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UA78M: being used to power a ISO308x and would like reverse polarity protection.

Josue Lambert
Prodigy 70 points
Part Number: UA78M
Other Parts Discussed in Thread: TPS709, TPS7A26, TLV767, , LM5118, LM5117

Hello,

I am working on a new design in a highly inductive environment with 48V PWM switching on solenoids. Our limited testing revealed that we have current flowing through the low voltage ground, causing our I2C bus to miss commands. (The current raises the ground high enough that with noise and ground swings we missed some I2C commands) In order to mitigate the problem, we plan to isolate the communications from the rest of the board using an RS485 with opto coupling (ISO308x). I was looking at the datasheet for the UA78M05CDCYR and it looks like it should have enough power to drive the RS485 high side. We plan to have 12V Differential signals and a +12V computer power supply at the input jack to the board.

With that in mind, I think this power regulator should be more than sufficient to power the RS485 chip, is this assumption true?

Also, for reverse voltage protection, I was considering using a schottky diode (ST P/N STPS1L40ZFY) but aren't sure how much negative current the voltage regulator can handle. I chose a schottky diode with a reverse voltage of ~30uA and a foward voltage drop of ~0.25V.

Any recommendations would be greatly appreciate!

Thank you,

Josue Cruz-Lambert

  • 0
    TI__Genius 10200 points

    Hi Josue,

    Looking at the datasheet specs for the ISO308x, the UA78M should not have any problems powering the logic-side or bus-side supplies. Based on my understand of your application's input voltage (12 V), output voltage (3.3 V to 5 V), and output current (less than 25 mA), I would also like to suggest the TLV767 or the TPS7A26 as these are some newer devices with additional features and improved performance. The TPS709 is another option that includes built-in reverse current protection. You can find more LDOs based on any additional requirements using the parametric search on the TI website here:

    http://www.ti.com/power-management/linear-regulators-ldo/products.html

    If any amount of reverse current is expected in your application, it is recommended to choose a Schottky diode with a forward voltage drop of ~0.3 V at 10 mA to 100 mA forward current. This will ensure that the Schottky diode protects the internal body diode of the LDO by clamping the output to input differential to 0.3 V. Looking at Figure 11 of the datasheet, the STPS1L40ZFY fits this requirement. 

    Thanks,

    Gerard Copeland

  • 0 in reply to Gerard Copeland
    Prodigy 70 points

    Hi Gerard,

    Thank you so much for you input and recommendations. We are building an amusement machine and have found a centralized power ground is causing leakage currents on our ground. The cabinet of the amusement machine host a RS485 controller while the are boards distributed about the cabinet that will have RS485 OptoIsolated transmitters chips.

    I think the TPS709 is the most attractive option for it's built in reverse polarity protection. We expect our customer to force plug in a cable backwards at some point since these are customer serviceable parts. Going forward with the TPS709 would also remove the need for a Schottky diode correct?

    We are planing for two dedicated 48VDC inputs on our board, one is low current, regulated to 5V for the RS485 Opto Isolated chip only, while the other 48V is a high current that will be regulated to 5 & 12. I could use a recommendation for the 48V @ 12.5A reverse polarity. I seen some note on TI comparing a smart diode to a PMOS with it's gate tied to ground.

    The planed structure is to have the power regulated from 48V to 12V. From the 12V we would regulated again down to two 5V. My thinking is that the MCU would benefit from the high precision output of the TPS7A26. The other 5V output would be about 10A peak with nominal 3A used for LEDs & Servos. I am not sure on what the best practices would be for the 5V @ 3A.

    12V would be used by stepper motors, DC motors, and N-Mos Gate Drivers. I am thinking of using the LM5118 for 48V to 12V regulation The Gate drivers are driving mosfets to control various solenoids at 48V, with inrush currents of 12A & transients around the 4-8A @48VDC.

    I started reviewing some of TI DRV chips but they all seem to handle much lower current than a 4ohm, 5.1 mH inductors we have.

  • 0 in reply to Josue Lambert
    TI__Genius 10200 points

    Hi Josue,

    You are correct, the TPS709 would not require an external Schottky diode for reverse current protection. Please keep in mind that the minimum recommended input voltage (2.7 V) is required to ensure that the reverse current protection circuitry is fully functional. When using the 5 V output version, the maximum voltage that can be applied at the OUT pin is 6.5 V. If these requirements cannot be met in your application, I suggest using the TLV767 with an external Schottky diode. This LDO has similar accuracy specs to the TPS7A26 with better PSRR which will be necessary to suppress the switching noise generated by the preceding 48 V to 12 V DC-DC regulator. 

    For better efficiency and thermal performance, switching regulators should be used instead of LDOs for any conversions involving large step-downs and high output currents. As you have been looking at the LM5118, I will be looping in someone from the boost converter and controller solutions team to provide additional advice and any other recommendations for your power design. 

    Thanks,

    Gerard

  • 0 in reply to Gerard Copeland
    Prodigy 70 points

    Gerald, 

    Thank you so much for you input. I am considering using the LM5117 to step down the noisy 48V to 12V. From the 12V, I plan to use an LM5118 for the 5V high current and a TLV767 for the 5V Micro controller/IC devices. We will also us another TLV767 for the RS485 isolated side. Lastly we plan to use TLV75533 for the 3.3V being feed from the 5V TLV767. I am thinking of using the LM5056 as feedback for the Microcontroller to read the power output at the inductors. I am going to review some of the forums for similar applications for these chips. Should I open separate post for each of these chips?

    Also, we are planning to order evaluation boards for rapid testing of the chips performance in our application. I look forward from hearing from your boost converter and controllers solutions teams advice/recommendations.

    Sincerely,

    Josue