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I'm experiencing some high-pitched squealing noise from my boost converter, I'm getting some resonating frequency when I start increasing the load current.The inductor is a 47uH inductor. The part number for the diode is MBRD640CTT4G.

This is my schematic:

schematic [1]

I checked the power line with the scope and this is the 12V line @ 100mA using AC Coupling:

scope trace #1 [2]

As you can see, the ripple noise isn't so bad, I'm getting about 200mV peak to peak.

But when I run it at 500mA load this is what happens:

scope trace #2 [3]

The ripple is horrible, I'm getting about 4V p-to-p.

Any ideas what is going on?

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  • \$\begingroup\$ Try putting the scope on the 12V source; it may be failing to deliver enough current. Otherwise, it looks like the regulator's control loop is going unstable, but I can't immediately see why this would be the case. \$\endgroup\$ – Dave Tweed Oct 20 '12 at 0:34
  • \$\begingroup\$ Forgot to mention, those measurements are taken at the Vin pin. \$\endgroup\$ – Gustavo Corona Oct 20 '12 at 0:43
  • \$\begingroup\$ Dave, I am using a BK Precision Bench power supply, and it has enough juice for this circuit. I'm capping it at 8A, so no problems there. \$\endgroup\$ – Gustavo Corona Oct 20 '12 at 0:44
  • \$\begingroup\$ If you suspect that control loop goes unstable, try adding a small capacitor between Vout and FB. Somewhere between 100pF and 10000pF. BTW, the datasheet for LT1268 is only 4 pages. No "Principles of Operation" chapter. No "Application Info" chapter. This is uncharacteristic of Linear Technology. \$\endgroup\$ – Nick Alexeev Oct 20 '12 at 0:57
  • \$\begingroup\$ I fixed it. I will add my answer to my own question. \$\endgroup\$ – Gustavo Corona Oct 20 '12 at 0:58
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I went into AN19 http://cds.linear.com/docs/Application%20Note/an19fc.pdf and it has a section for Frequency Compensation on pin VC. I simply increased the capacitance to 1uF and kept the resistor on 1K and I no longer have this problem. I am going to talk to Linear Tech to better fine tune this circuit and see if there is a more optimal value for that Capacitor (C112).

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  • \$\begingroup\$ I was about to bring up that point next! :-) \$\endgroup\$ – Dave Tweed Oct 20 '12 at 1:02

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