all about writing 0.01V to the register will be accepted as valid data or not. Output voltage will be VDC_COMMAND + Vdiode, which is always larger than 0.35 V. 

Could you please tell me what happens if I write 0.01 V to the VDC_COMMAND register?

Thanks...

Hi Fatih,

As you've pointed out in your highlighted image, I'd expect the VOUT_COMMAND to be treated as invalid since this is below the VOUT_MIN stored value. Writing a "0.01V" on the VOUT_COMMAND should result in the VOUT_COMMAND setting the output to the next lowest available stored value for output voltage which in this case would be 0.35V (VOUT_MIN).

Regards,

Jimmy

Hi Jimmy,

My understanding was that we can set the VDC_MIN to any value we want but I just was the following part. 

If VOUT_SCALE_LOOP = 1:
• default: 0000 0000 1011 0011 (binary) 179 (decimal) (equivalent VOUT_MIN = 0.35 V)
• Minimum: 0000 0000 1011 0011 (binary) 179 (decimal) (equivalent VOUT_MIN = 0.35 V)
• Maximum: 0000 0011 0000 0000 (binary) 768 (decimal) (equivalent VOUT_MIN = 1.5 V)

I have decided to connect the remote sensing points right across the VOUT as shown in application schematic in the datasheet. As you can see there is not 49.9 ohm resistance as we have in TPSM846C23EVM Schematic. Would you recommend 49.9 ohm resistance? I just wanted to make sure. Thanks...

Hi Fatih,

Generally I would recommend you have a Kelvin connection with 10ohms to the output capacitor bank. I don't see any issues using 49.9ohms either.

Regards,

Jimmy

Hi Jimmy,

Thanks for your help. I am pretty much done with my final schematic but I just want to make sure one more thing. If my board has electrolytic bulk capacitors already do I still need 2 × 330 μF electrolytic bulk caps at the input voltage of TPSM846C23? 

Thanks...

Hi Fatih,

If you have an electrolytic bulk cap upstream that connects to the input of the TPSM846C23 then you can do without the 330uF you have shown in your schematic. It is important to keep the ceramic caps local to the device to provide proper bypassing of the input.

Regards,

Jimmy