Low power opamp for battery cutoff, zener operation

Discussion in 'General Electronics Chat' started by Vindhyachal Takniki, Sep 8, 2015.

  1. Vindhyachal Takniki

    Thread Starter Member

    Nov 3, 2014
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    1. this is a battery powered application & have to very low power.

    There are two power supplies, task is when one goes below +5V, I have to send a high signal to output. So that I can turn off a switch.

    2. Attached a ckt using TLC3702. Whenever V1 goes below 5V, I have set output high. V2 max is 12.6V. I have also grounded channel 2 pins, since it is not used so better to pull them low.

    3. Now TLC3702 have current consumption of 20uA itself.

    Voltage divider of V1 has total 1M of value. V1 varies from 5V-20V. So max current to ground will be 20uA here also. Can I set high voltage divider value to TLC3702 pins?

    Also now main current path is V1 with zener path. I = (12v-2.7)/100k = 93uA.

    Can I increase R1(100k) further, so that zener can still maintain 2.7V across it & don't have any affect on TLC also
     
  2. dl324

    Distinguished Member

    Mar 30, 2015
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    Current in the voltage divider should be at least 10X higher than the input bias current of the opamp.
    No. You didn't mention the part number for the zener, but 1N5242B should be operated at 20mA to be past the knee of the IV curve. At 250nA, it's dynamic resistance is 600 ohms and it can't be used as a stable voltage reference; 93uA is far too low...
     
  3. kubeek

    AAC Fanatic!

    Sep 20, 2005
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  4. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    [​IMG]

    For an Ultra-low power voltage reference, then use a low power voltage reference - this one uses only 5 uA.

    For your 5v voltage divider, it depends on the bias current of your op amp. You can calculate what is acceptable. You gave no indication of what batteries you are using or what your target battery life is so we can calculate (or you can tell us) what the power (or current) budget is.

    Finally, find a single op-Amp for lowest power consumption. The unused Op Amp in that package will also consume power. If you are stuck with this one, look at the datasheet for how to wire unused section - I don't think your option is the common suggestion and may lead to oscillation/heating.
     
  5. Vindhyachal Takniki

    Thread Starter Member

    Nov 3, 2014
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    Hi,

    I have redesigned my circuit. Some of its specs are changed.
    Now I am using TLV3701(push-pull comparator) & TPS715-5V.
    I have to turn off system at 5V & turn it on at 7V. Circuit is attached


    1. Voltage V1, when falls below 5V then cut till it rises above 7V.
    V1 vaires from 5V-20V. It may also be removed from circuit.
    Rest of circuit is powered by V1 whhich varies from 9V-13V.

    2. Now first V1 is stepped down, so that at 5V it gives 1.39V & at 7V it gives 1.95V.

    3. U1 which is powered by +5V is used here as inverting schmitt trigger.
    R2,R9 & R6 forms a network so that it acts as schmitt at desired levels.
    It gives low when voltage is above 7V & high when voltage is below 5V.
    (as it is inverting)

    3. Since I need to turn on a mosfet at around 9V gate voltage, so U2 is added. Non-inverting pin has arounf 1/10th voltage of V2 i.e 0.9 to 1.3V

    When U1 sends high(5V), then U2 output is low. i.e when V1 is below 5V
    When U1 sends low(0V), then U2 output is high. i.e when v1 is above 7V

    4. So I can turn on/off mosfet as desired where load is connected.



    Queries:
    1. What I am thinking, will the circuit behave the same way?

    2. At page 5 of datasheet of TLV3701, min voh = vcc-0.450 & max vol = 0.3V
    Although I am taken care of these values, can these values increase/decrease further for full operating range?

    3. This is final circuit. Is there any need more decoupling caps?

    4. U1 acts as schmitt whose output is fed to U2.
    Can U2 disturb the schmitt action of U1?

    5. Circuit is designed for battery powered application. At places current is in acceptable range except R2,R6 & R9 where I want to reduce current by further increasing resistor in 100K ranges.
    Current for this circuit I have taken help here:
    http://pcbheaven.com/drcalculus/index.php?calc=st_nonsym_sp
     
  6. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    The 2N6659 seems rather old and has huge Rdson, why not use some modern part like IRLML2502 for example? What voltage and current are you switching?
    Both mosfets don´t need that second comparator and will happily work with just 4-5V at the gate, but the IRLML2502 has only 0.045 ohm Rdson. The R8 seems rather high, some 470 ohm should be ok.
    R2,R6,R9 should be much higher if you don´t want your battery to die soon.
    You should add some more capacitance at the input of the TPS715, say a 1uF ceramic wouldn´t hurt.
     
  7. Vindhyachal Takniki

    Thread Starter Member

    Nov 3, 2014
    348
    6
    that mosfet as for only reference.
    Can you tell me how to increase the resistance values
     
  8. dannyf

    Well-Known Member

    Sep 13, 2015
    1,780
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    Buffer it with a CMOS opamp or a jfet.

    Google windowed comparator too.
     
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