EDIT at the bottom.
Referring to a previous question which I solved, it was related to a inverting boost configuration of the MC34063 with the following arrangement:
I realized my circuit and calculations were right, and the question was solved. What remained open, though, was another issue: why from the measurements the Vsat of the internal switch was 1.6V or so, while by following the components selection, it should still be in the range of less than 0.5V (from datasheet).
To recap the component selection:
- R1 = 0.33 Ohm
- C1 = 47μF (electrolytic)
- R2 = 49.5 kOhm
- R3 = 2.49 kOhm
- C3 = 680 pF
- D1 = V4PAN50-M3/I
- L1 = SRN1060-221M
- C6 = 47μF (electrolytic)
I had the correct 50mA at the minimum input voltage of 6.5V, obtainable from the following formula:
\$ \frac{t_{on}}{t_{off}} = \frac{|{V_o}| + V_{diode}}{V_{in}-V_{sat}} \$
Measuring the on/off time ratio at the worse case scenario of the minimum input voltage with the maximum load (50mA), I discover that the minimum input voltage is 6.5V, since the on/off time ratio is 5.2 and the Vsat is 1.6V. Putting values in the formula will actually verify what is happening.
My question is why the chip keeps an on/off time ratio close to the minimum in the datasheet with all the 6 parts I tried, and why the Vsat is so high (way out of spec).
This is the scope acquisition of the pin 3 on channel 1 (timing capacitor) and the pin 2 on channel 2:
And here on channel 2 and 4 are, respectively, the pin 2 and the pin 1 (shorted with 7 and 8), showing the input voltage at around 6.5V (on the acquisition is lower at 6.1V though...), and during on-time the inductor voltage is around 4.5V, see acquisition:
The acquisition is taken with a light load, as you can see the inductor current goes to zero and voltage oscillate at a certain resonant frequency before decaying to zero.
Is there anything obvious here?
EDIT:
- There was a complain about not having the original schematic that I have used, so here it is.
- Also, in an answer I've been pointed out that the max Vce sat is 1.3V, while I wan mentioning it was 0.5V. Indeed I was wrong, but is also important to mention that this still does not explain my reading of 1.6V which is above the absolute maximum.
- A scope acquisition was not convincing, so since I have no access to it now, I will update with a better one a bit later.
EDIT 1:
- Scope acquisition updated posted in this EDIT1:
The natural ringing of the oscillator now shows that there is no fundamental error in my setup/calibration.
The open question, considering the observations I received of the Vce_sat, is the following:
Why the V_ce stays around the maximum value from the datasheet (max 1.3V, measured 1.6V), far from the typical? And, from previous question, why the t_on/t_off ratio stays around the minimum value of 5.27, where minimum is 5.2? Is this related somehow? Did I missed something else?
I just wonder if someone else had a similar issue with similar specs. I am using a PCB I designed. As I have no problems in posting the grounding of the top of the test board I made (the bottom is just an entire GND plane), and also a 3D view, I hope I can provide some more insights now:
Also, the effectively mounted component, are the one shown in the initial schematic.