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TLV1117: TLV1117-50CDCYR design review

Manjunath G14
Expert 2075 points
Part Number: TLV1117
Other Parts Discussed in Thread: TLV743P

Hi TI team,

Attached is the Our design using TLV1117-50CDCYR and TLV74333PDBVR LDO's for one of our Home automation applications,

Before We release the design for fabrication, We want this design to be reviewed by TI technical team, 

Hence Please review the design and share your important point on this.

Thanks

  • 0
    TI__Guru 55940 points
    Hi Manjunath,

    While the connections follow datasheet recommendations, due to the large headroom (Vin - Vout) across both the TLV1117 and the TLV743P, I am curious as to your intended output current. Keep in mind that linear regulators and LDOs are power dissipative devices so thermal performance should be considered as well.

    Very Respectfully,
    Ryan
  • 0 in reply to Ryan Eslinger
    Expert 2075 points

    Hi Ryan,

    Thanks for the comments,

    1. Input to TLV1117 is constant 7.5V and output is 5V, So Is this difference(Vin-Vout) is huge for power loss you meant to say?

    2. The output of TLV1117  (5V) is only connected to one relay where the current consumption of that relay is 80mA and the output of  TLV743P LDO(3.3V) will be supply to few controllers, I think at the max current consumption of 3.3V will be 100mA-200mA. Hence I thought the current ratings of both the LDO is fine. Kindly confirm it.

    3. and regarding the power dissipation for both LDO's, I thought of giving copper pads exposed with lots of via under the tab pins. Is this is sufficient for both the LDO's?

    4. Any concern should We take at the output of 5V LDO(Like keeping large capacitor) for this design?.

    Please share your review comments.

    Regards,

    Manjunath G

    Hardware design Engineer

    Crevavi Technologies pvt ltd

  • 0 in reply to Manjunath G14
    TI__Guru 55940 points
    Hi Manjunath,

    You are correct that you are well within the current ratings for both devices; however, we still need to consider the thermal performance.
    Since your maximum current consumption is 200 mA, we can estimate the junction temperature using the thermal information table for both devices. While this data is modeled on a JEDEC Hi-K board, it is still good to use as a first pass calculation until you have your board layout.

    First let us calculate for the TLV1117 which has a Rtheta_ja of 104.3 C/W in the DCY package:

    Tj = (Vin - Vout) x Iout x Rtheta_ja + Ta
    Tj = (7.5 V - 5 V) x 0.2 A x 104.3 C/W + Ta
    Tj = 52.15 C + Ta

    As you can see for the JEDEC Hi-K board, your junction temperature will be ~52 C higher than your ambient temperature. Keep in mind that TLV1117C is designed, characterized, and tested for an operating junction temperature from 0 to 125 C.

    Now let us calculate for the TLV743P which has a Rtheta_ja of 228.4 C/W in the DBV package:

    Tj = (Vin - Vout) x Iout x Rtheta_ja + Ta
    Tj = (5 V - 3.3 V) x 0.2 A x 228.4 C/W + Ta
    Tj = 77.656 C + Ta

    As you can see for the JEDEC Hi-K board, your junction temperature will be ~78 C higher than your ambient temperature. Keep in mind that TLV743P is designed, characterized, and tested for an operating junction temperature from -40 to 125 C.

    You are correct that layout is critical to thermal performance. The primary heatsink for both devices is the PCB copper local to the IC. As such by maximizing the PCB copper local to the IC, you will achieve the best thermal performance in your application.

    There is not a concern with using 11 uF (10 uF || 1 uF) on the output of TLV1117. If you expect the possibility of reverse current in your application due to Vout exceeding Vin, you will want to add external circuitry to protect the regulator from the reverse current. This is most commonly done by placing a Schottky diode from OUT to IN. The diode provides a robust path for the reverse current.

    Very Respectfully,
    Ryan
  • 0 in reply to Ryan Eslinger
    Expert 2075 points

    Hi Ryan, 

    Thanks for your detailed explanations,

    I Understood the concept,

    Can you please help me to select a suitable schottky diode for reverse current protection, Just give me some spec to select the schottky diode.

    Thanks

    Regards,

    Manjunath G 

    Hardware Design Engineer

  • 0 in reply to Manjunath G14
    TI__Guru 55940 points

    Hi Manjunath,

    For TLV1117 the diode does not strictly need to be a Schottky as the datasheet actually references a 1N4002 diode for this purpose.

    Newer LDOs will often have an Absolute Maximum rating for Vout of Vin + 0.3 V or as in the case of TLV743P will have an Absolute Maximum rating for Vout that is lower than the Vin range.  Schottky diodes are commonly used for reverse current protection due to their low forward voltage.  If choosing a Schottky for reverse current, we are generally looking for a diode with a forward voltage at 0.3 V at 10 to 100 mA.  You should be able to find these specifications in the diode's datasheet and/or a parametric search at the diode manufacturer's website or distribution partner.

    Very Respectfully,

    Ryan

  • 0 in reply to Ryan Eslinger
    Expert 2075 points

    Hi Ryan,

    Thanks for reviewing the design and also for the detailed explanations.

    Regards

    Manjunath G

    Hardware design Engineer