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LMX2492 Power Supply Architecture

Alber, Moon
Intellectual 720 points
Other Parts Discussed in Thread: LMX2492, LP5900

Hello,

I'm Alber as a FAE in Arrow Korea.

My customer tested LMX2492 device on their board.

Following picture is their application.

Case 1 is their simple schematic.

They use a resister to voltage drop from 5.0V to 3.3V.

at that time, LMX2492 did not operate.

Customer tested again after modified at Case 2.

at that time, LMX2492 were woking correctly.

If Power Rail in LMX2492 use at Case 1, is there any problem?

  • 0
    Intellectual 720 points

    Hello

    Would you please give any feedback?

  • 0 in reply to Alber, Moon
    Intellectual 720 points
    Hello
    Would you please give any feedback about my question?
  • 0 in reply to Alber, Moon
    TI__Expert 5370 points
    Dear Alber,

    I am afraid you can not use this form of regulation to lower the voltage to 3.3 V.

    The requirements on most modern devices with Vcc =3.3 V is to have a fairly low output impedance with a contant voltage able to source the required current. By placing a series resistor of 680 ohms, you will see the voltage moving a lot with respect to the current load. I am not surprised it does not work. please consider a voltage regulator or a seperate power supply.

    In term of equivalent circuit you are looking for a voltage source with the lowest output impedance (series z with it) you can get.

    I hope this helps,
    Regards, Simon.
  • 0 in reply to Simon Damphousse
    Intellectual 720 points

    Hi, Simon

    Thank you for your feedback.

    But the customer's board is quite small about 12mm x 12mm or 9mm x 9mm.

    They have to design VCO circuit, PLL circuit(include charge pume) and MCU within this size.

    They can not design the independent LDO in this size.

    Is there any way to supply 3.3V PLL IC from one soruce voltage(5.0V) without using LDO?

  • 0 in reply to Alber, Moon
    TI__Expert 5370 points

    Dear Alber,

    I understand the constraint you are working with. I suggest to reach out to one of the voltage regulator team and see what can do to provide a small form factor and the required voltage supply. I would not not be surprised if they have a small single package regulator with dual output ...

    Regards, Simon.

  • 0 in reply to Simon Damphousse
    TI__Expert 5370 points
    a second thought, I would try webench. regards, Simon.
  • 0 in reply to Simon Damphousse
    Intellectual 720 points

    Hi, Simon

    Wolud you please give any advice for my customer design? without using additional LDOs....

  • 0 in reply to Alber, Moon
    TI__Mastermind 28001 points

    Alber,


    The PLL charge pump can run at 5 volts, but you must supply 3.3 V to the other PLL, not 5V.  

    If the customer is unwilling to use an LDO, theoretically they could use a series resistor or some diodes to drop the voltage, but the current consumption through the device will change based on how it is configured and if it powers down, the voltage a the pin would go back to 5V if you use the series resistor.

    Although the customer says they do not want an LDO, have they considered all the options?

    The LP5900 has excellent performance and comes in a 4 pin package that is about 1.1 x 1.1 mm and it requires 2 caps.

    REgards,
    Dean

  • 0 in reply to Dean Banerjee
    Intellectual 720 points

    Hi, Dean

    Thank you for your feedback.

    I checked you said it on EVK.

    Using Serise R about 33 ohm, after configured LMX2492, the VCC Level is about 3.34V.

    And LMX2492 were working OK that the Lock LED go on.

    But, as you mentioned, before configured LMX2492, there was about 4.8V.

    Customer don't use 'Power Down Mode'.

    After Power up, only one time configured.

    So, 4.8V supply at Vcc Pin in very short time(about few seconds).

    After configured, 3.3V will supply without changed soruce voltage(5V).

    Of course, I know that there was decribed the absolute Vcc Voltage is 3.6V on datasheet.

    This will be a problem? such as MTBF or damage to device... and so on...