Dear people from LTC sent me an evaluation board for the LTC3864 DC/DC Controller with 100% Duty Cycle Capability. My intention is to use it as pre-regulator in the new design. The complete regulator circuit is really tiny and it’s located on both upper and bottom layer. PCB has 4-layer.

The board could deliver 5 V, 2 A with broad input voltage from 5 to 55 VDC. I tested it first without changing anything with load of little above 2 A (3.3 and 8.2 Ohm connected in parallel). The output (yellow) and input (cyan) ripple and noise is shown below (Vin=20VDC):

Improvement is visible with additional LC filter at the output (8.2uH + 22uF):

Next step was to modify a board a little to be able to deliver variable output depending of the post-regulator output. Taking into consideration the rating of the power inductor, mosfet and diode my intention was to not go over about 24 V with load of not more then 1.5 A. To accomplish that I first replace original voltage divider’s resistors used in feedback loop to set new output voltage. I got 24.5 V with 4.7K + 140K combination, and had a chance to desolder and solder 0402 size components for the first time :). Tested, everything works as expected so that in next step a “tracker” circuit (Q2) can be added.

Schematic of the modified evaluation board looks like this:

Additionally Q3 is added to test 100% duty cycle. It simply disconnect tracker from the feedback voltage divider (R3, R5) that has to be set very high – few volts above the max. input voltage. For mentioned combination that gives 24 V at the output, input voltage should not be higher then 20 V to insure that the controller will enter the 100% duty cycle mode. A breadboard is used for connecting additional tracker components:

For testing how LTC3864 board is working with tracker I’m using my PSU where first channel is set to 20 V and second one is used to control board output. First, I made a screenshot when no power is applied (to see "reference" noise):

On the following screenshot is shown output after the filter and switch node signal when control voltage is set to 10 V (load is 0.3 A):

And now when Q3 is disabled LTC3864 goes into 100% duty cycle mode. Switching frequency is zero and output ripple and noise are as clean as possible.

I also tested how external switching frequency sync is working. Here is an example when external frequency (blue trace) is set to 390 kHz (Vin=33 V, Vset=9 V):

Now, I have to make a PCB that should reproduce comparable results. Hopefully I'll succeed regardless two obvious restrictions: everything has to be done on a 2-layer PCB and with larger components. Both of them could make a whole thing more noisy and unpredictable, but could lower the total cost and offer more flexibility for experimenting with higher voltage and current. My intention is to deliver up to 40 V (+ few Volts) and min. 3 A.