MC34063 - Inverting conf not working properly

[Moriambar]

Hi,
I'm trying to design a circuit around the inverting configuration of MC34063, a bit for fun, a bit to understand the basics for a new project I am undertaking.
The idea is "simple" I have a 3.7V power supply (currently: my bench psu) and I have to provide (-)3V on the output capable of 300mA. 500mV ripple would be great.
Using a bit of math and mostly an online tool I came out with the following values for the components of the standard circuit:

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Ct=223 pF
Ipk=1356 mA
Rsc=0.221 Ohm
Lmin=11 uH
Co=30 uF
R1=1.3k R2=1.8k (2.98V)
and designed a first draft circuit (which achieves a bigger output voltage, but I currently don't have the required resistors)


The problem is that the only output current I can get is around 20-30mA before the voltage collapses.
What have I done wrong?
thanks

[capt bullshot]

10uH for the inductor appears a bit too much on the low side to me.
The 34063 is from the generation of converters that usually work with 100uH to 1mH inductors.

[Moriambar]

10uH for the inductor appears a bit too much on the low side to me.
The 34063 is from the generation of converters that usually work with 100uH to 1mH inductors.

Hi, thanks for your reply.
I tried swapping the inductor for bigger ones (up to 10mH). Negligible changes...

[mikerj]

Both the inductor and the 47uF output cap are very low in value for this configuration, should be ~10x that.  Are your inductors rated for the peak current they will see without saturating?

[Moriambar]

Both the inductor and the 47uF output cap are very low in value for this configuration, should be ~10x that.  Are your inductors rated for the peak current they will see without saturating?

Hi, I tried putting a 1mF and 220uH components to no avail.
And regarding the inductors… that's a good question, probably that's the main problem: I have only some generic ones bought from aliexpress some time ago.
Thanks, I guess. I think I'd better try and move on to the next circuit, which will have an actual use, and buy better components.

[kevin original]

Try adding more input capacitance on Vin, like 100uF or more. The inverting configuration draws lots of peak current on the input.