mc34063 question

[hedley]

I recently hooked up an electronic load - and am not sure if I am doing something wrong , as in circuit for the past 20 years we have had this device delivering 600mA @ 5V without a problem . I have the setup as per the attached spreadsheet , but as soon as I set ma MA on the load , after a few minutes the DUT shuts down ( no output ) , with a 250ma load as the input voltage reduces then the DUT seems to forget about the 5V output and track the input voltage with a +- 0.8V difference .

Just seems strange and I am sure I am missing something really basic

[rteodor]

As far as I know this converter has a (very discretely documented) limit on duty cycle of 84%. That would be 5.95 V at the input in 3mA test.
In the 250mA test case it could be the over-current protection resistor. Limiting should occur at 0.3V. Hard to say without a schematic or a at least a value for that resistor.
No idea for for the 400mA test case.

[hedley]

Thanks for reply - I did some more measurements , looking specifically at duty cycle ( as measured on an oscilloscope vs the calculation ) .

The resistors ( over current protection )   I have  4 * 1 Ohm in parallel = 0.25 ohm and I also tried with three in parallel to get 0.33 ohm .
If the input voltage is raised to 16 Volts , I can get +- 600ma on the 5V output - which is what is needed for the circuit that is powered . BUT ->
I took an IR photo and the device MC34063,  and the inductor ( PE 53606 [114uH, 940ma max )] )  outer case is +- 65 Degrees C .

Ideally I could find a more robust device that could accept input range of 12 VDC -> 17VDC and output 5VDC at up to 1.2 Amps - clearly the worst case 12V drop across the device @ 1.2 Amps would require dissipation of 14 .4 watts of heat ,

[PGPG]

Ideally I could find a more robust device

MC34063 is very, very old construction. When I was looking at DCDC for the first time in 80s I got uA78S40 datasheet (it is practically the same as MC34063).

Distributors like Digikey or Mouser have good selection tools.
After selecting by parameters sort elements by price and then look at stock (if huge than element is popular) and read datasheet.

10 years ago I was selecting DCDC IC to be used and I have big table of ICs I made those time, but it is 10 years old and I was searching for up to 1A ICs.
I've selected LM5017 (direct step down to 3V3 from 7.5V-100V). I like COT (with Ton being f(Vi)) controller mode.

[mikerj]

the inductor ( PE 53606 [114uH, 940ma max )] )  outer case is +- 65 Degrees C .

Regardless of anything else the inductor does not have sufficient current rating for your 5v, 600mA requirement.  In buck configuration the peak switch current is twice the load current, so 1.2A in your case.