I wanted to make a fast pulse generator like the one designed by the great Jim Williams1. Unfortunately, the required chip for the high voltage power supply (LT1073) is not available where I live. I found several similar circuits using different approaches to generate the required voltage. I particularly liked the one in Dangerous Prototypes2, in which an astable multivibrator made with transistors is used to drive a step-up and It got me thinking ¿could be possible to “make” the entire LT1073 with transistors?
I looked for the datasheet3 and found a block diagram of the device.
Comparator A1 compares the feedback pin voltage (Fb) with the internal voltage reference. When the feedback drops below 212mV the comparator A1 switch on the oscillator. The driver amplifier boosts the signal level to drive the output NPN power switch Q1. The switch cycling action raises the output voltage and the feedback pin voltage. When the feedback voltage is high enough the comparator turn off the oscillator.
I made the oscillator with an astable multivibrator (like the one in DP). I opted for the version with waveform correction4.
I added a transistor to control the oscillator. When the feedback voltage is lower than the reference the comparator goes low and the oscillator is activated in order to rise the output voltage. According to the datasheet the oscillator is set internally for 38µs ON time and 15µs OFF time. I calculated the resistors needed using a 1nF capacitor but later on I tweaked the values in the breadboard until I got close enough to the required ON and OFF time.
For the reference I made a variable zener with a pair of transistors5.
I set it to 1V and made some measurements to see how stable the reference was:
The measurements were done with the converter working making the reference voltage quite noisy. Without a 1uf capacitor between Vref and ground (not shown in the schematic) it looked even worse:
These are the measured values in a much compact graph:
I build the simplest comparator I could find:
For testing I connected the (-) input to a potentiometer and the other input to the oscillator output:
Moving the potentiometer up and down changes the comparator output width:
It is not the best comparator you could find but it is good enough for this application. I’m not using hysteresis like the comparator inside the Lt1073 does.
Tried first with a single BC548 but couldn’t get more than 30V, adding a 2N2222 in a Darlington configuration I could reach a little more than 50V:
I also added a diode-capacitor voltage step-up network (as in the original circuit with the LT1073) and this is what the final circuit looks like:
and the block diagram of my version:
Then did some test to see if the whole thing was regulating properly and how the output changed with voltage supply variations.
Again the values from the previous oscilloscope captures in a single graph:
I also made some captures of the output from the oscillator as the supply voltage decrease:
It can be seen how the pulse trains increase with the reduction of voltage supply. With voltages lower than 1,7 V the converter can’t regulate properly.
I was able to source locally the 2N2369 used in the original application. I connected it in the protoboard to see if the converter could get it to avalanche. The blue trace (Figure 16) shows the voltage in the capacitor, it charges until the voltage is high enough to avalanche the transistor rapidly discharging and generating a fast rising pulse shown in the yellow trace.
All this testing was done on the protoboard so is no surprise that the pulse looks like crap:
I used a 22pF capacitor but even without using one the parasitic capacitance in the protoboard where high enough to make it work anyway. Now I need to build a proper board.
1. Application Note 72, APPENDIX B, Measuring Probe-Oscilloscope Response
2. Avalanche pulse generator, and some scope porn
3. LT1073 Micropower DC/DC Converter Adjustable and Fixed 5V, 12V
4. Transistors Tutorial, Part 7: “Oscillators”
5. Simple Transistor Circuits For Experimenting, Fun and Education. Variable Zener Diode