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LM2775-Q1: Allowable operating ambient temperature

Expert 3775 points

Replies: 7

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Part Number: LM2775-Q1

Hi Champs,

 My customer is interested in LM2577-Q1 to supply 5V for TCAN1042 device from 3.3V rail.

System requires Ta=105C, and TCAN1042 consume 180mA at maximum when bus fault.

 While in clause “6.3 Recommended Operating Conditions” Ta is specified as 85C max, is it possible use LM2577-Q1 under Ta=105C condition if its Tj <125C is satisfied?

Best regards,

Nobuo

  • Hi Nobuo,

    LM2775-Q1 implements a thermal shutdown mechanism to protect the device from damage due to overheating. When the junction temperature rises to 150C(typical), the part switches into shutdown mode. The device releases thermal shutdown when the junction of temperature of the part is reduced to 130C(typical).
    It's hard to calculate the temperature rise because it's related to the PCB layout, copper weight, and if there is other hot device close to this device.

    I used WEBENCH to simulate the thermal rise and result is IC die temp is 139C if using the 1OZ EVM as the model. If use a 2OZ EVM, the IC die temp could reduce to 132C, which is more safe. If the device could work well and not trigger the thermal shutdown, letting the device work at such high temperature condition continuously will affect IC life and that's not recommended.

    Regards,
    Zack Liu
    Boost Converter Solution

  • In reply to Zack Liu:

    Hi Zack,
    Thanks for the reply.

    To improve efficiency of LM2775-Q1, is it reasonable to put Schottky diode (VF around 0.5V) between 3.3V rail and Vin of LM2775?
    Regards,
  • In reply to NOBUO FUJIHARA:

    Hi Nobuo,

    What's reason of putting the Schottky diode?

    Regards,
    Zack Liu
    Boost Converter Solution

  • In reply to Zack Liu:

    Hi Zack,
    According to Figure 11 and Equation (5) in LM2775-Q1 datasheet, efficiency increased if Vin clsoe to 2.7V, expecting lower Power dissipation at LM2775-Q1 and lower junction temperature increase.
    Best regards,
  • In reply to NOBUO FUJIHARA:

    Hi Nobuo,

    The figure 11 is the efficiency curve in the condition of load current is 100mA. Actually if you look at the page 1 Description, it says "The
    LM2775-Q1 can supply up to 200 mA of output current over a 3.1-V to 5.5-V input range, as well as up to 125 mA of output current when the input voltage is as low as 2.7 V".

    Regards,
    Zack Liu
    Boost Converter Solution

  • In reply to Zack Liu:

    Hi Zack,

    Thanks, I missed the point.

    Can I move to off-line for further discussion via e-mail?

    Best regards,

  • In reply to NOBUO FUJIHARA:

    Hi Nobuo,

    I'll close this thread and continue discussing ti via email. Thanks.

    Regards,
    Zack Liu
    Boost Converter Solution

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