MintyBoost! - Small battery-powered USB charger

Step 5 The Process: Boost chip selection

So now it's time to design the boost supply. Since I don't have much space, I'm going to try to make my circuit as tiny as possible but still be easy to solder. That means I want a boost chip that is 8-DIP (smallest though-hole), with an internal MOSFET switch (1 less part) and is high frequency (to keep the inductor small). I also need to be able to supply 100mA at 5V and it should run on as low as 2V input. Also I want to be able to buy it online from a common supplier.

• 8-DIP package
• Internal FET switch
• 100mA output @ 5V
• 2V minimum input voltage

OK, lets search Digikey. I start with "DC/DC converter 8-DIP" and check "items in stock"

I then select 1 output, 8-DIP (to differentiate between 18-DIP) and select all the current-outputs >=100mA and apply the filter. There's till about 40 options. So then I select the all voltage input ranges that start with 2V or less. Also I select all the Adjustable, and 5V-inclusive output voltage options

Looking over this list, it looks like I have a lot of options so I'm going to go back and select only the chips that can be preset to 5V (as opposed to adjustable ones that use 2 resistors to set the voltage). 5V is very common so every reasonable DC/DC chip will be available with such an option.

Now there are about a dozen options.The LT1073, LT1111, LT1173 and LT130x as well as the MAX751 & MAX756. They're all pretty much the same, so I basically make my choice based on price at 100 pieces (since I'm planning to kit it up). I also know that Maxim is great about sending samples so I decide to go with the MAX756 (datasheet) which is $2.32/100. Note that I could have gone with any of them, so this a somewhat arbitrary choice.

According to the datasheet, I can supply up to 200mA @ 5V, run off input voltages as low as 0.7V and the efficiency is about 85% with 2 AA batteries. The chip also runs at 500KHz which is pretty fast and means that the inductor can be pretty small (~22uH) Anyway,I've used this chip before and its worked out well for me.

Comments

[boostergold1]

There is nothing like a some good Nord Lock Washers to keep a bolt from coming out. Just wish they were easier to come by.

[chinnerz]

I need some help :(, I can't seem to find any of the chips you said we could use. can I use the "MC34063", it looks right but I am a total noob when it comes to this sort of stuff.

[swami]

hi i tryed to find that chip here in Australia but i cant find it would this work?http://www.ferret.com.au/articles/46/0c02e046.asp

[Jedibowler]

tried to find a MAX756 in the UK without any luck, would LT1111CS8 work OK as you mention above this model of chip

[erckgillis]

Completed IC "charge" pumps are also available, require NO inductors and samples are available for free ... http://focus.ti.com/docs/prod/folders/print/tps60110.html

[ladyada]

uh, most charge pumps top out at 50mA, definately can't supply 250mA. also they can ONLY double voltages, so you have to regulate on top of that