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LM2587 not working

Shiladri Chakraborty
Prodigy 20 points
Other Parts Discussed in Thread: LM2587

Hi,

I am using the LM2587 to obtain a +12V,-12V and 5V supply.

The circuit is the same as Fig. 29 of the datasheet.

However, I am facing the following problem :

As I slowly increase Vin, initially nothing happens. However, after Vin reaches 5V, the input current

suddenly increases (about 2-2.5A) and hits the current limit of the input supply. 

Any ideas what may be the problem?

I have double-checked the circuit so I am pretty sure there are no errors in the PCB as such or with

components. 

Initially, I am operating without connecting any external load-resistor. Do I need to operate with a load

resistor connected always?

  • 0
    TI__Expert 3870 points

    Hello Shiladri,

    To be able to better help you please attach a copy of your schematic and PCB layout files.  No, you don't need a load to operate and the device should not be hitting current limit without a load.  

    In the mean time please try the following to isolate root cause:

    1) Please remove the diodes of all 3 stages and power up again

    2) If you still see the problem, then check the transformer connection, IC shorts, or input cap shorts

    3) If you don't see the issue that means the problem is in one of your isolated rails

    4) Turn-off the power and place back the diode on one of the rails

    5) Turn the power back on and measure input current

    6) Repeat the problem until you find the rail that's pulling the supply down

    Best Regards,

    -Juan

  • 0 in reply to Juan Paulo Fung
    Prodigy 20 points

    Dear Juan,

    Thanks for your reply. I tried the things you mentioned.

    -First I removed all the secondary diodes but still the problem remained. Clearly this
    meant the problem is on the primary side.

    -Then I removed the transformer altogether from the board (virtually opening the primary)
    and then the current limit was not being hit

    -Next I connected a simple inductor between the terminals where the earlier transformer primary was
    and again on giving supply the problem reappeared.

    Where do you think things are going wrong?

    The schematic and PCB layout are inserted for your reference.

    Regards,
    Shiladri

  • 0 in reply to Shiladri Chakraborty
    TI__Expert 3870 points

    Hi Shiladri,

    So if the problem is clearly on the primary side, please check for short-to-ground connections with your board powered off.  It sounds as if you have a passive component that's damaged and pulling the supply down or there's damage to the IC already.  So ohm-out the IC pins for any short to grounds and see if that's the problem.  If so replace the IC and step by step populate the components of the secondary.

    Best Regards,

    -JP