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TPS92515HV: Low output ripple / large inductor

Steve Widener
Genius 11010 points
Part Number: TPS92515HV
Other Parts Discussed in Thread: TPS92515

We are looking to design a ~ 48V input, 34Vout/250mA LED string with minimum ripple - say 0.2%

The dimming ratio is important (10000:1 or so) so the shunt would be used

The desired frequency is around 200kHz operating.

Are there any issues with this low ripple, imagine using a ~ 1mH inductor.

  • 0
    TI__Mastermind 28655 points

    Hello Steve,

    I just ran a few quick calculations.  At 200 KHz switching frequency, Vin = 48V, Vled = 34V, Iled = 0.25A and 1 mH the current ripple percent will be approximately 10%.  To get to 0.2% the inductor value would need to be around 50 mH (standard value probably 47 uH).  This will be a fairly large inductor and difficult to find a standard one.  Note that at this low ripple the peak current detect may cause the on time to vary since the current sense voltage change is so small.

    What frequency do you plan to shunt FET PWM dim?

    Best Regards,

  • 0
    Prodigy 10 points

    Hi Irvin,

    The current plan is for the shunt FET PWM frequency to be in the 120HZ to 250Hz range, but higher frequencies could be considered if shunt FET dimming performance can be improved.

    Any suggestions or input in implementing shunt FET dimming would be helpful as the information in the TPS92515 DS is minimal.

    Thanks,

    George  

  • 0 in reply to George Smith
    TI__Mastermind 28655 points

    Hello George,

    At 120 Hz PWM at .0001 duty cycle the on time is 833 ns.  This is only a portion of one 200 KHz switch cycle.  With low ripple it shouldn't be noticeable however keeping current regulation close may be difficult.  It will require setting the second Roff resistor to operate shunted similar to the unshunted current.

    Higher shunt FET pwm frequencies can be used to see if there is any benefit however at low inductor current ripple I would think it won't matter much.

    Best Regards,