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LM2937: Is it ok to use 6V as input to generate 3.3V?

Howard Hu
Prodigy 10 points
Part Number: LM2937

We had one IT part LM2937IMP-3.3/NOPB to generate 3.3V with 12V input.

Now we plan to reduce the input voltage from 12V to 6V, in order to get the higher efficiency.

I checked on your website and looks good as its input could be 6V~26V. Would you help to double check and make sure enough design margin if we change it from 12V to 6V as input?

  • 0
    Prodigy 10 points

    Is 6V input to generate 3.3V, so that to get the maximum efficiency? The output current will be 10mA to 100mA. 

  • +1 in reply to Howard Hu
    TI__Genius 12511 points

    Hi Howard,

    The dropout voltage for the device is well below the 2.7V of headroom that you are operating under. Additionally there is no UVLO for this device so there is not a risk of triggering a UVLO condition. Finally, from the ROC the minimum VIN that we recommend is VOUT+1V:

    So 4.3V would be the minimum that we could recommend using the part at. With 6V of input you have clear headroom and the device will operate well for your 10-100mA load.

    As for efficiency, LDO's are inherently dissipative devices. Efficiency is not usually a concern for the LDO itself as Pin-Pout will be a result of system design and not the LDO's efficiency. Decreasing input to the minimum will maximize the efficiency of the system as you minimize the dissipation across the LDO device. Maximum efficiency would be found in switchers with an LDO at the output to clean up the switcher noise. However moving VIN from 12V to 6V will increase the total efficiency of the system as the LDO will burn less power.

    Regards,

    John

  • 0 in reply to John Schneider
    Prodigy 10 points

    Hi John,

    Sounds exciting and a valuable improvement to enhance the efficiency. We used a 1394 cable to power the module, so the efficiency's improvement will finally reduce the voltage loss through the cable. 

    According to your description, even the 4.3V would be acceptable, because the output is 3.3V. Right? And now I realized that there was an ORing diode MBR0540T1G, with the maximum Vf 0.62V, so the input voltage should be no less than 5V.

    If I use 6V as input, there will be still enough design margin more than 15%.

    Thanks for your support.

    Please help to confirm or point out any misunderstanding.

    Regards, Howard 

  • 0 in reply to Howard Hu
    TI__Genius 12511 points

    Hi Howard,

    Yes, that is correct according to the datasheet the part is recommended to operated down to 4.3V = VIN in your particular case. With the diode, this would be 4.92V. Seeing as the setup is currently at 6V VIN there is more that +-15% on 4.92V (5.658V).

    Regards,

    John

  • 0 in reply to John Schneider
    Prodigy 10 points

    HI John,

    Below is more result from my colleague: 

    " I just double checked the voltages in the circuit, and it has more voltage drop than considered before. But the required input voltage is still above the specification. It’s about 5.55V instead of 4.75V. Could you confirm this with TI?

    I don’t expect that the behavior changes that much over temperature and component variations. So, 7V supply voltage appear to be enough but I’d suggest using 8V.

     

    Measurement point

    Supply voltage 6V

    Supply voltage 6.5V

    Supply voltage 7V

    Supply voltage 8V

    A

    5.98V

    6.48V

    6.98

    7.98

    B

    5.64V

    6.13V

    6.63

    7.63

    C – LDO in

    5.10V

    5.55V

    6.05

    7.04

    D – LDO out

    3.187V

    3.29V

    3.29

    3.29

     

     "

  • 0 in reply to Howard Hu
    TI__Mastermind 43275 points

    Hi Howard,

    John is OoO today. He will get back to you tomorrow. 

    Regards,

    Nick

  • 0 in reply to Nick Butts
    TI__Genius 12511 points

    Hi Howard,

    The datasheet specifies that this dropout specification data covers over operating temperature. Additionally reading 5.1V at the LDO input shows that you are operating with 1.8V of headroom, but are not seeing VOUT correctly regulate that implies that something is going wrong in the circuit. However looking at the datasheet, this should not occur, I will look for devices to see if I can test it in our lab to verify what is being seen in the field.

    I would definitely recommend 7V or 8V supply voltage as they are seeing performance match the intended use case, but if efficiency is the goal, at 6.5V you are getting optimal efficiency and the part regulating.

    Regards,

    John