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LTM8064 shows small output voltage error (voltage drop) under load.  What is the normal drop?  Programmed Vout = 29.5V, but with 3.5A load it drops to ~28.5V

I have 3 of them in a load sharing circuit.  Under modest loads, everything it OK, but when I increase the load to ~3.5 - 4 A, there is a slight (~3%) drop in output voltage.  That seems a little high for a feedback regulated supply.  Sharing of load is not perfectly equal, but seems reasonably close based on the Imon outputs.  I didn't see any load regulation information in the data sheet (did I miss something?).  Output voltage "should be" 29.5V (and it's right on the money with light load) but drops to ~28.5V with the load.

I noticed the feedback voltage is slightly lower with the load (about 1.18-1.19 V with load, 1.22 without).  These voltages could be a little off due to the ground point I'm suing may have a slight voltage drop relative to the ground plane of the modules because of the high currents in the circuit board.

  • Make sure the Vout is measured directly across the output caps (close to Vout, bank3). There should be enough copper around the Vout on the board (especially if the load is not near the Vout). Check the current limt setup (CTRL1, CTRL2, input current limit). Since the power is pretty high, make sure there is no thermal issue. Thanks. 

  • Chaz,

    Thank you very much.  The drop is on the p.c. board between the LTM and the board terminals.

    Ray

  • Chaz,

    Solved and unsolved.  Your suggestion showed up a defect in the p.c. board layout - that's very helpful.  But I added wires to bridge over the inadequate traces.  I was surprised to find the problem was still there.  Under load, the output drops and the feedback voltage also drops a little. I don't think it's in current limiting because if I increase the "demand" signal (i.e. pull down on the feedback pin a little more) the output still goes up.  It acts almost as if it needs a little more internal gain, but I assume you have an integrator in the error feedback path, thus "infinite" gain?

    So I'm still perplexed.

    Ray

  • Hi Ray,

    The DC gain should be high enough. Not sure what's your Vin. Check the Vin waveform. Make sure it's high enough. Check CTRL1/CTRL2 voltages. Try to tie CTRL1/CTRL2 to Vref if possible.

    Chaz

  • Chaz,

    I have 48V input.  It's bypassed as in the layout in the data sheet.  For each of the three modules, I have 2 x 4.7 uF ceramic and 2 x 33 uF Aluminum poly bypass capacitors.  I don't see any substantial sag on the inputs when I scope it.  I can't tie the CTRL1/CTRL2 voltages to Vref for the two slave LTMs because there would not then be load sharing.  I also have a 3 phase clock sync (was not active in the above case) to reduce load ripple.  I'll let you know if anything else shows up.

    Ray

  • Chaz,

    Also, I'm running it near the high end of its switching frequency range.  Rt is 115 k.  I don't know if this has any effect.  Would it be better to dial down the frequency a little?

    Regards,

    Ray

  • Hi Ray,

    The 115K Rt should be fine. Check the CTRL1/CTRL2 voltage at least. Make sure it's high enough.

    Chaz

  • Chaz,

    Hope you had a nice weekend.

    CTRL1/CTRL2 as follows:

    Master: CTRL1/CTRL2 connected to VREF

    Slaves: CTRL1 = 0.388V

             CTRL2 connected to Vref.

    Also, I have found another difficulty.  When I apply the SYNC signal (at something like 450 kHz - I need to recheck the exact frequency) I get a sinusoidal looking ripple of approximately 2.6 kHz and approximately 1.68 V p-p.  I'm wondering if this is somehow to be expected.  I have 3 sync signals, all at the same frequency but 120 degrees apart.  I was hoping to use this to cause the 3 LTMs to run as a 3 phase supply to thereby lower the ripple amplitude, both on input and output ripple.  Any hints on this?

    Thanks for your help.

    Ray

  • Chaz,

    I found the cause of the 2.6 kHz oscillation - not a result of the SYNC operation.  It was a ground loop in the signal controlling the LTMs.  I'm brining up another example of the circuit board so I'll see if the maximum load issue persists.

    Again, thanks for the help.

    Ray

  • Hi Ray,

    When the CTRL1 voltage is low, the output current will be limited. (And the Vout will drop.) Please see data sheet page 11 for details (curves).

    Chaz