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TPS82085: Low Dropout Operation (100% duty cycle)

Carlos Moreno1
Intellectual 550 points
Part Number: TPS82085
Other Parts Discussed in Thread: TPS22919

Hi,

Section 7.3.2 (page 8) of the datasheet talks about a low dropout operation, described as the device "entering 100% duty cycle mode".

Since the device offers no user interaction for configuration (other than the EN pin), I figure that the device internally decides to enter that mode when it has to?

So, from my perspective, do I simply set up the resistor voltage divider so that it sets up a target Vout ⩾ Vin?

Thanks,

Carlos
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  • 0
    TI__Guru**** 151530 points

    Hi Carlos,

    Normally, the TPS82085 is used to create a lower, regulated output voltage from a higher, possibly not-so-well regulated input voltage.  Thus, as the input voltage drops close to the output voltage, the IC would enter 100% mode, if needed, to maintain Vout at the desired voltage.  If the input voltage drops further, the output voltage will also drop. 

    Do you need a regulated output voltage from the TPS82085 or are you using it differently?

  • 0 in reply to Chris Glaser
    Intellectual 550 points

    Chris Glaser said:

    Do you need a regulated output voltage from the TPS82085 or are you using it differently?

    Thanks Chris for your reply.

    To answer your question, it's sort of both:  I have a 3.3V source that is regulated (comes from a DC/DC converter).  Then, I am using several TPS82085s, to generate several lower-voltage rails.

    One of the rails that I need is 3.3V, and I was initially planning to use the source itself.  However, the recommendations for power-on sequencing call for the 3.3V rail to be powered-on last!

    Although I have the option to use a MOSFET to connect or disconnect the 3.3V to the output, I would need two MOSFETS (one n-channel to invert the PG output from the previous TPS, and one p-channel as the on/off switch), and each MOSFET is already the same size as the TPS.  Plus, the TPS has soft start and overcurrent protection.

    From your description, it sounds as if it should work reasonably well if I just set up a target voltage of 3.3V  (with an R1:R2 ratio of 25:8)

    Follow-up question:  if I do set up a TPS82085 to permanently work in 100%-mode, can I reduce the output filter capacitor, say, down to 4.7uF or 2.2uF instead of the prescribed 22uF?

  • 0 in reply to Carlos Moreno1
    TI__Genius 13760 points

    Hi Carlos,

    I since you will not be regulating the voltage on this 3.3V rail I believe a load switch would be the best solution here. The load switch integrates the discrete solution you described with 2 MOSFET's in one single package. If the over current protection is something you would need, I would suggest taking a look at the TPS22919 or the TPS22948. Both devices also incorporate the soft start feature to manage inrush current when enabling the switch into a discharged load. Do you think a switch like this would work in your application?

    Best regards,

    Andy Robles

  • 0 in reply to Andy Robles
    Intellectual 550 points

    Andy,

    I have to say:  ten out of ten!!   This is a great suggestion, and I think I will definitely go with the 22919  (higher current and lower Ron resistance).

    I'm a bit surprised that the switch is an N-channel MOSFET .... but then, the datasheet mentions that the controller has its own charge pump, I guess to generate a gate voltage above the supplied input voltage  (i.e., above the voltage at the MOSFET's source terminal).   So, I guess a kudos is in order!

    Best regards,
    Carlos
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  • 0 in reply to Carlos Moreno1
    TI__Genius 13760 points

    Hi Carlos,

    I'm glad you like the device!

    If you have any more questions on the TPS22919 please create a new post and tag the device.

    Best regards,

    Andy Robles