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?

  • \$\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, 2012 at 0:34
  • \$\begingroup\$ Forgot to mention, those measurements are taken at the Vin pin. \$\endgroup\$ Oct 20, 2012 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\$ Oct 20, 2012 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\$ Oct 20, 2012 at 0:57
  • \$\begingroup\$ I fixed it. I will add my answer to my own question. \$\endgroup\$ Oct 20, 2012 at 0:58

1 Answer 1


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).

  • \$\begingroup\$ I was about to bring up that point next! :-) \$\endgroup\$
    – Dave Tweed
    Oct 20, 2012 at 1:02

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.