• Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

High current switching regulator LM274X

Status
Not open for further replies.

tnay

New Member
Hi,

I have been checking several regulators circuit to drop the voltage from 7.4V to 6.0V. My application is likely to draw about 8-12A of current, so I figure that I need a switching regulator to increase the efficiency.

I come across numerous regulator, like LM2742, and other LM274X series, while some site mention the LM3477, 2642, 3075 and etc.

Unfortunately, I don't have enough electronics knowledge to understand the difference of this chip, and still struggling to read through the datasheet and application note.

I like to ask, if anyone has deal with this chip before? What is the selection criteria? Many thanks in advance! :)
 
Last edited:

dougy83

Well-Known Member
I had a quick look at the LM2742, LM2642 & LM3477. The LM2742 provides a driver for synchronous rectification, which improves efficiency. The LM3477 does not.
The LM2742 has an adjustable operating frequency. The LM2642 is for providing dual outputs and is not a good option for your project.

Of those 3, I would use the LM2742. The mosfets should have low RDS(ON), gate charge is not as important when compared to the large conduction current. The mosfets will be on for similar times due to the low in-out voltage difference, and the same sort could be used for high & low side. I would also put a low forward voltage schottky diode across the low side fet to marginally improve efficiency.
 
Last edited:

indulis

New Member
A 7.4V to 6V buck will have a duty cycle of ~81%, so they will not be on for similar times. As for gate charge... if a MOSFET is under driven, switching losses can get higher than you might think relative to the power dissipated due to I²R (don't forget to multiply by the duty cycle when calculating dissipation)
 

tnay

New Member
Hi all, thanks for the quick reply. I'm using Lipo cell, and full charge would give me about 8.4V, and stabilise at 7.4V. It's very tricky. I have consider using linear regulator, but it would dissipate too much heat. Diode would only give me a fixed power drop, not an ideal application.

Running time is not actually a problem to me, (and efficiency is not a major concern), but weight is the constraint. I like to keep component count (and weight) down to as reasonably minimal, so using large heatsink to dissipate the heat is not be great idea. Hence I drop the linear regulator.

I have yet to check out the electronics store here. The reason I put a few options so that I could go for another one if they don't have what I look for.

I will try to post up my circuit design shortly. Thanks again.
 

Diver300

Well-Known Member
Most Helpful Member
You can run Lithium-Ion batteries down to about 3.4 V per cell, so you will be down to 6.8 V.

What is it that you are driving? If it is an electric motor, a PWM speed controller will sort it all out and handle the 8.4 V.

It seems odd that weight is a concern, but run-time isn't.
 

tnay

New Member
I'm using that to run 12 servos on a walking robot. Normally I'll be running it from DC supply, only on certain occasion (i.e. demonstration) when I need it to be mobile (and wireless), I'll need the battery. :)
 

smanches

New Member
Why are you trying to drop the voltage? Are you also powering control circuitry from it as well that needs 6v? Or is this only for the motors themselves?

If it's for the motors only, then I would go the diode route. The only other practical solution is to use a DC-DC converter which for that much current would not be small.

Depending on the capacity of the battery, the voltage drop from the 8-12A load might bring it down to 6v anyway. You might only need a single diode which will increase efficiency.
 

tnay

New Member
I'm using 5V microcontroller, PIC. But that would come from a separate LDO regulator, from the same supply. I need this switching especially for the 12 servos. When load, one servos could draw up as much as 1A.

I read that Lipo cell are very stable and can provide high current with minimal/no voltage drop. Looks like diode would be the best option for my case? Any diode recommendation? Would 2x 5A or 6A diode good for the job? I like to split it up to be safe, or should I be concern?

Will there be any heat problem?
 
Last edited:

smanches

New Member
Any typical silicon power diodes should work. Not sure if you could put multiples in parallel to increase the current rating for this application or not. I know there are issues with putting diodes in parallel, but not sure how they apply to such a simple application.
 

tnay

New Member
What I mean is to arrange the diode in such way that 1 diode is for 6 servos, while another diode for the remaining 6 servos. If otherwise, I could even breakdown to 1 diode for each servos, but that is just too much :D
 

smanches

New Member
There will be some heat from the diodes as they will have to dissipate 0.7V x 12A = 8.4W. Shouldn't be too hard to dissipate it though.
 
Status
Not open for further replies.

EE World Online Articles

Loading
Top