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LM211-EP: Strobe and Balance

Gerald Horgan
Prodigy 100 points
Part Number: LM211-EP
Other Parts Discussed in Thread: LM311-N, LM211, LM111

I need to use the Strobe function, but not the Balance function.  The note in Figure 12 says that if balancing is not used, then BALANCE and BAL/STRB pins should be shorted together.  Yet Figure 13 only shows BAL/STRB being used during Strobing.  Should the BALANCE and BAL/STRB pins be tied together when using the Strobe function and not the Balance function?  What are the pros/cons of using BAL/STRB and leaving BALANCE open vs using BAL/STRB with BAL/STRB and BALNCE shorted together?

  • 0
    TI__Guru 59846 points

    Hello Gerald,

    See page 12 of the LM311-N datasheet:

     http://www.ti.com/lit/ds/symlink/lm111-n.pdf

    In short - shorting them minimizes noise pickup, which can cause oscillations or false triggers.

  • 0 in reply to Paul Grohe
    Prodigy 100 points

    If they are shorted together, what current is needed to strobe the device?  The datasheet indicates a typical 3 mA needed with only the BAL/STRB pin used.  If both BALANCE and BAL/STRB are used does this double the current needed?  Also, in looking at the schematic of the part, there is 2.4KOhm + 450 Ohm between Vcc and BAL/STRB.  The Electrical Characteristics indicate typical 3 mA with Vcc at 15V and V(strobe) at 0.3V..  This indicates the BAL/STRB should be typically 5 mA instead of 3 mA, and that shorting the pins together would require 10 mA of current to strobe the device.

  • 0 in reply to Gerald Horgan
    TI__Guru 59846 points
    Hi Gerald,

    The specs seem to be for the single input. I do not think they considered the tied balance pins.

    I agree that the current will double when the pins are tied together, paralleling the 2.4k and 450R resistors.

    Remember, the 3mA is the typical amount of current needed to put the output into strobe, and not the typical current that is flowing out of the pin. So it is a threshold to cross and not a static current.

    So "typically" the threshold is 3mA, but can be as high as 5mA. So you really want to draw current just above 5mA out of the strobe pin.

    When the transistor and 1k resistor is used, with a "TTL" input, that places the collector at about 4.2V, so you get about 4.3mA.

    My guess is that they did not consider both a trim pot and strobe at the same time - so no examples. Tying extra circuitry to one side of the balance pins naturally "unbalances" it (ruins the thermal tracking due to leakage).

    They did not specify a maximum current, but I would keep it under 20mA total. Also watch power dissipation.
  • 0 in reply to Paul Grohe
    Prodigy 100 points
    The datasheet says strobe current is typically 3mA at test conditions V(strobe) = 0.3V. If 3 mA is going through the emitter resistor of the 2N2222, then the emitter is at 3.0V. The collector and V(strobe) can not be 0.3V in that case. With the increase in current required due to shorting the BALANCE and BAL/STRB, should the emitter resistor be lowered from the 1K value shown in Fig 19 for the strobing both input and output stages?
  • 0 in reply to Gerald Horgan
    Prodigy 100 points
    Paul, can you reply to the latest question?
  • 0 in reply to Gerald Horgan
    TI__Guru 59846 points
    Hi Gerald,

    The 0.3V is a production test condition. During final test, a current source is used to force a current since the STOBE/BAL pin cannot be directly grounded. The "transistor" circuit is not used. This is a go/no-go test during production.

    As long as you pull more than 5mA out of the (single) strobe pin, then you are assured that the output is "strobed". Paralleling the other BAL pin will require pulling more current since it will "steal" some current from the STROBE/BAL pin.

    If the transistor base is at 5V (TTL), then the emitter and collector will be at about 4.2-4.4V due to the VBE of the base.

    As to your original question. Pulling just the "BAL/STROBE" pin just disables the output. Also pulling the other "BAL" pin also fully disables the input stage, which kills the input bias current. This is why the figure 23 "Sample and Hold" parallels both inputs to also kill the input stage to reduce the input bias current to reduce the "drift" on the sample cap during hold. I have not confirmed it, but I would assume this also increases the time coming out of strobe ("un-strobe") as now the input stage must now recover.
  • 0 in reply to Paul Grohe
    Prodigy 100 points

    Paul,

    If the required current to pull out of the BAL/STRB and BALANCE doubles when they are tied together, then the 1K emitter resistor needs to be lowered due to emitter voltage being at 4.2-4.4V for a TTL input at the base.

    It does not seem that the application circuit for strobing both input and output stages simultaneously is correct.  It would only draw about 4.3mA total, so half of that from each pin, which is less than the specified typical 3mA.

    Do you have a SPICE model of the LM211 that includes the function of the BAL and BAL/STRB pins?

    The slcj005.zip file available for download does not contain these pins in the .SUBCKT model.

    Thanks

  • 0 in reply to Gerald Horgan
    TI__Guru 59846 points

    Hi Gerald,

    The existing model is old (and very simple). We do not have any other newer LM111/211/311 models. This is an older device and not on the priority list, so there is little chance of getting once created in any reasonable time.

    However, there are other "analog" companies that have a similar device with a model...though it follows the original National (300 & 1.3k ohm) design and not the TI design (450 & 2.4k ohm)...