This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

  • TI Thinks Resolved

TL331: voltage divider resistor in V+/V

Guru 11785 points

Replies: 9

Views: 455

Part Number: TL331

Hi Sirs,

Sorry to bother you.

We are using a voltage divider resistor in V+/V-, is there any suggestion range need follow? (ex : 1~10k or Mega ohm level of partial pressure...)

 2/11/20 [LE]>Schematic clip removed

  • Hello Shu-Cheng,

    The only thing to watch out for is the bias currents of the inputs creating a small error across high value resistors.

    This is a bipolar device, so there will be 18 to 40nA (higher currents at lower temps) flowing OUT of the inputs. This is shown in Figure 2. This current will cause an extra voltage drop across the input resistance. Larger resistors (megoms) will have more error, and smaller resistors (kohms) will have less error, but at the cost of a higher divider string current.

    In your case, the biggest error would be across the ~33k input impedance of the inverting input divider. 33k*40nA=1.3mV. So you may see about a 1.3mV change over temp in the reference voltage due to bias current. You can drop the resistor values by a decade to 10k and 4.99k to further reduce the error contribution, but you will then burn more current in the divider string.

    You can attempt to "null" the bias effects by balancing the impedance at the inputs so that the bias currents cancel. To do this, you would make divider resistors similar to the values of the temp sens resistors *at* the threshold temperature. So if you want the trip point at 85C, then the divider resistors should be 6.2k and 1.45k. Doing this, you can scale-up the size of the resistors.

    You may also wish to add hysteresis, as temperature is usually a "slow" signal, and you can get "chatter" as the temperature slowly approaches the threshold.

    Regards,

    Paul Grohe

    TI Comparators (CMPS) Applications Group

  • In reply to Paul Grohe:

    Hi Sirs,

    Thanks for your reply.

    If our NTC is divided voltage and it's for two comparator input, is there any problem with this? Is there a problem with insufficient current drive capability?

    Here is our ideal schematic~~

    [LE] 2/11/20 > Schematic clip removed

  • In reply to Shu-Cheng LIN:

    Hello Shu-Cheng,

    You are just doubling the bias current as seen by the resistors, which would double the error. So now you have 36 to 80nA of current, or about 2.6mV max error. As long as you allows for this in the deign margin, you should be fine.

    Otherwise, you would want to switch to a CMOS input device where the bias current is negligible. Unfortunately, we do not have a CMOS input device with the same pin out as the TL331. But if you can change your footprint, see the TLV7021.

    Regards,

    Paul Grohe

    TI Comparators (CMPS) Applications Group

  • In reply to Paul Grohe:

    Hi Sirs,

    Thanks for your reply.

    According to this suggestion, my circuit will change to TLV7021

    Because it is CMOS architecture, so I have a larger adjustment of the piezoelectric group (10k~100k Range) then cancel OPA (U28) (Old schematic)

    Can help us see the following circuit (New schematic), is there any suggestion? All good?

    Old schematic: 

    2/11/20 [LE]> schematic clip removed    

    New schematic: 

    2/11/20 [LE]> schematic clip removed    

  • In reply to Shu-Cheng LIN:

    Since you are wiring the outputs together, there is no reason to have two pullup resistors.  You can just eliminate one per pair of comparators.  Other than that, everything looks good.  Best of luck on your circuit.

    Chuck

  • In reply to Chuck Sins:

    Hi Sirs,

    Thanks for your reply.

    We have idea that use one LM2903 to replace two TLV7021, is this feasible? LM2903 Input  is CMOS architecture?

    The circuit is drawn on the first page here, mainly the one as below

    2/11/20 {LE]> schematic removed    

    After the control signal from the NAND Gate, we want to turn off the Load Switch in the figure below.

    Our schematic:

    2/11/20 {LE]> schematic.pdf removed  

  • In reply to Shu-Cheng LIN:

    Shu-Cheng

    I will review this more closely in the morning but I’m a little concerned about using the lm2903 as shown in the schematic. The biggest concern is that the lm2903 does not have a rail to rail input stage. It is a bipolar input stage. The tlv7021 that you mentioned is cmos and has a RRI. I suggest you look at the tlv7032 which is very close to being released. The datasheet is already available on the web and can be samples. This device is push pull output but you are not level shifting the output, so the push pull output may work for you and save you 2 pull up resistors. If you need open drain the tlv7042 would be what I would recommend. That part is also listed in the datasheet. I’ll review more tomorrow and send you another response. I just wanted to give you another option to consider while you were waiting.

    Chuck

  • In reply to Chuck Sins:

    Hi Chuck,

    Thank you very much.

    Please let me know if have any update in morning.

    Have a pleasant night!!

  • In reply to Shu-Cheng LIN:

    After reviewing this further, I suggest you use the TLV7032 (dual, CMOS comparator with push-pull output stage) which reduces your package footprint by only having one package and it eliminates the need for the pull-up resistors.  Everything is powered at 3.3V, so there is no need for an open-drain output stage on the comparator.  As I mentioned before, the LM2903 input stage is not rail-to-rail, so you need to keep the input voltage 2V below the supply rail.  Since the supply rail is 3.3V, the maximum input voltage is 1.3V.  The resistor divider values you are using in the schematic will cause the circuit to violate this input common mode range.  The TLV7032 will solve these problems and save you on power consumption.

    Chuck

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.