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TLV767-Q1: What is the required value for the pull down resistor on EN pin to keep the device inactive when floating?

Matthew Pole
Intellectual 340 points
Part Number: TLV767-Q1

Hi,

I am putting this device to power CAN transceivers on my PCB. The supply to the device will be constant and I would need to enable the device when leaving sleep mode on my PCB through the enable pin. As this device is on by defaulyt when enable is floating what value resistor should I use to make sure it is OFF when floating and then I can control with my 5V signal?

Also, can this device dissipate 1.5W? Will only hit this in a fault condition but wanted to see if it could

Many thanks

  • 0
    TI__Expert 6640 points

    Hi Matthew,

    I'm looking into this and will have an update for you about the resistor value in about a day.

    Regarding your question on power dissipation, the short answer is yes, but it depends on system ambient temperature.

    Since the thermal resistance value of TLV767-Q1 is 51.9C/W, 1.5W of power dissipation will result in an LDO temperature rise of ~78C (1.5W * 51.9C/W) on top of ambient temperature. The maximum LDO junction temperature for this device is 150C. An LDO junction temperature >150C will cause the LDO to go into thermal shutdown and stop regulating. 

    So, if the ambient temperature is < 72C, there should be no problem (78C + 72C = 150C). If the ambient temperature is higher, however, you may need to thermally optimize the PCB, which will result in a lower effective thermal resistance of the LDO (i.e. Rθja < 51.9C/W). For more guidance on thermally optimizing the PCB, we have an App Note that shows actual board measurements and quantifies the changes in Rθja for different types of board layouts. As a general rule, an optimized PCB layout can see a reduction of 35%-55% in the Rθja listed in our datasheets. App Note here: https://www.ti.com/lit/an/slvae85/slvae85.pdf  

    Regards,

    Kelsey

  • 0 in reply to Kelsey Farr
    TI__Expert 6640 points

    Hi Matthew,

    The recommended resistor value to EN to ensure the device is not enabled while EN is floating is 10kΩ, which is what we typically see used.

    Sizing the resistor is a matter of achieving a voltage drop across the resistor < VEN(LOW) . You can do this by looking at the enable pullup current (IEN) and VEN(LOW) , which are both spec'd in the datasheet's EC table. If we're designing for the maximum IEN value, that would be 800nA, and VEN(LOW) , is 0.4V. If you use a 10kΩ resistor, the voltage across that resistor will be 8mV. So, since that's significantly < VEN(LOW), it will keep the LDO disabled when EN is floating.

    You can see that by the same logic, if you used a 1MΩ resistor, the voltage the EN pin sees would be 0.8V (800nA*1MΩ), which is > VEN(LOW)  of 0.4V and would cause unexpected EN behavior.

    Regards,

    Kelsey