Is my Boost Converter schematic alright? - Page 1
Forum section: electronics / projects

author: EdimarDigital
author_rank: Contributor
timestamp: January 22, 2017, 07:05:51 pm
content: Hello everyone, I'm new to electronics and i want to create a digital watch just like the ChronodeVFDhttp://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 TPS61070http://www.ti.com/lit/ds/symlink/tps61070.pdfto 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.

author: Benta
author_rank: Super Contributor
timestamp: January 22, 2017, 07:40:41 pm
content: 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.

author: EdimarDigital
author_rank: Contributor
timestamp: January 22, 2017, 07:53:31 pm
content: And if i want 5V?

author: Benta
author_rank: Super Contributor
timestamp: January 22, 2017, 07:56:43 pm
content: Increase R1 accordingly.R1 and R2 form a simple voltage divider. For any desired output voltage, FB must be at 0.5 V.

author: Spuddevans
author_rank: Unknown
timestamp: January 22, 2017, 07:58:06 pm
content: Quote from: EdimarDigital on January 22, 2017, 07:05:51 pmActually you can get microcontrollers that work all the way down below 1V, but loads are available that work @1.6V, go tohttp://www.microchip.com/ParamChartSearch/Chart.aspx?branchID=1012scroll right on the selections and select 0.7v and/or 1.6v, the result will surprise you.Tim

author: EdimarDigital
author_rank: Contributor
timestamp: January 22, 2017, 08:17:53 pm
content: 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

author: Benta
author_rank: Super Contributor
timestamp: January 22, 2017, 08:39:24 pm
content: QuoteCorrect, if R2 is 200 kohms.Like I said, it's a voltage divider:VFB= R2/(R1 + R2) x VOUTYou know R2, VFBand VOUT. Getting R1 is easy.

author: EdimarDigital
author_rank: Contributor
timestamp: January 22, 2017, 09:07:17 pm
content: QuoteIt did gave 0.5V, thanks man.

author: Phoenix
author_rank: Frequent Contributor
timestamp: January 22, 2017, 11:37:23 pm
content: I suggest you also read section 11.2.2.3 on the input and output capacitor requirements.

author: EdimarDigital
author_rank: Contributor
timestamp: January 23, 2017, 12:33:36 am
content: I find it hard to define which output capacitor to place since i don't know what switchingfrequency and output current on the circuit is gonna be to complete the equation.

author: timb
author_rank: Super Contributor
timestamp: January 23, 2017, 12:40:33 am
content: 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.

author: EdimarDigital
author_rank: Contributor
timestamp: January 23, 2017, 01:10:32 am
content: 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.

author: timb
author_rank: Super Contributor
timestamp: January 23, 2017, 04:19:35 am
content: Quote from: EdimarDigital on January 23, 2017, 01:10:32 am6.6e6 == 6.6uFThough 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.

author: Phoenix
author_rank: Frequent Contributor
timestamp: January 23, 2017, 06:57:31 am
content: Quote from: EdimarDigital on January 23, 2017, 12:33:36 amFrom 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.