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Power conversion ICs

Discussion in 'Electronic Design' started by David Collier, Aug 24, 2006.

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  1. I'm looking for some more cost-effective alternatives to a couple of LTC
    chips.

    1. 2-car-battery to 5V ( LTC1976 )

    I need a chip which is safe to use on a truck *( with additional input
    protection ) So runs at the alternator voltage for a truck, of about
    30V, and doesn't die during spikes of up to 40V or so.

    I need about 3A out at 5V(ish)

    2. LiIon to 3V3 ( LTC3443 )

    that's all really - LiIon in, from 4.2 down to 2V5ish as it discharges,
    3V3 out until the battery gives up.
    500mA plus would be great, I'd think about one that did less. I may want
    to power a radio transmitter ( BT, 800MHz, Zigbee or something so can't
    go much below half an amp burst capability )

    LTC want $11 for the pair at 100-off, which is making my budget a bit
    sick. Any suggestions that don't start with the letters MAX?

    David
     
  2. PeteS

    PeteS Guest

    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
     
  3. linnix

    linnix Guest

    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.
    3A is not a problem.

    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
     
  4. PeteS

    PeteS Guest

    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:
    http://focus.ti.com/docs/prod/folders/print/tps40055.html

    $1.35 in 1k qtys. At 3A, you won't find many integrated solutions, but
    look at this one:
    http://focus.ti.com/docs/prod/folders/print/tps5430.html

    $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.
    http://www.linear.com/pc/productDetail.do?navId=H0,C1,C1003,C1042,C1032,C1063,P2310

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

    For the Li+ to 3.3V, TI shows this:
    http://focus.ti.com/docs/prod/folders/print/tps61131.html

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

    I am sure National and Analog Devices do similar stuff.

    Cheers

    PeteS
     
  5. linnix

    linnix Guest

    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.
    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.
     
  6. PeteS

    PeteS Guest

    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:
    http://focus.ti.com/docs/prod/folders/print/bq24103.html

    $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
     
  7. linnix

    linnix Guest

    We are designing a product for 12V auto and thus 30V max.
    Cost (<$10) and power efficiency (>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.
     
  8. PeteS

    PeteS Guest

    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
     
  9. Joerg

    Joerg Guest

    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.
     
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