Synchronous Buck Constant Current Source with LT3741

[S0syk]

Hello Everyone,

I curently working on adjustable constant current source with LT3741. Design specifications are:

Input voltage: 10 V - 15 V (lead battery)
Output current: 0 A - 3,3 A
Maximum output voltage: 5 V
Switching frequency: 200 kHz

Schematic:


Parts were chosen with datasheet equations and advices:
http://cds.linear.com/docs/en/datasheet/37411fd.pdf

Without a load i got nice 5.06 V ouput over whole input range.

Voltage waveform of switching node at high and low input volatage:


Problem is when i connect load i get waveform like this with audible noise from inductor.


With 1.8 ohm load I tested from lowest input current (Vctrl1 set to minimum). Power supply is working fine till about 1A (also regulating well over input range). Over 1 A there are again oscilations like on the picture and supply dont ragulate well. Are these subharmonic oscilations? LT3741 using average current mode control and gm OPAMPs.
I used equations from datasheet to calculate R14 and C11.

I think problems can be caused by:
a) bad layout (huge noise)
b) bad compensations parts (i know i need to set the downslope of the current signal comming into PWM comparator to be lower than slope of the PWM ramp, but i dont know if i did it properly)

I tried different values for R14 and C11. Nothing changed.
Here is layout:
top
http://www.dropbox.com/s/a4hfaaefjeu9z6o/top.png
bottom
http://www.dropbox.com/s/qxu0jzsfmqe9o17/bottom.png

Anyone have experience with average current mode compensation or with LT3741? Any advices? Thank you

 

[ronsimpson]

Your two top and bottom links are the same.
What is the part number on the inductor?

 

[S0syk]

Your two top and bottom links are the same.
What is the part number on the inductor?

Edited, Thank you.

Inductor is this DR125-180-R from coiltronics.

 

[ronsimpson]

It is hard to see from here. I think you are OK on the volts-seconds on the inductor.
The inductor should not have more than 19VuS. (19volts for 1uS)
The inductor saturates at 5.32A. I have not checked but you are probably close to that.
The inductor can handle 3.8A DC. So you 3.3A plus the AC losses will make the coil good and hot.