Power conversion ICs

Try TI, AD and National. I'm using their parts (and some LTC, although I agree they are pricey).

As I am not sure of the complete picture of your system, you'd probably be best using the parametric searches.

Cheers

PeteS

It's possible to build it, but not with a single chip.

I would suggest a 30V (get expensive beyond 30V) switching regulator with a voltage divider to drop from max of 40V to 30V. You only need to deal with about 200mA for the controller itself. You can get a 60V power MOSFET for the by-pass current. I would set the ouput to 6V to 7V.

Now hook up a micro-controller to run secondary switches based on the NiIon or NiMH voltage. I suggest 3 cells for a range of 3V to 5V. Connect the switches in parallel with power diodes (for proper voltage drops) to provide a 3V to 3.5V range. I.e. 0V, 0.7V,1.4V, etc in power drop across the diode network.

It can be built for less than $10, depends on qty.

PCB: $2 Switching controller $1 Power Mosfet: $1 Microcontroller: $3 Load switches: $2 Diodes: $1

A voltage divider? There are better ways (an emitter follower with base tied to a zener comes to mind) for controller power to keep the source impedance down. With a divider you'd be dissipating huge amounts in the divider. You'd still be dissipating a fair amount on the follower, but not nearly as much.

Besides, there are 40V parts out there that aren't too expensive For example:

$1.35 in 1k qtys. At 3A, you won't find many integrated solutions, but look at this one:

$1.85 1k qtys. Vin (max) 36V. Use a snubber along with a decent transorb and you'll be ok. You'll have to have protection against load dump (80V in a truck typical) anyway.

Stick a SM8S33 on the front (or maybe a sharper cutin device) and an LC filter and you're all set. Most of the cost is in the FETs and the inductor, anyway for external switch devices.

Linear Tech shows no such animal as a LTC1976 on their website.

For the Li+ to 3.3V, TI shows this:

designed specifically for 3.3V out from a single Li+. $2.05

I am sure National and Analog Devices do similar stuff.

Cheers

PeteS

OK, a voltage divider with darington drivers. You don't need good regulation for the controller input (10V to 80V, 200mA). Yes, you should design for 80V, not 40V.

Same issue with 80V.

Agree.

You also have to design the charging circuit for the Li+ cell. We use a microcontroller to monitor the cell voltage and switch on/off the charging source. A few weeks ago, we had a discussion here of how dangerous over-charging are. That was before the news on Dell/Sony's laptop matches.

I have a number of products in vehicles, so I deal with this issue daily :)

My regulators can withstand 40V and I ensure the front end won't exceed that with some extra circuitry. The FETs (I have to supply 10A impulse currents on one unit) are rated at 60V. Never had a failure due to front end damage.

I use TI BQSwitchers for my LI+ chargers. A number of outfits make them. Page at:

$2.00 for the standalone versions. Add an inductor and a few passives. Works precisely as advertised, but can't handle much in the way of ESD and don't exceed Vin(max). Power it from the 5V supply and you're in great shape. It's actually easier with a single cell - my units have two series cells so I have to provide 9.5V min.

I have about 20k units in the field with these and I have had zero problems with the charger or the batteries.

The usual suspects make equivalent devices, of course.

Cheers

PeteS

We are designing a product for 12V auto and thus 30V max. Cost (90%) are important. We budget for $10 including an AVR.

Our solution is (30V->6V->3.3V). Your solution (30V->5V->1.2V->3.3V) would cost more than $10.

My systems must be able to go into either 12V or 24V vehicles, which makes the design a little more 'interesting' and a little more expensive. I also have to design for Pet Regs (hazardous load) vehicles, which puts more 'stress' on the design, so to speak.

The current batch operate from 10.5 to 36V in (nominal) plus all the usual certifications.

If I had only to deal with a 12V vehicle things would certainly be simpler.

Cheers

PeteS

Hello David,

As others have said it can be more than 40V if something big disconnects or takes a dump.

Not in a single chip, really. Take a look at the National LM5005. 75V max in and 2.5A (but not under all conditions).

I'd consider a design around a smaller normal PWM chip and a separate FET.

There should be lots of chips for that at TI and others.