LT1171 Single Cell Boost Mod

[Akya2120]

I found this tonight and it seems to fit the bill.
This is the high efficiency chip(don't know how much more efficient yet)
Digi-Key - LT1171HVCT#PBF-ND (Manufacturer - LT1171HVCT#PBF)
This is just the regular one
Digi-Key - LT1171CT#PBF-ND (Manufacturer - LT1171CT#PBF)

So I signed up with linear and ordered 4 samples, two of each. They sent me an email, I will have them in 2-7 business days. Now I need to figure out how to setup the circuit diagram. I don't know how to design a circuit so I'm doing some reading. All help will be greatly appreciated.

 

[CraigHB]

Looks like a nice easy to use part. Uses current mode control to simplify loop compensation which saves a huge headache.

One disadvantage is it runs at a relatively low frequency so you need a big inductor. When inductors get into those higher values, they get large when rated for higher currents. With boost, you can pull a lot of current through the inductor so you have to design for that. For a comfortable margin, I'd shoot for a minimum of 10A on inductor saturation and 8A on current rating.

For boost, you need a battery with high drain capacity since you're going from lower voltage and higher current to higher voltage and lower current. I would plan for at least a 5A drain. Probably need to use something like an IMR 18650 or some other high drain battery. The current limit on the protected ICO 18650's would likely kick in when using those.

Boost needs a switch with high current capacity. A 10A switch will provide a liberal margin. However, the LT1170 has a 5A switch which is probably okay, but you'll definitely want the part well heat sinked. The LT1171 has a 2.5A switch and the LT1172 has a 1.25A switch (reference Ilim on the 4th page of the data sheet). Though, you can use the LT1171 to engineer your regulator, then upgrade it to the LT1170 once you get the circuit running well.

To give you a comparison on specs, I'm running my single cell boost controller at 600kHz with a 1u inductor. The switch (external MOSFET) is rated for 11 amps and the inductor saturates at 16A with a current rating of 9A. My battery has a max drain of 10A. The controller can throw out 5A at 6V before it starts current limiting. That puts the battery right at its drain limit. Not that I'd ever actually vape at 30W, but for testing, you always want to check your limits and make sure your device doesn't go boom.

 

[pmos69]

How about the LTC3113 ?
It has a programmable frequency from 300kHz to 2MHz
Output V max is 5.5V though.

 

[CraigHB]

That's a really cool part, the way it has all 4 FETs built in and a wide programmable frequency.

Though, I would not recommend it for layman use. There's a couple reasons. For one, it uses voltage mode control and requires an external compensation network, hard to design. Reference page 17 of the data sheet. You can try the reference design and even the formula they give, but other factors like PCB layout, parasitic impedances, and component characterisitcis can effect compensation performance. The reference design or formula they give may or may not work right out of the gate for your particular circuit. You often have to tune that sort of thing on the bench or via simulation and it's not a trivial task, even for a professional. Also, the output ratings are a little low for our application. It probably would work, but it would be running close to or at its limits.

 

[pmos69]

That's a really nice insight.
Thanks Craig.

Sent from my GT-I9000 using Tapatalk

 

[cumuluslimbus]

how about this?
DC-DC Boost Converter Step Up Power Module 3V to 5V 3A | eBay

should work with no mods needed.

 

[CraigHB]

It would do the job, but....

It's rather large.

It's not variable.

It's probably not that efficient. Looks like it's asynchronous which means it uses a diode rectifier instead of a MOSFET. Probably around 85% efficient with an atomizer load. It says up to 92%, but that's peak which means it only applies to lighter loads at higher input voltages. A good synchronous booster has around 98% efficiency peak and 92% minimum.

In any case, it's not like boosters with higher outputs are easy to come across so it has that going for it. It may be possible to mod it and make the output variable, but even if you could, the controller might go tango uniform if you try to drive the output up to 6V.