Hi, I'm hoping someone might be able to help me.

I am trying to simulate a circuit involving the MC34063 that's illustrated in the datasheet (attached as JPG) using LTSpice. It's a simple 12V -> 28V DC-DC converter.

I downloaded the model (attached) for the MC34063 so that I could use it in LTSpice. It seems to be ok when I tell it where the files for the model are (C:\DATA)

Have also attached the LTSpice model.

The problem is that I don't get 28V at the output - I get around 11V! I'm very new to this so have probably done something silly. I'm also not sure what values the input and output capacitors should be.

Any help is very much appreciated.
Thanks

 

R3 should be 47K; not 471Ω
C2 should be grounded
R1 works better at 100Ω
Make R5 (the load) 100Ω to see what happens under load.

Now the biggie:
Add the STARTUP qualifier to the simulation directive. This will start the oscillations by preventing the initial DC solution from reaching a stable (non-switching) initial state...

 

Hi

Also set the tran to 30m or greater:

.tran 30m startup

eT

 

Wow, thank you both so much!! All working fine now!

I didn't know the 'startup' directive existed - could you please tell me under what circumstances it's used so I don't make this mistake again?

Thanks again, really appreciate it

 

Wow, thank you both so much!! All working fine now!

I didn't know the 'startup' directive existed - could you please tell me under what circumstances it's used so I don't make this mistake again?

Thanks again, really appreciate it

As I said:

"Add the STARTUP qualifier to the simulation directive. This will start the oscillations by preventing the initial DC solution from reaching a stable (non-switching) initial state... "

Particularly useful when simulating oscillators (what is a switcher?).

 

Thanks Mike.

I wanted to get the example running so I could then adjust for what I want to do:

Boost a Vin (3-4V) into a Vout of at least 30V. Don't need much current - less than 50mA is fine. I also need another output of -30V

I have attached an ASC file that can get 4V to 40V but it won't work for 3V. If you could suggest any adjustments that would be great!

I also want to be able to do it easily on a breadboard!

I have got something working with the MC34063 but I was wondering if you might be able to suggest a better option?

Thanks again!