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dv/dt for TPS27082L?

Matt Shira
Expert 2180 points

Other Parts Discussed in Thread: TPS27082L, TPS27081A, TPS22967, TPS22910A

Hello,

Is TPS27082L/TPS27081A suitable for intense pulse applications?

When looking at their datasheet, they appear like just like basic P-Channel FET (Q1) controlled by another N-channel FET(Q2), packaged inside a single IC.

However, its classification under “USB power switch” somewhat confuses me: what its relationships with USB, since nothing seems to be related to the 5V VBUS power requirement of USB devices. In addition, the “Application Examples” are all about in-rush protection, standby module, power sequencing, which also has little relationship with USB.

I wonder whether it is usable as a generic fast switch?

In our application an LED is going to be flashed with 200μs pulse at high current (1A) with supply from a super capacitor, and we originally consider making the switch using N-channel MOSFET. However, after viewing some documents, it seems that dv/dt poses serious challenges for high switching frequency. Without experience in analog electronics we found it difficult to understand is the effect of the intrinsic/parasitic BJT transistor. Several documents

  1. Fairchild AN9010 MOSFET Basics (http://www.fairchildsemi.com/an/AN/AN-9010.pdf)
  2. Power MOSFET Tutorial (http://www.microsemi.com/document-portal/doc_view/14692-mosfet-tutorial)
  3. Power MOSFET Basics (Vrej Barkhordarian, http://www.irf.com/technical-info/appnotes/mosfet.pdf)

stressed the importance of concepts like {dv/dt capability, diode recovery, Diode Recovery dv/dt}, and it appears that the solutions are not trivial.

In TPS27082L datasheet “Configuring Turn-ON slew rate”, it mentions that “switching a large capacitive load CL instantaneously results in a load inrush current”. Since our load is an LED which is more close to being “resistive” than “capacitive” (correct?), is it free from the in-rush current problem?

Additionally, do we need to consider {dv/dt capability, diode recovery, Diode Recovery dv/dt} when driving this LED in fast switching mode using TPS27082L/TPS27081A?

 

Matt

  • 0
    Expert 2180 points

    Hi Matt,

    This should not change the rise/fall time for the TPS27081A version.  These passive components aren't used to 'program' the device.  The rise time is primarily determined by how quickly the input (VIN) charges the Output Capacitance (C1) - fairly straight forward.

    The other rise time factor is how quickly the FET turns on and its ON resistance (as determined by VGS).  The VGS is a voltage divider between R1 and R2, so the VGS term is factored out of the final equation.  For the TPS27081A, using the same R1 and R2 in a parallel configuration would result in the same VGS as a stand-alone situation.  Since the VIN and C1 would also be the same, there will be no difference in timing.

    The TPS27081L version would see a difference in behavior since the R2 is integrated as Rs.  In this case, the Rs resistors would be in parallel, resulting in an R2 value equal to 1/2*Rs (12.5kohm).   You should be able to use the equations from the A version to find the expected timing since the L version equations assume 12.5kohm.

    Let me know if this makes more sense!

    Best Regards,

    Adam Hoover

    Load Switch Applications Engineer

    Adam,

    Both ICs (tps27082L/tps27081A) are specified as 3A device. If we do have a large current (at least 1A as described in dv/dt for TPS27082L?), and that Table 1 shows we need only very small caps for faster rising time.

    Does it mean that: after the small value C1 is charged to VIN voltage, the load is then going to be supplied directly by VIN?

    If this being true, can we disconnect C1? What is its purpose?

    Fall time:

    Additionally,

    "The rise time is primarily determined by how quickly the input (VIN) charges the Output Capacitance (C1) - fairly straight forward."

    Does this explanation also apply to fall time case, such that fall time is just the time required for C1 to get discharged? How are

      1. toff> R1 C1 (tps27082L)
      2. toff > 2 R1 C1 (tps27081a)

    derived?

    If we have load with RL which however usually much smaller than R1's default 125KOhm, do we need to include RL into the calculation?

    The formula gives only lower bound. Do we have an accurate bound for VOUT to fall until 10% of VIN?

    Why the toff for the two devices differs by a factor of two?


     

    Matt

  • 0
    TI__Expert 5275 points

    Hi Matt,

     

    This device is included under the USB load switches, just because that is where people are likely to search for this type of device.  Other than that, there is nothing that makes it specific for USB.  The datasheets make no mention of USB. 

     

    Also, I wasn't able to view the documents you provided, but all design considerations should be included in the datasheet:

    Fairchild - "Requested page was moved or not available"

    Microsemi - took me to the home page

    IRF - "Server Error 404 - File or directory not found"

     

    As for the inrush, this is a very real design concern and is dictated by the amount of output capacitance and rise time.  The TPS27082L will allow you to adjust the rise time, but at the cost of additional RON resistance.  Since your load is mostly resistive this will be less of a concern, but surely there is some capacitance even if only parasitic.

     

    By the way, have you considered one of our integrated N-Channel Load switches.  The TPS22967 will allow you to adjust the rise time without increasing the RON.  The TPS22910A (P-Ch) would also provide very quick rise time (~6us).

     

    I will now respond to the new questions you have just posted.

     

    Regards,

    Adam

     

  • 0 in reply to Adam Hoover
    Expert 2180 points

    Adam,

    Sorry for the broken links. I just updated them in the original post.

    Could you please help to evaluate the suitability of using TPS27082L, TPS27081A, TPS22967 and TPS22910A on the particular LED pulsing application? In particular we want the pulsing times to be accurate, so rising/falling time is an important concern.

     

    Matt

  • 0 in reply to Adam Hoover
    Expert 2180 points

    Adam,

    Very particularly, since

    1. the LED load is not capacitive, hence inrush is unlikely to be extreme

    2. LED itself is a diode which prevent reverse current, so it seems the body diode/BJT within the current passing MOSEFT (internal Q1 27082 & 27081) is not possible to have any reverse current due to the presence of LED.

     

    So I guess that perhaps the

    1. dv/dt capability
    2. diode recovery
    3. Diode Recovery dv/dt

    problems which are important for high-speed and how voltage MOSFET switching applicaitons, are not really relevant here? Can we ignore them?

     

    Matt

  • 0 in reply to Matt Shira
    TI__Expert 5275 points

    Matt,

     

    Thanks for fixing the links, I read over the parts that discussed the dv/dt and diode recovery. 

    1. The dv/dt capability mentions if the VIN is brought up really fast (high dv/dt), there could be capacitive coupling to the gate which would turn on the FET.   This is still possible (in extreme cases) with any of these devices, but I don't expect it will happen since the VIN is always present.  Also, input capacitance helps reduce the dv/dt seen by the device when VIN is first turned on.

    2 & 3. Both come into play when reverse current is passed through the body diode of the FET.  As you pointed out, the LED will block reverse current, so this is not a concern here.

     

    Any of the parts should result in consistent turn on/off times, but you can expect the turn on time to be longer than the turn off time.  The TPS22910A or the TPS27081A (R2 = 0 ohm) would give you the quickest turn on/off times.

    As for your questions about C1, a larger value will result in a longer turn on time because it will take longer to charge up.  The smaller you can make this value, the more quickly the output will rise.  Per the component table on the first page of the TPS27081A datasheet, C1 is optional (only required for load inrush current (slew rate) control).  Removing this capacitor will result in the quickest possible rise/fall time.

     

    Lastly, I don't see a reason why the turn off time equation would be different between the TPS27081A and TPS27082L.  I suspect the TPS27081A has a 2x factor because it came first and was more conservative.

     

    -Adam

  • 0 in reply to Adam Hoover
    Expert 2180 points

    Adam,

    Thanks deeply for taking time to read the documents and answer the questions.

    As for the fall time,

    toff > (R1 × C1 sec)

    Do we have an upper bound when it falls to 10% of VIN, after turning off ON/OFF pin?

     

     

    Matt

  • 0
    TI__Guru**** 174626 points

    Matt,

    The TPS27082L supports up to 3A continuous and 9.5 Amperes pulsed. You will need "R1" but no not install a "C1".  

    How do you control the LED current magnitude?

  • 0 in reply to Ron Michallick
    Expert 2180 points

    I don't think it is possible since if I have found any dedicated TI LED drivers can work up to this speed; they are good in adjusting static LED current but cannot do that during short pulses. So perhaps we just try out best to keep constant voltage.

  • 0 in reply to Ron Michallick
    TI__Expert 5275 points

    Hi Ron,

     

    Thanks for jumping in here.  Can you comment on the difference between the two fall time equations or an upper bound for this parameter?

     

    Thanks,

    Adam Hoover

    DLS Applications

  • 0 in reply to Adam Hoover
    TI__Guru**** 174626 points

    Adam,

    Yes, I am working on this.

    Matt has several posts running on the same devices.