Is my Boost Converter schematic alright?

[EdimarDigital]

Hello everyone, I'm new to electronics and i want to create a digital watch just like the ChronodeVFD http://www.johngineer.com/blog/?p=1595. I will be using a AA battery as my source but we all know micro-controllers need at least 3.3V or more to work and a single cell just delivers 1.5V which is not enough. So i selected a Texas Instruments TPS61070 http://www.ti.com/lit/ds/symlink/tps61070.pdf to deliver the necessary voltage. I created a schematic which i believe should output 3.3V according to the datasheets, but being the total amateur i am to this (since I'm only 17 years old with barely any experience) i need a professional to take a look at the schematic and point out the flaws and what can i do to make work efficiently.

[Benta]

According to the datasheet, R2 should be 200 kohms.
With R1 at 1 Mohm, you'll get a 3.0 V output. R1 = 1.12 Mohm will give 3.3 V output.

[EdimarDigital]

And if i want 5V?

[Benta]

Increase R1 accordingly.

R1 and R2 form a simple voltage divider. For any desired output voltage, FB must be at 0.5 V.

[Spuddevans]

we all know micro-controllers need at least 3.3V or more to work and a single cell just delivers 1.5V which is not enough.

Actually you can get microcontrollers that work all the way down below 1V, but loads are available that work @1.6V, go to http://www.microchip.com/ParamChartSearch/Chart.aspx?branchID=1012 scroll right on the selections and select 0.7v and/or 1.6v, the result will surprise you.

Tim

[EdimarDigital]

Thank you guys, i find it hard to understand the equations given to sort the components out.

If somebody could explain the equation given on the datasheet for choosing R1 I'll be very grateful.




Edit: NVM, i figured out the equation, if i want 5V, R1 needs to be 1.8Mohms

[Benta]

NVM, i figured out the equation, if i want 5V, R1 needs to be 1.8Mohms

Correct, if R2 is 200 kohms.

Like I said, it's a voltage divider:

V_FB = R2/(R1 + R2) x V_OUT

You know R2, V_FB and V_OUT. Getting R1 is easy.

[EdimarDigital]

R1 and R2 form a simple voltage divider. For any desired output voltage, FB must be at 0.5 V.
VFB = R2/(R1 + R2) x VOUT

It did gave 0.5V, thanks man.

[Phoenix]

I suggest you also read section 11.2.2.3 on the input and output capacitor requirements.

[EdimarDigital]

I find it hard to define which output capacitor to place since i don't know what switching
frequency and output current on the circuit is gonna be to complete the equation.

[timb]

Read the datasheet. It tells you the frequency for that part and you should have a good idea of your maximum output current.

You can also determine it experimentally. Start with a 4.7uF X7R Ceramic (0805 or 1206 size) rated for at least 6.3V (ceramic caps quickly lose capacitance as the temperature increases and you approach their rated voltage).

Look at the output ripple with a scope, if it's too much, move to a 10uF cap.

Be careful placing too much capacitance on the output as it can destabilize the control loop and actually make ripple worse. (You end up with a few mV of ripple super imposed on a slower sine wave that's a few hundred mV.)

The input cap can be any size, but should be low ESR. Start with 10uF. This needs to be ceramic as well.

[EdimarDigital]

timb, i tried reading and i attempted equations but i don't think 6.5809265944645006016847172081829e-6 is the minimum capacitance I'm looking for. Sorry that i do not understand how to work this. 

[timb]

timb, i tried reading and i attempted equations but i don't think 6.5809265944645006016847172081829e-6 is the minimum capacitance I'm looking for. Sorry that i do not understand how to work this. 

6.6e6 == 6.6uF

Though like I said, 10uF on the input and output are pretty standard values for fast (500kHz+) switching regulators. In fact, that equation is only a guide. Final values are best determined experimentally, really. Start with 10uF and see how it acts at different output currents with your scope.

[Phoenix]

I find it hard to define which output capacitor to place since i don't know what switching
frequency and output current on the circuit is gonna be to complete the equation.

From your maths 10uF looks pretty good. Maybe consider 2x4.7uF in //el to raise their self-resonant frequency.

Also note the 10uF+100nF input capacitance recommendation.