USB 5v to 7v?

[bhsmith]

I want to use USB to power a project I'm working on except it needs 7v and 50ma.  What I need to know is which is the best IC to do this?  Would it be a buck/boost IC or something else?  A specific model would be helpful.  Thanks!

[billhowl]

I'm using this "MC34063 Based Switching Regulator Adapter, Step-Up IN:+5V~+12V OUT:+7~+24V DC Max current 200mA " which not the best but it work.




http://www.nomad.ee/micros/mc34063a/index.shtml
http://www.onsemi.com/pub_link/Collateral/MC34063A-D.PDF

[bhsmith]

I've gotten this image from your link and input the requirement that I know and got this:

With this as values:

Ct=NaN pF
Ipk=165 mA
Rsc=1.818 Ohm
Lmin=NaN uH
Co=NaN uF
R=180 Ohm
R1=8.2k R2=39k (7.2V)

I'm not sure about the Vripple and Fmin to put into it for calculations.  Any suggestions, comments?

[Soeren]

Hi,

NaN means "Not a Numerical", so either you typed nothing, something wrong (like the letter "o" for a zero), or the script is broken.

Vripple is the ripple that you can tolerate at the output and depends on what you need it for.
The frequency probably doesn't matter for you, so try 100kHz (and/or try a few other values).

With input
5V, 7V, 50mA, 10mV, 100kHz

It spits out:
Ct=150 pF
Ipk=160 mA
Rsc=1.875 Ohm
Lmin=94 uH
Co=169 uF
R=180 Ohm
R1=3.9k R2=18k (7.02V)

Mind you, I just took the numbers you mentioned, you probably haven't included overhead in the 50mA, so rather go for 75mA..100mA.

Raising the frequency (within the limits of the chip) means smaller components, lowering it might mean less headache for a beginner.

[billhowl]

The Oscillator Max freq is 42kHz 

the typical value 33kHz used in the spec MC34063

With input
5V, 7V, 100mA, 10mV, 38kHz

It spits out:
Ct=395 pF
Ipk=320 mA
Rsc=0.937 Ohm
Lmin=123 uH
Co=888 uF
R=180 Ohm
R1=3.9k R2=18k (7.02V)

The capacitor Ct you can use 390pF and Rsc is 1 ohm and 15 ohms in parallel, you will get 0.94 ohms
The inductor is min value 123uH, so you can some higher value like 150uH
The output capacitor Co 888uF also min value you can use 1000uF
 

[Soeren]

Hi,

The Oscillator Max freq is 42kHz 

OK, didn't read the datasheet.
The calculator/wizard really should have limits to keep inputs within the allowed range.

*Bad wizard, go to your room*   

[bhsmith]

I appreciate all of the help.  I'll try this out and see what comes out.
Thanks!!!

[bhsmith]

So I put everything together and it doesn't work like its suppose to.  My input is straight from the usb port and my output is around .8v @ 20ma.  It wouldn't even turn on one of my small leds.  Any reason why the output is so low.  It's actually doing the opposite of what it is suppose to.  I appreciate all of the help.

[Soeren]

Hi,

So I put everything together and it doesn't work like its suppose to. 

How did you put it together (exact schematic) and what values did you use?

My input is straight from the usb port

Courageous!
A bit foolish perhaps, but you've certainly got balls and faith in your abilities to not screw up your USB port 

May I suggest using a separate power supply before your PC goes haywire - any adapter of (measured) 4.5V to 5.5V and 150mA up would do if you use a good quality cap before the switcher (yes, an extra cap).

and my output is around .8v @ 20ma.  It wouldn't even turn on one of my small leds.

LED's need a higher voltage to light.

Any reason why the output is so low. 

Yes, but you don't wanna hear it 

It's actually doing the opposite of what it is suppose to.

Electronics circuits work the way they're constructed to work, which is not always the way the constructor would have liked it to work.

Did you perhaps put the diode in the wrong way?

If not, scrutinize the board with a magnifier - is the soldering OK, or did you perhaps wire something up wrong (compare with schematic).

[bens]

If you want something that will just work, you could always use a Pololu Step-Up/Step-Down Voltage Regulator S8V3A.  It's a SEPIC regulator (meaning it can buck or boost on the fly) with an adjustable voltage output, and it can deliver up to 300 mA.

- Ben

[Admin]

Another device to add to the list:
http://dimensionengineering.com/AnyVoltMicro.htm

Use a female header or hitec connector so you can reuse this in future projects.