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tps2421-1: do I need the load connected?

Federico Battaglin
Intellectual 545 points
Part Number: TPS2421-1
Other Parts Discussed in Thread: TPS25921L, TPS2595

Hello,

For new design, I have to drive an external led lamp by mosfet: nominal voltage 12V and 350mA max of current. And during normal working, the lamp blinks with mimimum tOn (time in ON) about 200ms.

The first revision of schematic is composed by a 2 BJTs current limiter with 1 Ohm resistor and the analog supervision connection to microcontroller: if the analog value is wrong, I understand if the load is connected (in OFF) and if disconnected (in ON) and if there's the short circuit on load (in ON). Until here, it's ok. But, if themicrocontroller will be locked when the drive pin is in ON and there's the short circuit on load, I have the BJT have to dissipate 4W... ops... it's too much and the circuit will burn! And I don't wont this... it's clear.

So, my idea is to use a mosfet to driver the led lamp and use the "tps2421-1" di check the current limit and disconnect the load if the load is in short circuit. And until here, it seems all ok.

But, reading the data sheet, at section "9.2.1 Design requirements", I read "expected resistive load, Rload during start up: 15 Ohm".
What means? Do I need to have the load connected?
And if the load is disconnected or the lamp is broken, what happen? Does tps2421 work well?

Please, explain me....

Thank You and Best Regards
Federico Battaglin

  • 0
    TI__Guru** 105161 points
    Hi Federico,

    Thanks for reaching out!
    Do you have any specific requirement for leaded package device?
    I recommend to consider TPS25921L device for your application. TPS25921L has better current limit accuracy and lower turn on delay than TPS2421 device. Please check and let me know if you have any questions.
    Can you share more information on your project, block diagram and the end product?

    Best Regards,
    Rakesh
  • 0 in reply to Rakesh
    Intellectual 545 points
    Dear Rakesh,
    in first, thank You for your reply.
    Yes, I just considered the product suggested but there' re more components in BOM.

    The project is a driver led lamp powered at 12Vdc with consumption about 350mA.
    I'm evaluating the output driver and I have different solutions:
    1) simpler current limiter based on 2 BJTs and resistor (1 Ohm). But, if the pin of micro is high when the microcontroller will be broken and led lamp will be in short circuit, the circuit can dissipate all joule power and will burn. Ok, there're two faults in the same time, but I want to find other solution.
    2) use a eFuse (it's not cheaper and I need to use an external mosfet)
    3) intelligent mosfet driver (good solution, even if it's not cheaper solution). For example TPS4H160. But I don't know what's happen when I need to make a led flash (turn on 200ms) and when the led lamps are in short permantly. Please, clarify me...

    Thank You and best Regsrds
    Federico Battaglin
  • 0 in reply to Federico Battaglin
    TI__Guru** 105161 points
    Hi Federico,

    Thanks for the details.
    The new eFuse device TPS2595 should be able to match close to your option-1 on cost, however, the minimum current limit possible with this device is 500mA. It has short circuit, over load, over voltage and integrated thermal protection. Please send email for pricing details.

    Best Regards,
    Rakesh
  • 0 in reply to Rakesh
    Intellectual 545 points

    Dear Rakesh,
    in first, thank You for indication of new device.

    But, I'm considering another solution. Yes, because the eFuse needs the mosfet to turn-on and turn-off the load.
    So, I'm evaluating the chip TPS4H000A to drive 4 led lamps at 350mA each (these lamps blink and they are not in turn-on always). In this case I have only one chip for 4 channels and so I save space and BOM (because I no need the mosfet). And I have the diagnostic signals to report to MCU. But I have some question, and here I'm off-topic and so I will open a new ticket.

    Thank You
    Federico

  • 0 in reply to Federico Battaglin
    TI__Guru** 105161 points
    Hi Federico,

    Good. Please open a new post if you have any questions on TPS4H000A device.

    Best Regards,
    Rakesh