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LM3409HVEVAL: module stopped working after minor changes

felipe carvalho
Prodigy 150 points
Part Number: LM3409HVEVAL
Other Parts Discussed in Thread: LM3409HV, LM3409

I received the LM3409HV EVM and did some quick tests to make sure it was working okay,which it was.

I then replaced R6 for a 30k resistor and R9 for a 82m resistor. After switching the EVM on I didn't get the expected 36.5V output (with load connected), I measured 48V (Vin) instead.

I then reverted the parts back to their original values, but still no result.

As a last resort I tried replacing the IC, but still didn't get the expected voltage output.

It seems as if the EVM is working at 100% duty cycle because when I change the load, the current supplied by the EVM also changes.

I removed the pfet from the EVM and tested it independently and it seemed to be working fine as well.

What is really puzzling is the fact that I can't see any oscillation at the gate of the PFET. 

I have also noticed another result which I still haven't found an explanation...The voltage at the VCC pin sits at 42.5V (for 48 Vin). I replaced the capacitor connected between Vin and VCC,but that had no effect.

The reason I mention this is odd is because the VCC pin max rating is 7V.

It would be really helpful if there was a free spice model, or webench simulation, that I could use for reference of working behaviour.

My goal was to have the driver designed such that I could have a 36.5V / 3A (max) output with a ILpp ~ 150mA.

Any advice/ suggestions are welcomed.

Thanks.

  • 0
    TI__Mastermind 28655 points

    Hello Felipe,

    VCC is referenced to Vin, 48V -42.5V = 6.5V.

    Are R6 and R9 the only changes made?

    It probably should have worked, did you go through all of the design equations to make sure all of the parts on the board can handle the higher current?  The MOSFET is a bit small for this and will get hot if you run at 3A.  The inductor is near saturation but it seems it should still work.  Did you check to see if R9 is 82 mohm and also that it is not open?  What size of resistor did you use for R9?

    The LM3409 has been designed to run much higher in other designs so it's not limited.

    DId you check the MOSFET to see if it was shorted?

    Best Regards,

  • 0 in reply to Irwin Nederbragt
    Prodigy 150 points

    Hi Irwin,

    Thank you for your reply. R6 and R9 were the only changes I had made. I checked all the points you mentioned, but I still couldn't find any culprits. I even replaced the PFET as a precaution.

    Having said that, I think I "might" have found an error on my part...but I'm still missing some detail as the results I got during my new tests don't seem to add up.

    I'm glad that VCC is referenced to Vin, which means I haven't damaged the other 2 ICs I used previously. I automatically read VCC as being referenced to GND (but confirmed its not the case).

    Onto the issue at hand...

    To test the EVM the very first time, I connected a 29ohm load to draw a current close to the nominal 1.5A specified. Now I don't recall 100%, but, at the time, I'm sure I measured the voltage drop across the load at around 42V (for 48Vin).

    After the R6 and R9 changes, the voltage drop across the same load is 48V (Vin basically). This remained true even after reverting R6 and R9 back to their original values.

    However, if I connect a power supply to VADJ, I can modulate the output current/voltage correctly. Is this its correct behavior? i.e. with VADJ floating, duty cycle=100%  (i.e.Vo=Vin) and with VADJ connected to 0-1.24V, duty cycle variable.

    I will try to get the spice model running on LTspice to compare results, but I would appreciate it if you can provide further input.

    kind regards,

    Felipe

  • 0 in reply to felipe carvalho
    TI__Mastermind 28655 points

    Hello Felipe,

    A 29 ohm load at 1.5A is 43.5V.  At 3A it is 87V (not possible).  The LM3409 will just turn on and never hit peak current threshold.  If the peak threshold cannot be reached the MOSFET just stays on.  The load resistance needs to be lower, 15 ohms or less for 3A at 48V input.

    Viadj is meant for DC voltages, EN is meant for PWM dimming however if you PWM dim at 3A for a 29 ohm load you might as well just switch a MOSFET off and on at the PWM frequency since that is all the LM3409 will do.

    If you change Viadj with a DC voltage and turn it lower eventually the LM3409 will go into regulation, below 1.6A (depending on ripple current and output capacitor).

    Best Regards,

  • 0 in reply to Irwin Nederbragt
    Prodigy 150 points

    Hi Irwin,

    Thank you for your reply.

    Perhaps I should have made it clearer that the operation I mentioned in my previous post (29 ohm load) was AFTER I reverting R6 and R9 to their original values. In other words, as the EVM was delivered. I did this to go back to a "known state"

    Nevertheless, I will run a few more tests tomorrow to check if this solves the problem.

    Best Regards,

  • 0 in reply to Irwin Nederbragt
    Prodigy 150 points

    Hi Irwin,

    This morning I connected a 29ohm load to the EVM in its original form (i.e. as it was delivered) and the voltage at the output was 48V (also Vin). The current measured was just over 1.5A.

    When I inserted an even higher load (20ohms), the output voltage dropped to 31V, which is expected.

    This implies that the driver is working correctly, however, I don't understand why with a 29ohm load (~1.5A) the voltage output is not regulating to 43V.

    This whole issue arose because I didn't use a higher load in order to stay within the boundaries of the design and prevent damage to the EVM.

    It still not clear to me why with a 1.5A load the output did not regulate, unless 42V is the maximum voltage output possible before the EVM goes into 100% duty cycle.

    Once I hear from you regarding this latest result I will consider this case closed.

    Best Regards,

  • 0 in reply to felipe carvalho
    TI__Mastermind 28655 points

    Hello Felipe,

    From a few posts up "The LM3409 will just turn on and never hit peak current threshold.  If the peak threshold cannot be reached the MOSFET just stays on."  The LM3409 doesn't regulate average current.  It is a hysteretic controller that uses a constant off-time to keep the ripple constant.  For example if the output is 1.5A average and there is 500 mA peak to peak current ripple the peak current regulation threshold is 1.5A + 500 mA/2 = 1.75A.  This is an LED driver, driving LEDs isn't the same as driving a resistor, so when testing this with a resistor load the voltage ripple can be large (depending on the output capacitance).  So the LM3409 is turning on and staying on because it is not reaching peak current threshold.  You can have a resistor load you just have to account for that in the design.

    Best Regards,

  • 0 in reply to Irwin Nederbragt
    Prodigy 150 points

    Hello Irwin,

    Thank you for your reply and help.

    I did take into account the effects of a resistor load,but what was puzzling me was the fact that with 29ohm load I can get ~1.5A to be drawn from the EVM (ampere meter in series with load) and the output voltage doesn't regulate. However, when increasing the load (lower resistor) I can see that the output now regulates.

    I think I just got unlucky to have had my load just outside the value required. I started with a resistor load to make sure everything was set correctly to get the correct operation, before connecting LEDs as a load.

    In any case, I believe this has now been resolved and will close the ticket.

    Best Regards,