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TPS3838: Supervisor & Reset IC forum

Vinayak Nevpurkar
Prodigy 180 points
Part Number: TPS3838
Other Parts Discussed in Thread: TPS3839

Hi,

I would like to drive the out load to turn it ON & OFF . so I can use the TPS3838

Is Reset pin capable of providing 3 mA [Sourcing] in TPS3838A09 [cut off of 0.9 V] ? [Please check for 0.9 Volts part specifically] ?

 

 

 

  • 0
    TI__Genius 12630 points

    Vinayak,

    The ABS MAX for output current for TPS3838 is 5mA. The output current will depend on your VDD and the impedance of your load. You can also implement an external pass/protection FET so that the TPS3838 simply controls a FET to allow power to the load or to prevent power to the load.

    Does this answer your question? Please let me know if you need any additional support.

    -Michael

  • 0 in reply to Michael_DeSando
    Prodigy 180 points

    Hi Michael,

    When 2 mA was load, , Vdd 1.2 Volts, then Voltage of RESET dropped below 0.5 Volts.

    So I queried about this

     

    Now we cannot use FET as typ. VDD is self is 1.2 Volts [Max 2 Volts, Min 0.9 Volts]

    Do you have any recommendation of FET which can get on at such low voltage ?

    Please suggest

     

  • 0 in reply to Vinayak Nevpurkar
    TI__Genius 12630 points

    Vinayak,

    Please confirm if you are using TPS3839 or TPS3838.

    When load = 2mA, The VDD is both supplying the device and the load. Does the circuit still trigger at the expected voltage? Are you using any external resistors?

    Are you saying you want the device to trigger at 0.9V when VDD is dropping but it is actually triggering at 1.2V??

    -Michael

  • 0 in reply to Michael_DeSando
    Prodigy 180 points

    HI Michael,

    Sorry my mistake I am using TI P/N# TPS3839A09DBZR 

    yes I want to trigger the device at 0.9 V. so I can not use FET and need to use BJT probably  PNP transistor  with pull up resister.

    Also confirm if I am  using PNP transistor is there any need of pull up resister ?. pls suggest transistor .   

  • 0 in reply to Vinayak Nevpurkar
    Prodigy 180 points

    Few more inputs for your information.

    We have just started the design & testing.

     Meanwhile, I had one query

    In our application, it is needed that, Vout [Reset] should be same as Vin

     Our Application in brief as below

     We have a device which works on rechargeable battery of 1.2 Volts.

    1. There is one motor driver IC which runs small motor  is driven by 1.2 Volts battery
    2. This IC takes max 3 mA current
    3. But this IC does not have cut off
    4. So battery voltage tends go below 0.7 Volts which might damage the battery
    5. So, we are planning to get cut-off IC which will protect the IC

     What are problems in using Cut-Off IC as it is ?

    1. If we connect 3 mA load direct to IC, its output gets loaded
    2. So we decided to use transistor as driver
    3. Transistor as driver takes 50 uA considering gain.
    4. But as per given in datasheet on page no. 7, Vout may be less by 0.4 Volts than Vin

     

     How this may create the problem ?

     a)       Say if Voltage is 1.2 Volts, then Vout may be 1.2- 0.4 = 0.8 Volts. Here Transistor will work

    b)      But if Battery voltage goes as 1.0 Volts, then 1.0 -0.4 Volts = 0.6 Volts. Here Transistor may get Off at 1.0 Volts of battery.

    c)       So here backup will be compromised & product may get Off prematurely

     Query

    a)       Practically we have seen that, Vout is same as Vin [Just few mV Drop] at ambient temperature

    But, theoretically, Vout can come as Vdd-0.4 Volts

    b)      How we can reduce this ?

    c)       Do you have the same IC which have sinking capacity of 3 mA ? Or how to reduce [VDD-0.4] factor ?

  • 0 in reply to Vinayak Nevpurkar
    TI__Genius 12630 points
    Vinayak,

    There are 2 main ways to help make this work.

    1. Reduce the current required for the transistor to operate less than 50uA. This would make the voltage drop from VDD at the output lower than the 0.4V mentioned in the table above.

    2. Find a transistor that requires a lower voltage threshold to operate. This would allow the VDD-0.4V to still be OK to keep the transistor functioning.

    Solving either 1 or 2 will require finding a good transistor to use. I have been looking but haven't had too much luck since every transistor seems to be right at that operating border line.

    Can you provide me the part number for the battery you are using? I am curious to know what batteries are common to work down to 0.9V. I will also see if I can find a transistor that either requires less current or a lower voltage threshold to make this work..

    As far as another device working better than this one, I am not aware of any voltage supervisor that operates at as low of input voltage and requires as little supply current so I do think this is the correct device.

    -Michael