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TPS274C65: usable with 12V systems?

Matthew Costello
Prodigy 10 points
Part Number: TPS274C65
Other Parts Discussed in Thread: TPS274160, , TPS4H000-Q1

Tool/software:

I need a way to monitor switched outputs in a (12V) alarm system.
I got excited as the TPS274C65AS was exactly what I was looking for.
And if not for the fact that the undervoltage is designed for 24V only
it would really be perfect. The data sheet shows

Undervoltage Warning starting below 13.5.

Undervoltage Protection starting below 9.3

Will useful operation in a 12V system be possible?

  • 0
    TI__Mastermind 30555 points

    Matthew,

    While the device is designed to work with standard 24V systems- nothing is stopping it from working with a 12V system too. The faults can be masked out so you can simply ignore the undervoltage warning (it's just a warning, nothing turns off). How low would your voltage dip to?

    Best Regards,
    Tim

  • 0 in reply to Timothy Logan
    Prodigy 10 points

    The voltage drop is not a fixed number. If depends upon what is installed and the distance between nodes in the system.

    The alarm system is connected by a four-wire cable (GND,+12V(nom),RS485_A,RS485_B). The issue is the voltage drop at remote (~ 100 meters away) locations. The minimum current limit of the TPS274C65AS is 250 mA. That can result in a voltage drop of 1.0 Volts over 100 meters of 18 gauge cable. Most loads are 30-60 mA and will be verified with current monitoring, but they can add up. As a swag I assume a maximum voltage drop of 3 Volts. Note that most 12V alarm products have a usable power range of 9-15 Volts. Raising the +12(nom) voltage to 13.2V gives only a 0.9V margin above Vs_uvwf which is not enough for a life-safety application.

  • 0 in reply to Matthew Costello
    TI__Mastermind 30555 points

    Matthew,

    Understood on the limitations. Does it have to be a SPI device? We do have GPIO controlled high-side switches that would work at nominal 12V. TPS274160 would work in this regard.

    Best Regards,
    Tim

  • 0 in reply to Timothy Logan
    Prodigy 10 points

    Does not support the requirements:

    1. Output load current measurement (requires I2C or SPI)
    2. Programmable current limit with latching shutdown
    3. Detect open-load condition
    4. Supports at least 700mA per channel
    5. Operable in voltage range 9V-16V

    As far as I know the TPS274C65 is the only IC on the market meeting these requirements. The programmable current limit is the trickiest to meet; I want a 50 mA default current limit for outputs with the ability to raise the limit when necessary.

    The alternative is a 3-IC solution: 1) current-limiting power switch, 2) digital potentiometer to adjust current limit, and 3) I2C current reporting.

  • +1 in reply to Matthew Costello
    TI__Mastermind 30555 points

    Matthew,

    We wouldn't have anything integrated that would be able to support all those requirements unfortunately. The 50mA would be tricky as it is pretty low- but the TPS4H000-Q1 would be able to support that on the lower current end. It is a GPIO controlled device, however, so an I2C expander would likely need to be used.

    This might overcomplicate the solution- but if you coupled the high-side switch with a low-cost MCU such as MSP0L1306 you could likely achieve all of this. You could use the DAC to force an output voltage on the ILIM pin to change the threshold. This of course would involve software development which I am not quite sure is feasible in your application.

    Best Regards,
    Tim