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WEBENCH® Tools/TPS62136: forced pwm cannot use 1.5uH ?

Stefan Vranken
Prodigy 120 points
Part Number: TPS62136

Tool/software: WEBENCH® Design Tools

Hello, I am using TPS62136 to convert from 12V to various supply rails 1.0/1.8/3.3/5.0.

Webench simulator autoselects L=1.5uH for TPS62136 in PWM/PFM-mode (mode-pin=low).

 

To exclude 2.8% accuary of power-save mode (PFM) and always have 1% accuracy I would use "forced PWM" (mode-pin=high)

Webench simulator autoselects L=2.2uH for TPS62136 in PWM-mode (mode-pin=high).

When I change L from 2.2uH to 1.5uH then webench displays next warning :

 

The following limits have been exceeded which may cause instability in your design:

- The property L with value'1.5E-6 H' is lower than the minimum recommended value of '2.2E-6 H'

 

QUESTION:  Is it allowed to use  L=1.5uH if TPS62136  has mode “forced pwm” ?

Or do I have to use recommended 2.2uH ?

Notwithstanding this instability warning for L=1.5uH, my simulations (start, steady-state, loadstep) are stable.

  • 0
    TI__Guru**** 150900 points

    Hi Stefan,

    Any Webench model should always be following the D/S recommendations for component selection. Even though Webench can give you an initial design, I still recommend that you read the D/S to understand how the part operates.

    1.5uH is the minimum recommended value for lower Vouts, like your 1V and 1.8V rail, per page 13 of the D/S.

  • 0 in reply to Chris Glaser
    Prodigy 120 points
    Hello Chris,
    I thought it was ok to use 1.5uH (design with large Vin and small Vo) as D/S page-13 states not to use "smaller values than 1.5uH". Anyway, I will change to recommended 2.2uH for conversions with large offset between Vin and Vo (my case 12V to 1.0, 1.8).
    I intend to also make this change for conversions from 12Vin to (3.3, 5.0).
    >> Another warning I get in webench simulation is when I change Cin rated voltage from 25V to 16V: warning says min value = 17.64V ??
    Regards,
    Stefan
  • 0 in reply to Stefan Vranken
    TI__Guru**** 150900 points
    Hi Stefan,

    Thanks for the feedback. Yes, 2.2uH is safer.

    What Vin range did you enter for this design? There is some margin automatically applied to all component ratings in all designs. Most customers would not use a 16V component for a 15V rail, for example.

    Webench is designed to give you a safe starting point for a design. Feel free to tune/optimize it for your needs.
  • 0 in reply to Chris Glaser
    Prodigy 120 points
    Hi chris,
    I specified Vin range from 11.4 to 12.6 (12V niminal 5% tolerance)
    I will share a design.
    Stefan.
  • 0 in reply to Stefan Vranken
    TI__Guru**** 150900 points
    Thanks for sharing.

    Yes, the 17.64V is 40% on top of your 12.6Vin maximum rating. We should be reducing this margin to 10% in the next update. In any case, every customer has their own required amount of margin for their designs so no one default value will work for all customers.

    Did you have any other open questions on how to use the device?
  • 0 in reply to Chris Glaser
    Prodigy 120 points
    Chris,
    no more questions right now but i am still working on my design....close this ticket ?
  • 0 in reply to Stefan Vranken
    Prodigy 120 points

    Hello Chris,
    I now use 2.2uH instead of 1.5uH, can you review ?

    Worst case for inductor is buck from 12V to 1.0V
    inductor ripple current peak-peak = V/L*Ton  ; datasheet eq(10) uses fixed Ton 100ns ?

    Ton = 250ns * Vin/(Vin-Vo) ; datasheet eq(1)
    Ton = 0.25us * 12/(12-1) = 0.273us

    L Ipp = (Vin-max - vo) / L * Ton
    L Ipp = (12.6 - 1.0) / 2.2uH * 0.273us = 1.44 A   ;webench reports 1.431 A

    maximum inductor current
    IL = Io + L Ipp/2 ; datasheet eq(9)
    IL = 2.9 + 1.44 / 2 = 3.62 A ; webench reports 3.615 peak switch current in IC
    This is lower than internal FET current limit min = 4.8A allowing the use of TPS62136
    This is lower than inductor saturation current = 6.2A allowing me to use "wuerth 74438356022"

    WE-MAPI 74438356022

    Size 4.1*4.1*2.1mm

    Isat=6.2A (L-20%)

    Irms=4.7A (T+40)

    DCR max=35mohm

     

     

     

    Webench_1v0_2a9_68uF_2.2uH_pwm_report.pdf

  • 0 in reply to Stefan Vranken
    TI__Guru**** 150900 points
    Hi Stefan,

    Was there anything specific that you would like me to check in your design?

    Your Vin, Vout, Iout inputs are achievable with this IC. It looks like you changed the caps to custom ones, so I assume that you thought through those details. Those values look ok.
  • 0 in reply to Chris Glaser
    Prodigy 120 points
    Hello chris,
    I was looking for reassurance that my inductor is ok because it is not mentioned in datasheet, nor available in webench.

    Question : does IC operate internally with 4Mhz clock ?
    Ton = 250ns * Vin/(Vin-Vo) ; datasheet eq(1)

    Regards,
    Stefan.
  • 0 in reply to Stefan Vranken
    TI__Guru**** 150900 points
    Hi Stefan,

    Thanks for clarifying. Yes, that inductor delivers the required inductance with a high enough saturation current.

    Per the D/S, this IC switches at around 1 MHz. But section 9.4.1 notes the AEE feature. This reduces the switching frequency (strongly for lower Vouts) to improve the efficiency. You can see an example of what the frequency would be in figure 66.
  • 0 in reply to Chris Glaser
    Prodigy 120 points
    Thanks chris.
    I will close this ticket.