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TPS43000 Asynchronous

CraigHB
Prodigy 215 points
Other Parts Discussed in Thread: TPS43000

Hello,

New to the forum here.  Been using Ti stuff a while, but never had a need to post on the forum until now.  I have a qestion about a TPS43000 design.

Can I use the asynchronous configuration for a SEPIC toplogy with this chip?

If so, what is the approximate difference in efficeiency between asynchronous and synchronous control with an input of 3.5-4.2V and an ouput of 1 to 3 amps at 3 to 6V?  I need to determine if it's worth the extra cost in components to use the synchronous design.

Also, my design uses a variable output voltage. Will the compensation circuit work okay with a digital pot as RBias.

TIA

  • 0
    TI__Guru 56925 points

    Tia,

    Do you realize your user name is Craig?

     

    Back to your questions.  I presume when you say Asynchronous, you're refering to Non-Synchronous (1 MOSFET + 1 Diode) rather than Synchronous (2 MOSFETs)

     

    The efficiency benefit moving from Non-Synchronous (Diode rectifier) to Synchronous (Active FET rectifier) is the difference between the diode losses (Idiode(avg) * Vfd) minus the MOSFET losses (Ifet(rms) * Rdson + Ddrive * Qgate * Fsw)

    In a SEPIC, the diode carries the average load current (in pulses, but since diode forward drop is near constant, we're only concerned about average).  For a 1-3A load, the diode losses will be 0.4 - 1.5W depending on current and diode selection.  (You're losing 8-15% of the ouptut power depending on the ouptut voltage)

    With good MOSFET selection, you should be able to recover most of those losses.  I'd expect a 5% efficiency improvement at 6V output to a 10% improvement at 3V output.

  • 0 in reply to Peter James Miller
    Prodigy 215 points

    Sorry for the forum jargon there, been hangin out on too many casual forums too long.  TIA is "thanks in advance", so thanks. 

    I wasn't sure if the PMOS bypassed the "optional" diode shown in the typical application.  It wasn't clear to me.  The PMOS on current goes backwards from what I'm used to seeing, drain to source instead of source to drain.  Didn't know you could do that with a P-Channel.  In any case, if I still get RDSON going backwards, it will make a big difference, especially at higher currents.

    I think I've also found the answer to the other question myself.  As far as I can tell at this point, RBIAS does not come into play when calculating the compensation loop so there shouldn't be any problem using a digital pot there.  I couldn't find any maximum rating for the FB pin on the chip.  Do you know what the voltage tolerance is on that pin?  Concerned about transients with the digital pot, could spike FB with V0 on microcontroller reset if the POT shuts down before the TPS43000 receives a shutdown signal.

    Now, if I can just get the compensation loop figured out, I'll be in business.  It's a hairy task.  First time doing a compensation loop and to boot, a SEPIC with all those crazy poles and zeros.  Been trying to remember all that analog stuff I forget from college years ago.  By any chance, is there some kind of online app that allows you to just plug in the values?

  • 0 in reply to CraigHB
    TI__Guru 56925 points

    The PMOS on current goes backwards from what I'm used to seeing, drain to source instead of source to drain.  Didn't know you could do that with a P-Channel.  In any case, if I still get RDSON going backwards, it will make a big difference, especially at higher currents.

    In the "ON" state, MOSFETs conduct equally well in the forward and reverse direction  When turned OFF, they look like a zener diode (at Vbd) in one direction (Drain to Source for an N-Channel and Source to Drain for a P-channel) and a high voltage drop diode (called the body diode) in the other direction.

    "Forward" current is generally considerd to be against the flow of the body diode (Drain to Source in an N-Channel MOSFET and Source to Drain in a P-channel MOSFET) with reverse current being in the direction of the diode (since the MOSFET will conduct, even if the channel is "OFF")

    I think I've also found the answer to the other question myself.  As far as I can tell at this point, RBIAS does not come into play when calculating the compensation loop so there shouldn't be any problem using a digital pot there.

    You are correct.  Rbias helps set the DC set-point of the control loop, but in AC analysis, both terminals of the resistor are "ground" (DC voltages = 0V in AC analysis) so it has very little effect on loop compensation.  The biggest concern with using a POT in this location is the accuacy of the POT (since it sets the output voltage) and transients caused by sudden large changes in the resistance.

    The FB pin should not go lower than -0.3V or higher than 6.0V or VIN, whichever is less.

    Now, if I can just get the compensation loop figured out, I'll be in business.  It's a hairy task.  First time doing a compensation loop and to boot, a SEPIC with all those crazy poles and zeros.  Been trying to remember all that analog stuff I forget from college years ago.  By any chance, is there some kind of online app that allows you to just plug in the values?

    I'm afraid I'm not aware of one.