LT1108 in a dc step-up configuration

[BeatleManiac]

Hi!

I'm what Dave would refer to as a "New Player"  and I have a question concerning the LT1108 DC to DC converter.  Here's the data sheet:

http://www.linear.com/product/LT1108

I'm using it in a Step-Up configuration and my question concerns the drawing labeled Figure-1 on pg 8.  I understand how to get the desired output voltage using the resister divider connected to the FB pin, and how to choose the inductor using the provided formulas.  However, there is no mention of how to choose the correct diode D1 and capacitor C1.  I have various values of zener diodes so I'm assuming I'll be ok as long as I pick a reverse-bias voltage somewhat larger than V+.  There is no mention of a procedure for choosing C1.  I know I can just find one that works with trial and error but I'd like to understand the correct procedure.

Thanks in advance for any help!

Steve Hurd
Floyds Knobs, IN

[Jay_Diddy_B]

Hi Steve,

Welcome to the Forum !!

The LT1108 is older part.

The Diode D1 should be a 1A Schottky. The capacitor C1 is typically a 100uF to 330uF low ESR electrolytic or polymer capacitor.

There is an LTspice model for the LT1108.

You might be better off using the LT3580.

Link: http://www.linear.com/product/LT3580

This part will switch at a higher switching frequency and be a smaller solution size.

What is your input voltage?

What is the output voltage?

What is the maximum output current?

Regards,

Jay_Diddy_B

[BeatleManiac]

Hi Jay_Diddy_B!

I'm wanting to use the lt1108 as a way to feed around 7V into my Arduino (no more than 400ma) so it can be assured of enough power.  I'm using a 10A 5v DC source that will drive several programmable color-changing LEDS that will line the perimeter of my bass amp.  I only want to siphon enough current off to power the arduino and supply +- 7v to my op amp that is in front of my bass signal and leave the rest of the current to drive the leds wich can take up to 30mA each.  Op Amp output will then be dc offset so it doesn't dip lower than 0V and be fed into an input pin on the Arduino.  Using a separate IC for the +7 to -7 conversion.

I originally calculated and bought inductors for 5V into the Arduino when I saw that its input voltage dropped to about 4.5V after the LED strip was turned on.  I only recently found that the Arduino prefers about 7V in to make sure it can run all the components at 5V.  I was concerned about putting extra voltage into the board that the internal regulator would need to dissipate as heat since the ultimate home for this project will be in a guitar pedal.

Hope this helps!

Steve Hurd

[BeatleManiac]

Where do you get the 100uF to 330uF value from?  I'm trying to understand the theory behind the values as I go along.

Thanks!

Steve Hurd