I am building a simple buck converter circuit with a current-mode control IC, the UC3843.
In the application note of the UC3843, a sample circuit is given:
I am completely clueless about how these values should be set up. Also, is this even the correct circuit to follow?
I want an oscillating frequency of 100 kHz, which this circuit includes. Desired input voltage 24 V; desired output voltage 6 V; desired output current 1 A.
I remember seeing this application circuit many years ago but never gave it a try. The thing is the poor efficiency considering the source/sink resistance of the driver (around 10 ohms). So it may be ok with sufficient input voltage, minimizing the drop.
One cool thing, however, is that it provides free synchronous rectification as the low-side transistor bypasses the freewheel diode during the off-time.
Honestly speaking, I would question the reliability of such an arrangement considering the negative swing at the DRV pin during the off-time. It's a known fact that ICs do not like pins swinging below ground - hence the max rating of -0.3 V in data-sheets - so I would be careful and check the IC does not latch up when operating. Ok for tinkering an experimental buck converter but not for mass production in my opinion.
Anyway, I have assembled a SIMPLIS schematic with my UC384x models and ran a simulation to a) obtain the loop gain and b) automate components calculations for a type 3 compensator:
The components values computed by the macro are given as an example with the adopted \$LC\$ for the converter. I shot for a 5-kHz crossover with a good phase margin and the below picture shows that it works well:
Again, this circuit has no cycle-by-cycle current limit - expect inductor current overshoot at power up - and its output stage has been designed to drive MOSFET transistors, which means pulsed currents during the transitions and not kind of continuous currents during on- and off-times so check out dissipation.
You can freely download my 100+ ready-made SIMPLIS templates which include buck topologies, also auto-compensated. They work on the free demo version, Elements, and will let you calculate different compensation strategies. Other compensators are also available from my webpage for download here.
Design of DC-DC converters it's not an easy thing. Not only you need manufacture specs of components used on the demo board they sell, but also you need the layout of the board where all the components are placed and manufacture special considerations (in case they will).
Vast majority of DC-DC converters designs uses a 4 or 6 layer pcb stackup to reduce EMI caused by switching noise, so take this into account at the moment of designing your board.
I found two links that I hope it helps you:
the first one is a pdf archive with an official Texas Instrument design of a DC-DC buck converter using UCC38C43 ic. I suggest you to use this ic because it has the same characteristics of UC3843 and it has a reference design.
the second link is a pdf containing an application note and in page 6 you can see a comparation between UC3842/3 with UCC38C42-3 ic.
