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LM2676: LM2676S-ADJ 3A alternative

Galtié Julien
Prodigy 40 points
Part Number: LM2676
Other Parts Discussed in Thread: LM2673, , LM22676

Dear support,

We need an alternative solution pin to pin compatible for the LM2676SX-ADJ and I found that the LM2673S-ADJ could be similar with same performances (we need 3A without current limiting).

I only note that the LM2673 proposes a current limit pin and a softstart function. 

On our current the LM2676 design, current limit pin is let open and softstart function is on/off function controlled by microcontroller. Can the LM2673 be controlled transparently by this LM2676 hardware without affecting the output current behavior and the enabling?

Many thanks in advance.

Best Regards,

Julien

  • 0
    TI__Mastermind 30550 points

    Hello Julien,

    The external componets for stabilility (output filter) should remain the same between both devices as it looks the same compensation is utilized for both devices.

    In regards to using the LM2673 with a LM276 schematic, it does look like you need to consider pin5 (current adjust) and pin7 (ss), which differ from LM2676 pin defintions.

    you can leave SS pin open per the datasheet.

    I will ask the designer for the behavior of LM2673 if pin5 is left open.

  • 0 in reply to Marshall_Beck
    TI__Mastermind 30550 points

    Hello Julien, I evaluated on the EVM and with RCL open, it seems the current limit is extremely low. For the given evm BOM at 12Vin, I was only able to get about 15mA before it started current limiting. 

  • 0 in reply to Marshall_Beck
    Prodigy 40 points

    Dear Marshall, many thanks to take time on this issue for testing and giving feedbacks. Is it a normal behavior this very low current limiting? If RCL is open, according to the datasheet shouldn't the current be non limited?

    Also regarding pin (7) SS, on LM2676 we set pin (7) ONOFF high and low depending of the enabling. What is the behavior of this control on LM2673? Is it enable every time? Is there any behavior on the starting of the IC?

    Thank you.

  • 0 in reply to Galtié Julien
    TI__Mastermind 19070 points

    Hello Julien, 

    Marshall is out of the office but I will follow up on his behalf. 

    In terms of soft start, the datasheet states "If soft start is not required this pin must be left open circuited." If you drive the pin directly LOW/HIGH as ON/OFF, I think you will affect the soft start function of the part and this could result in overshoot on the output at start up. This is something you will have to check with your specific loading in the application. 

    I will double-check on the current limit function and whether the pin can float. I hope to provide some more feedback in a day. 

    Regards, 

    Denislav

  • 0 in reply to Denislav Petkov
    Prodigy 40 points

    Hello Denislav,

    Thank you for your informations. We will try to let pin 7 floating or cut in order to not affect the behavior of our application. Do you have any news regarding the current limit function?

    Best Regards,

    Julien

  • 0 in reply to Galtié Julien
    TI__Mastermind 19070 points

    Hello Julien, 

    Yes, the part needs to have a resistor on the current limit pin to set the threshold. 

    The current limit is = 37125/Radj as described in section 7.3.5.

    If Radj is open the current limit will be very low. You can see this in the expression above, by setting Radj to a very high value (e.g. open). This also aligns with Marshall's test above and simulation (webench) confirms it as well.

    Regards, 

    Denislav

  • 0 in reply to Denislav Petkov
    Prodigy 40 points

    Hello Denislav,

    Thank you for the answer. My layout doesn't integrate this resistor and I would like to avoid modification. Resistor value is really necessary or if I solder directly the pin 5 to pin 4 (GND), it will work at full power without added limitation as on LM2676?

    Best Regards,

    Julien Galtié

  • 0 in reply to Galtié Julien
    TI__Mastermind 19070 points

    Hi Julien, 

    Understand on the layout, you are trying to fit the device into the LM2676 schematic. Unfortunately, it looks like you will need a resistor. Otherwise the current limit will be very low and you won't be able to get any output current. 

    You can also take a look at the LM22676, which is a more modern converter with the same pinout for the surface mount parts. It may fit, but the switching frequency is nearly double so you will have to check stability with your existing L and Cout values. 

    Regards, 
    Denislav