MC34063 DC-DC Step Up/Down IC question

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Hi,

This is quite an old chip. Rather use MCP1400, but, I can't get one from anywhere. Mouser.com has it, but you have to buy 1500 pcs, not single pieces.

So, I am thinking of doing a breakout board that step up a 3V input (2xAAA or 2xAA batteries) to output a 5V and 50mA.

I have been reading the below, and can't seem to understand the Ipk (sensing peak current) of pin 7. Is this the current I am to provide or a source current from the IC itself? If I am to provide, I guess I would use the some sort of transistor to have a 220mA current for pin 7. I use a tool to calculate out all the variables to achieve a 50mA/5V output.

http://focus.ti.com/lit/an/slva2...

Thanks

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Ipk is the peak current through the inductor, pin 7 measures this using an external current sense resistor.

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Paragraph 1.3 starts by telling you (and me) that to implement current limiting, you need to connect an external sense resistor to pin 7. The chip monitors the voltage dropped across this external resistor and the peak value of the current is set by the chip's own design to be 330 mV/the external resistor.

Cheers,

Ross

Ross McKenzie ValuSoft Melbourne Australia

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... and perhaps this picture might help. Here the external resistor is 0.22 ohms and is between pins 6 and 7.

Cheers,

Ross

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Ross McKenzie ValuSoft Melbourne Australia

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Thanks, all for the responses and verification.

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You should probably run at least 3 batteries and not just 2. The 34063s operating range starts at 3V, and the 2 1.5V in series will soon drop below that.

The 34063 also is a bit of a dog for this low input voltage boost application, expect to see quite low efficiency.

oddbudman