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TPS92641: TPS92641:

jimmy kan
Prodigy 130 points
Part Number: TPS92641

What is the voltage The Vout  PIN  of  TPS92641  ?

I found the value is 2.5V in the Application of spec.  but In the TPS9261EVM-slvu823 doc, the value is  2.0v.

I don not know which value is right?

please tell me how to select it and  design the output voltage?

  • 0
    TI__Mastermind 28655 points

    Hello,

    The Vout pin is used for over voltage protection as well as setting the ramp threshold for switching frequency.  Section 7.3.12 on page 17 shows the calculation for OVP using 3.05V.  This is 22% higher than 2.5V which is reasonable for OVP depending on the design requirements.  Section 7.3.2 on page 12 explains how the switching frequency is set by Ron, Con and the Vout divider.

    As for 2.0V or 2.5V, both work it's just one will have a higher over voltage trip threshold percentage (52% versus 22%)  with respect to the LED string voltage.  Note that the TPS92641 does not regulate voltage, it regulates current.  There is nothing in the IC that happens at 2.0V or 2.5V in regards to the Vout pin.  It sets the voltage for the internal ramp for switching frequency and will shut down if it hits 3.05V, over voltage protection.

    Best Regards,

  • 0 in reply to Irwin Nederbragt
    Prodigy 130 points

    Hi，Thanks for your reply.

    but my LED sting typ. voltage is 33V  3 A, then how to design the value of resistors as Rvout  Riadj  and so on?

    please give us some advices.

    thank you so much.

  • 0 in reply to jimmy kan
    TI__Mastermind 28655 points

    Hello,

    First I would set the OVP threshold.

    Example:  Set OVP 20% higher than 33V (pick the percentage you want).  Vovp is 3.05V, 20% higher than 33V is 39.6V, this ratio puts the Vout pin at (33V/39.6V)*3.05V = 2.54V (this is close to 2.5V as the datasheet uses).  The datasheet chooses the bottom resistor at 10K, I'll chose the top resistor as 100K and calculate the bottom (it doesn't really matter which method).  (39.6V - 3.05V)/100K = 0.3655 mA.  The lower resistor would be 3.05V/0.3655 mA = 8.34 Kohm, 8.25 Kohm is close or 8.45 Kohm.

    Calculate these values before calculating the switching frequency where Con and Ron can be varied with known Rvout1 and Rvout2.

    Riadj can be pulled to Vref with a resistor, 100K or so, or it can be used as a divider.  If it is used as a divider it will affect the Rcs calculation.  At 2.54V the Vcs will be 0.254V.  For 3A currrent the Rcs value would be 0.254V/3A = 0.0846 ohm.

    Read through the datasheet, it has all the calculations needed, check the EVM as well.

    Best Regards,

  • 0 in reply to Irwin Nederbragt
    Prodigy 130 points

    thanks  a lot!

    I found the EVM is different from the datasheet. please check it.

    it is ok or not.

    thanks!

  • 0 in reply to jimmy kan
    TI__Mastermind 28655 points

    Hello,

    It is fine, like I said it depends on what you want OVP set to.  TPS92641 does not regulate voltage so all that pin does is set OVP and it is a portion of the switching frequency calculation.

    Best Regards,

  • 0 in reply to Irwin Nederbragt
    Prodigy 130 points

    hello,

    I got it.  but I want to know if no load,  what is the output voltage?

    thanks!

  • 0 in reply to jimmy kan
    TI__Mastermind 28655 points

    Hello,

    No load as in a disconnected load?  It will go to OVP and shut off until the Vout pin drops 0.13V.  So from 3.05V to 2.92V.  The TPS92641 is a current regulator it does not regulate voltage.  The voltage will go to whatever is allowed by the load and the current regulation set-point.  If the TPS92641 is set to regulate 3A it will drive 3A to the load.  Don't think of it as an output voltage, it is an output current.  It is also a buck converter so if you set the OVP to be higher than the input voltage the output will only be able to go as high as the input voltage with an open load.  This also assumes there is an output capacitor is present.

    Best Regards,