LM5088-2 BUCK, 34-44V IN, 12V OUT. Not Regulating at open load?

Hey Ian,

I looked over your schematic and have noticed couple things. 

For instance, the Rt value of 25.5 Kohm sets the switching freq. around 250kHz, For 250Khz, your inductor value is too small.  it needs to be around 22uH.    Also, for Vout greater than 5V, it needs a Rramp resistor.  Please see page 17 of the datasheet.  

My suggestion is to use the WEBENCH Designer Tool or use the quick calculator excel to check your component calculations again. I have included the links below and I hope this information is helpful.

http://www.ti.com/product/LM5088

http://www.ti.com/tool/lm25088-5088quick-calc

Hi Ian

In case that pre-charge switching is not enough to keep BOOT-SW voltage (C_BOOT voltage) greater than 3V BOOT UVLO. IC stops switching until  C_BOOT voltage is greater than 3V BOOT UVLO (Usually restarts when Vout<5V).  Adding a small minimum load (390R in your case) can prevent this behavior. Also, in some cases, increasing C_BOOT and C_VCC together helps.

Regards,

Eric

Hi Howard,

    My design is using a Ipp of 40% of full load as the basis of the inductor calculation (Hence 10uH instead of 20uH when using a 20% Ipp), I noticed the lack of ramp resistor and have rigged a 220k ramp resistor to the board (between RAMP and VCC) in its current configuration but it made no difference to the output??

I havent had much luck with the WEBENCH deisgner as it wouldnt accept my entry for the switching frequency?? I did use the excel spreadsheet however in the early stages.

Best Regards,

Hi Eric,

      I have uploaded the scope traces of the BOOT pin both in a 2.5ms timebase and 5us timebase. Seems like it stays above 3V most of the time(??), i have taken into account the maximum gate charge stated in the datasheet for Q1. I will at some point try a larger Cboot capacitor (470nF??) and see if it helps?

Best Regards,

I used this part before and also have problem with light/no load.  The mark up in my schematic is the change that make my circuit to work with no load.

Hi Tony,

     Just looking at your markup now, It looks like you increased Cramp to 2200pF, increased Ccomp to 0.027uF and added a 1000pF capacitor (Chf) at the comp pin. Did this 1000pF capacitor connect in parallel with both Ccomp and Rcomp or did it connect ONLY across Rcomp??

Hi Ian

I might be wrong, but I meant BOOT to SW voltage not BOOT to GND voltage. If you have a differential probe, you can measure BOOT-SW voltage.

Regards,

Eric

only across Rcomp. 
Please response to see if this change work for your circuit.

Hi Eric,

I have taken a scope trace of the two voltages subtracted (BOOT - SW) at 2.5ms and 5us timebase.

Looking back over the Chb calculation for the bootstrap capacitor i get the following equation:

Chb ≥ Qg / ΔVhb

Where Qg is the gate charge of Q1 (maximum of 86nC) and ΔVhb is normally 5% of "The Vcc Voltage". Now basing off the Excel spreadsheet I took "The Vcc Voltage" to be the max gate-source voltage of Q1 which is 20V. Hence 86nC / (20 * 0.05) = 86nF.

Should I have based ΔVhb on 5% of Vcc on the LM5088 which is 7.8V?? If so that would suggest my Cboot is far too small (about half the size it should be).

Hello all,

I feel the phrase RTFM being thrown at me already but......:)

See the new schematic which details the changes to the circuit. I have swapped out the components I felt were incrrectly calculated and added the Rramp resistor mentioned earlier and the output is now stable at open load!

Many thanks for all your helpful replies as it has made me backtrack into my calcs again.

Please note the original schematic should have read 440uF for the output capacitance instead of 220uF.

I still feel I might investigate increasing Cboot as a result of what I saw in the earlier MATH trace.

Best Regards and thanks again. :)

EDIT: Once I have done some more testing at higher currents I will report back the progress. :)

Hi Ladies and Gents (Again!)

I have now managed to get some production testing done on  batch of these and we have noticed that the output voltage drops once there is some load on the device, generally beyond 2A we are atleast 5% out of regulation.

I have already suggested the Chb (or Cboot) is increased from 0.1uF to 1uF as the calculations suggest we need atleast ~250nF. This has resulted in slightly worse performance.

I have someone at the moment increasing Cvcc also since the quick start spreadsheet seems to imply it needs to be larger in proportion to Cboot.

I will post back the results of this next change but is there anything you people could suggest?

BTW the output even when out of regulation is quite stable with hardly any ripple, it is just too low! :)

Best Regards,

Hello again all,

The board has been tested again with increased Cvcc also and to no effect, infact the voltage is slightly worse on the output at load.

Any suggestions? (I will be taking some scope traces in the meantime).