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LM2674: Alternative for LM2674MX-5.0/NOPB

gilles mourier
Prodigy 230 points
Part Number: LM2674
Other Parts Discussed in Thread: LM2671, LM22675, LM22672

We have several designs using the LM2674MX-5.0 and this part seems very difficult to source at the moment.
I am looking for an alternative part pin to pin compatible which could be used as a replacement of the LM2674-5.0.

I found several TI parts which seem compatible (LM2267...) with a higher switch current and higher switching frequency.
At the moment I am thinking using either the LM22675-5.0 or the LM22672-5.0 which seem available.
I think the highest switching frequency 500kHz instead of 260kHz and the highest switch current 1.5A instead of 0.8A are not a major problem.

My main concern is the exposed pad at the bottom of the IC which needs to be connected to GND.
Our PCB using the LM2674 do not have a footprint to connect the bottom PAD of the LM2267... to GND.

Below are some questions:

1) Are they any problems in using a controller using a higher switching frequency and higher switch current?

2) What are the consequences of not connecting the exposed PAD of the LM22675-5.0 or LM22672-5.0 to GND?

3) What are your recommendations for an alternative to the LM2674MX-5.0 without modifying the PCB? 

Thanks for your support.

  • 0
    TI__Mastermind 33525 points

    Hi Gilles,

    Q1-----The inductor and output cap must be adjusted per data sheet description as LM2267X is voltage control mode.

    Q2-----better to check IC case top temperature at max Vin, Iout and Ta, there also should be entire ground plane under the PAD.

    Q3------how about LM2671?

    B R

    Andy

  • 0 in reply to Andy Chen
    Prodigy 230 points

    Hi Andy,

    Thanks for your reply. You are right, the LM2671 would be a better alternative, but unfortunately it does not seem to be more available than LM2674.
    That is why I first searched which IC compatible with the LM2674-5.0 were available without delay and found the LM22672 and LM22675.


    My main problem is the exposed PAD at the bottom of the IC as it would not be connected to GND. The footprint used on our existing PCBs does not have a pad for it.
    If that is not a problem to leave it floating, as long as the power dissipated does not cause an overheating of the chip, I would consider which components need changing.

    I would be happy to use any other reference as long as it could be sourced without delay and could be used as a replacement of the LM2674-5.0 on our existing PCBs.

    Regards

    Gilles 

  • 0 in reply to gilles mourier
    TI__Mastermind 33525 points

    Hi Gilles

    Pls check your PCB layout, there must be entire ground plane under the PAD, no noise trace like BOOT or SW routed under the PAD, otherwise the thermal pad will couple noise to small signal circuit of chip and may cause unstable operating.

    B R

    Andy

  • 0 in reply to Andy Chen
    Prodigy 230 points

    Hi Andy,

    Thanks for your reply, I am now a bit worried using a voltage control mode IC instead of a current mode, I did not realize it until you wrote it. Concerning the tracks under the IC, we have GND and the input voltage (see below picture). Do you think that could result in unstable operating?

    I really don't know what to do as it seems very difficult to source any IC directly compatible with the LM2674-5.0.
    I was even considering using an SO8 SMT header to fit a small daughter board with another PWM chip.

    What are your recommendations?

    Regards

    Gilles

  • 0 in reply to gilles mourier
    TI__Mastermind 33525 points

    Hi Gilles,

    There is no ground plane under the IC, the VIN trace has some noise but still OK. you can test IC with thermal PAD at highest VIN and load current to see if any unstable switching. you can add 5--10 ohm resistor in series with BOOT cap to reduce the SW noise.

    B R

    Andy