Boost converter cant handle loads below 80kohms

[Dmeads]

Hello! hope everyone's week is going well!

I am trying to build boost converters to step up 5v to 20v to power an operational amplifier, and I have two questions.

1.) the boost converter I built can only step up the voltage effectively when the load is above 80kohms, so I was wondering if anyone knows the supply impedence of an op amp? can you find this on the data sheet? I found input impedence but I want to know the supply impedence because if it is low, then my boost converter wont be able to power the op amp.

2.) if the supply impedence on an op amp is low (<80kohms), then how could I re-design my boost converter to still provide the stepped up 20V to smaller loads?

I have attached an LT spice simulation, but when i actually built the circuit I used an IRFZ34 N channel MOSFET instead of the FDS4410 in the simulation, but the build still worked fine. I used a 555 timer from TI to drive the mosfet which switched at about 500kHz but I can adjust it using a trimmer.

thanks!

[David Hess]

Operational amplifiers do not have a supply impedance in any useful way.  What they do have is a quiescent current which is continuously drawn between the positive and negative supply inputs which depends on supply voltage, temperature, and sometimes other things like common mode voltage and saturation condition.  Further, the load on the output of the operational amplifier also contributes to the positive and negative supply currents depending on if the output is sourcing or sinking current.

So in practice what you can do it look up or measure the operational amplifier's supply current at 20 volts, add the maximum load current, and decide whether your boost converter can provide it.

Depending on how your boost converter is configured, it may be a good idea to add like a 24 volt zener diode across the supply to protect the operational amplifier from an over-voltage condition.

[unitedatoms]

You can choose PWM ratio to have as high voltage as you need. Try 2.5us/3us ratio, it will double the voltage comparing to 1.8us/3us ratio.

It looks like the voltage ratio is more defined by PWM ratio than the impedance. However for accuracy, less dependency on load impedance and to lower output impedance of supply, practical circuits of DC-DC converters include feedbacks, references, etc.

[Dmeads]

Awesome thanks peeps!