Boost Regulator Soft Start

I am using the soft start circuit more-or-less straight from the LM317 datasheet. 

Soft Start using an LM317. Takes 300msec to reach 9V.

I started by adjusting the resistors to give a 9V output and use 1mA of bias current. It turns out the LM317 requires 10mA of output current to regulate. It's right there in the datasheet, but I missed it until I started to put the circuit together and couldn't figure out why it wasn't regulating. So I went for 10mA of current. Which seems like a lot. Maybe I should make an improved 317.

10mA. Ouch!

My intention is to soft start both the power supply for the compensator and the reference voltage. I used a diode model I had handy; I don't think the diode itself is particularly important.

Soft Start simulation results.

This entry is just a quick note. I ordered parts from Mouser and have started actually building the LED driver. I had trouble finding comparators of the needed speed at a price I could stomach. I'm going to try using the op-amps and only buy comparators if I need to. 

The approach to building the circuit will be to make the linear regulator with soft-start and use it to provide references. Then make the triangle-wave generator and see if I can really get a 400kHz triangle wave out it. Then make the boost regulator, but drive it from a pulse generator to check it out. Then pull the whole thing together. 

Then design a battery charger and interface and an on/off switch. Then install the beast in the trailer. 

I intend to publish photos of the building and verification process next week. I will also publish the schematic as it now is. I switched to direct current control and didn't document the change. It's not a big change, but it lowers the operating voltage and therefore the power requirements.

Late addition. I soldered the linear regulator and soft-start (and resoldered after finding I needed more LM317 current as explained above). I'm actually hitting 9.2V for input voltages above about 11V, that's less than 5% error.  The scope shot shows lower because my waveform generator only goes up to 10V. The soldering is not the best I've done. I could blame the re-solder, but I'm out of practice. I will have to do better when I put the triangle wave generator on here. I'm trying to decide if I use sockets for that part. It would make soldering safer.

9V regulator with soft start.

Soft start in operation. A bit faster than expected, but close enough. 

Miscellaneous tidbit for this week. I use this alias in my .cshrc.
alias big 'find . -type f -exec ls -s1k {}  \; | sort -gr | head'

And this one in a .bashrc.
alias big='find . -type f -exec ls -s1k '{}'  \; | sort -gr | head'

I don't remember now if the syntax is different for the different shells or just how they have drifted.
big gives a sorted list of the ten largest files in the current hierarchy. 

Bruce McLaren

IC Design Interview 3: Bandgap Reference

Life has intruded so not much update on the boost converter. I do now understand there is a brand new shiny zero that has to be taken care of. I also now understand the gate drive needs to forced to oscillate at some duty cycle regardless of the output value. I need to rework the feedback compensator to account for the boost AC characteristics instead of the buck that is in there now. Until I have that ready, here's some more interview notes.


The bandgap equations must be derived sometime during every integrated circuit design interview. At least it seems that way. It is a standard, useful circuit found in nearly every chip. I think the discussions about the circuit are better able to gauge a candidate's knowledge and experience, though. 

Start with a schematic like the one shown below. 

That last equation isn't the final answer, though I've never been asked in an interview to go beyond it. V_BE has a negative temperature coefficient and the V_T has a positive temperature coefficient so appropriate values of resistors and bipolar multipliers can be found.In general R₁ and R₂ are made equal. Then the bipolar multipliers are set based on layout. Pick a rectangular array that allows Q₁ and Q₂ to be arranged in a common centroid configuration. Then set the ratio of R₁ and R₂ for zero temperature coefficient.

There are at least four other sub-topics that I will put off until a later blog entry.

• Current conveyor instead of an op-amp
• Lower voltage reference
• Producing a VPTAT (Voltage Proportional To Absolute Temperature)
• Start-up issues

Bruce McLaren