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I need help determining the minimum supply voltage specification for the MC34063E in step-up (boost) configuration. I can't tell for sure by reading this datasheet.

First, on page 1 they say:

Features:

  • ...
  • Operating from 3 V to 40 V

Then, in page 7, table 8, under section "Electrical Characteristics - Total Device", which I copy below, they say that the device start-up voltage (\$V_{START-UP}\$) is 1.5V, which is the "minimum power supply voltage at which the internal oscillator begins to work."

MC34063 - Table 8 - Electrical Characteristics - Total Device

I understand that the start-up voltage is not necessarily the supply voltage at which the device starts regulating properly. It's the voltage at which oscillation starts. But then, what's the relevance of this information? Why would I need to know that the device starts oscillating at 1.5V if it is only guaranteed to regulate voltage properly at inputs above 3V? If I apply the lowest possible input voltage (3V), that's already above the start-up voltage (1.5V) so it will get the oscillation started anyway.

I understand that this parameter (start-up voltage) is relevant for other devices (such as the MCP1640) in which the start-up voltage is higher than the minimum input voltage. That means we must apply the start-up voltage when the device is initially powered on and then its input voltage may be lowered.

The specs for the MCP1640 are:

  • Low Start-up Voltage: 0.65V
  • Operating Input Voltage: 0.35V

But that's not the case for the MC34063E. Its start-up voltage is lower than its stated minimum input voltage:

  • Start-up voltage: 1.5V
  • Operating from 3V

My interpretation is that minimum supply voltage for the MC34063E is really 1.5V and not 3V as stated. Is that correct?

My question is: What is the minimum input voltage for the MC34063E to operate within its specifications in the step-up (boost) configuration?

Another thing that is annoying me is that the 3V minimum supply voltage doesn't appear in any other part of the datasheet. It only appears on what I consider to be the "marketing" section of the sheet. Is that spec stated in any other "more technical" part of the datasheet, such as a table under Electrical Characteristics section and I'm not seeing it?

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    \$\begingroup\$ such as the MCP1640) in which the start-up voltage is lower than the minimum input voltage. -> higher ? \$\endgroup\$ – Russell McMahon Jan 21 '15 at 16:51
  • \$\begingroup\$ @Russell - Right: higher! Sorry, I'm all confused after posting this. Corrected. \$\endgroup\$ – Ricardo Jan 21 '15 at 16:54
  • \$\begingroup\$ @Russell - or I left the error intentionally to make sure you guys are paying attention to what you're reading :D You passed. \$\endgroup\$ – Ricardo Jan 21 '15 at 16:56
  • \$\begingroup\$ I think this question should be closed because it's about reading data sheets and not about electronic design. Right ? :-) :-) :-) :-) :-) :-) :-) . +1 anyway :-) \$\endgroup\$ – Russell McMahon Jan 21 '15 at 17:04
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    \$\begingroup\$ FWIW I have 100,000+ of these working at down to a bit under 3V OK. They were probably mainly LRC brand = Leshan Radio Corp = classic Motorola IP due to a joint venture. At 3V they seem fairly happy. Even slightly below that they start to stop. (Or stop to start?) \$\endgroup\$ – Russell McMahon Jan 21 '15 at 20:30
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Never trust a 'typical'. My experience tells me that typical values in a datasheet are the most atypical.

Seriously though, these start-up values are cited as typical with no minimum or maximum. As Russell pointed out, many things are spec'd at 5V, and the threshold voltage line regulation parameter agrees with the front-page spec (3 to 40V).

If you ever get into a spec-lawyer discussion with a supplier, the most restrictive conditions in the datasheet will apply, measurement conditions matter, and typical values are utterly redundant. (Trust me.)

I would read this datasheet as, "I should probably start at 5V, maybe go as low as 3V for supply voltage: it may start sucking juice below 3V but the outcome may not be nice. Consider external UVLO. Unit-to-unit variation is possible between 3-5V."

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    \$\begingroup\$ Oh, boy! That 1.5V is listed under the typical column and I didn't see it. Well spotted. Thanks for the answer +1 \$\endgroup\$ – Ricardo Jan 21 '15 at 17:17
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    \$\begingroup\$ Talk about the outcome not being nice, as you mentioned. Just got a batch of MC33063E's from ST and they behaved wildly at input voltages below 2.6V in my step-up converter. Although I had it adjusted to output 5V, the voltage spiked up to 25V without a load. I wasn't expecting that at all, as I had a bunch of 063A's from another manufacturer behaving completely differently. Sometimes we think "how bad could it be if I use this device just a little bit outside its spec". Now I know that bad, unexpected things can happen. \$\endgroup\$ – Ricardo Jan 21 '15 at 22:14
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    \$\begingroup\$ Oh, the 063E from ST behaved nicely above the 2.6V threshold. Only now I realize what you meant by "consider external UVLO". So when input voltage drops, I don't get this sort of behavior (voltage spiking up). Just now I realized the significance of that. But that's a dangerous thing for a regulator to do (voltage spiking up) in a low voltage condition... \$\endgroup\$ – Ricardo Jan 21 '15 at 22:18
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If you wish to obtain datasheet wisdom by adumbration you could look at other hints as well :-)

Note fig 12 where they could easily have taken Vin under 5V and have chosen not to do so.

5V is used as reference test Vcc in many cases.

Note that at any sort of current the internal darlington saturation eats a significant amount of your lunch at < 3V Vcc.

That's an ST data sheet but I think it looks like a redraw of the very old Motorola version. What do other manufacturer's data sheets say?

In practice I have found that MC34063xxx curl up their toes at slightly under 3V Vcc. I've run them from 3V on up with good results.

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  • \$\begingroup\$ +1 for the answer - it's really helpful - and for adding a new word to my vocabulary :D \$\endgroup\$ – Ricardo Jan 21 '15 at 17:11
  • \$\begingroup\$ By the way, I thought that reading a datasheet = obtain datasheet wisdom by adumbration. Sometimes I feel that reading datasheets is more art than science. \$\endgroup\$ – Ricardo Jan 21 '15 at 17:11
  • \$\begingroup\$ The reason I posted this question was that I was able to get 100mA @5V output with 2AA NiMH cells (~2.5V input) from the MC34063A and wanted to go lower, but I was afraid I was in the "make my day" area of your classic chart. Now I know I'm certainly in that area. \$\endgroup\$ – Ricardo Jan 21 '15 at 17:20
  • \$\begingroup\$ +1 for "a dumb ration"... it's almost like "oxy moron", I'll start calling people that way immediately! \$\endgroup\$ – user20088 Jan 21 '15 at 21:16
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My interpretation is that minimum supply voltage for the MC34063E is really 1.5V and not 3V as stated. Is that correct?

I read it like this - you are stepping up a supply voltage and the manufacturer says that the chip will work as low as 3V. It says 3V on the front page and that is nearly always a typical figure because the front page of the data sheet tries always to say the nicest things about the device without being dishonest.

Later on it says 5V is the test voltage for the main body of the data sheet's written values (page 6 and 7). This then concerns me as to how far the device will be guaranteed to work under 5V.

I then see "Figure 12. Supply current vs. input voltage" and it does not show operation below 5V. That's sealed it for me - I can't expect this device to work as expected below 5V 100% of the time and for 100% of every chip I might buy. In other words I can't realistically design it to operate below 5V unless I'm prepared to test it below 5V and expect failures.

But, then I notice Figure 17. Voltage inverting converter and it shows it operating at 4.5 volts. I then have to make a decision based on how this circuit works as to whether I can transfer it's good news (4.5 volts) to my circuit (step-up). Maybe I can.

Lastly, I go back to where it says the oscillator may start at 1.5 volts and this to me is a warning that maybe if I dropped the voltage to that sort of area strange behavior will occur and I worry because at 1.5 volts any old thing may happen. Take the 1.5 volt operation as a warning that things could go stupidly wrong at too low of a supply voltage.

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