• State
  • Locked Locked
  • Replies 4 replies
  • Subscribers 63 subscribers
  • Views 695 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

LM3429 Boost Unstable Iout and GATE sw issue.

Ricardo Carballo
Prodigy 30 points
Other Parts Discussed in Thread: LM3429

Hello We are using the LM3429 Chip

Vin 10.3 min 28 Vin max

V outmax =40V I out =2.1Amps

When we tested the unit we receive a screech from the inductor, looking at the waveform, the inductor is bouncing from discontinuous to continuous conduction mode.  Looking at the Drain wave form the voltage indicates that the duty cycle is changing for no reason ( no Vin change) When this occurs it discharges the inductor and so it goes DCM and then tries CCM for little bit.

Since we are using analog dimming we have 1uF on the compensation Capacitor,

1 K ohm for each of the RHSN and RHSP,  RCHS is 12.4K ,  Cin is 20uF and Cout is 20uF. We have tried with 5uH 10uH and 20uH inductors.

Another effect that is seen is that during a particular voltage say 16V , the output stabilizes but outputs far more current than the calculation says should be possible, to ensure that this was not an effect of the trace resistance various resistors R150 R130 R100 have been used but both overshoot the 2.1Amp limit ( This should take a resistor of 0.047). If we exceed 20V , then the current drops to 600mA total, but is stable again but in DCM.  Using Webench we matched exactly what we had in our circuit and we still get the same results.

Thank you for your assistance.

  • 0
    TI__Expert 3495 points
    Ricardo:
    What inductors are you using? Are they big enough to handle the current? Can you show me the schematic?
    David Zhang
  • 0 in reply to David Zhang
    Prodigy 30 points
    Hello David, The inductor we are using is a 10uH 9A rms inductors with sat current at 16A. We have placed two of these in parallel as it yielded closer to new calculations allowing for approx 35% ripple from Lirms. The stability improved slightly from changing to 5uH but the issue remain. As for the schematic, essentially it is the Webench schematic for the boost , we have incorporated some proprietary circuits as well, I am trying to filter those out, but the WEbench sim is exact for the boost side of the circuit, even the Rsense of 0.047 is correct. We did decrease the I limit to 20A just in case as before it was 10A with 0.025 ohm resistor. this still did not provide any greater efficiency increase. Is there a way that I can send you the layout privately, as this may perhaps helps to indicate a layout issue.

    Thank you.
  • 0 in reply to Ricardo Carballo
    TI__Expert 3495 points
    Richardo: You can send the layout to me david.zhang@ti.com.David Zhang
  • 0 in reply to David Zhang
    Prodigy 30 points
    Thank you David, I have fwd the email along, We have tried placing chip ferrites on the RHSP and RHSN to clean up some of the noise, and it has helped, but there is still some instability at certain Vin's and the Iout increases dramatically above 20V. Reading the forums, someone had suggested adding a resistor to C comp ( from Agnd to C Comp), currently I have a 200ohm resistor and this is what has helped the most for stability, I am slowly lowering this value which gives the best results in comparison to no resistor. From my analysis it seems that doing this will slow the discharge of the capacitor and also help prevent logic from sending a high value to the SR flip flop which would have created the null that was seen during operation.