How to design an SMPS boost converter with MAX1771

Question

I am trying to design a switch-mode boost converter using a MAX1771. As far as I know, when designing this kind of converters there are a few parameters that must be set. In my case these are:

• Vin = 12V
• Iin (max) = 2A
• Vout = 210V
• Iout = 5-6mA (with pretty much no variation)

Once this is set it is important to decide whether we want the Boost converter to work in DCM o CCM (trying to avoid to work on the frontier between both modes). Accordingly to this, an inductor value is chosen. Let's say that in my case a 100uH inductor with a switching frequency of 300KHz would work.

This is how I would design it with an IC like TPS4021x or a 555 timer (just to stay a couple of examples). Please correct me if it is a bad method. The problem is that I can not work out a design with MAX1771 using this process since I can not choose the switching frequency nor duty cicle.

my questions are the following.

1. Which process do I have to follow to design a switch mode boost converter using a MAX1771 integrated? How can I control the circuit parameters and know how is it working?
2. How do I choose a proper Rsense value for such a high output voltage? (No information is provided in the datasheet for these Vout values).

Answer 1

Once this is set it is important to decide whether we want the Boost converter to work in DCM or CCM

The chip will handle both as per this: -

When the load gets heavier it runs in CCM and, should be fairly seamless so no need to worry about that: -

Regards operating read this: -

And that tells me you can use "normal" boost converter formula for this or, my crappy website calculator to get an idea of what mode you might be in for a given set of input parameters: -

As you can see, the peak inductor current is 0.826 amps so, set Rsense accordingly to about the value that gives this current. CS pin voltage level is 100 mV. I chose an operating frequency of 50 kHz and an inductor value of 68 μH for this simulation.

I chose 68 μH because that inductance shouldn't be so high that self-resonance prevents the high output voltage being reached. Concentrate on finding a really good inductor that has a self-resonance at least ten times higher than the operating frequency (if not more).

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Conclusion

In any circuit design you have to do checks once they are built and, this is particularly important here. Personally, I'd be thinking of modelling the circuit if I wanted it to run in DCM else, I'd force it to run in CCM by choosing an inductor that was hundreds of micro henries.

Either way, I'd be wanting to check this circuit out very carefully when it was built. I say "very carefully" because this particular device is not that clear on what it actually does i.e. no timing waveforms or timing examples (unlike other devices that appear more well-defined).