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TPS92515HVEVM-749: TPS92515

Conny Do
Prodigy 250 points
Part Number: TPS92515HVEVM-749
Other Parts Discussed in Thread: TPS92515

Hi,

I am trying to use a TPS92515HVEVM board to control a LED COB.

ILED max 700mA and LED Vf: 35V, Vin = 50V ,PWM dim, 2kHz signal from a MCU. 

I use the TPS 92515 Design Calculator to get related component values. The result is not perfekt, the LED can be controlled with PWM dimming but it is flickering at low level. I use obviously the integrated NFET

 

While updating the EVM with new components I found that the compnent values on the EVM have not any related Design Calculator sheet,  I would like to know how TI did the calculation of the EVM. 

  1. Can I  get that design calculation sheet?
  2. How can achieve a smooth dimming at low light level? 

 

Thanks for the support!

Conny

  • 0
    TI__Mastermind 28655 points

    Hello Conny,

    What PWM frequency and what duty cycle are you running at low light level (or what is the pwm minimum on-time and pwm frequency)?  What switching frequency is the TPS92515 running at?  What inductor value was chosen?  Is there an output capacitor (capacitor across the LEDs), if so what value?  What is Roff and Coff?

    The design calculator gives one method to calculate component values for a design.  When I create a design I just calculate using buck converter equations as well as some of the datasheet equations such as for Roff.  This is probably how the EVM was created.

    I can also calculate the desired design as well.

    Best Regards,

  • 0 in reply to Irwin Nederbragt
    Prodigy 250 points

    Hi Irwin!

    Below are the parameters you wonder.

    The PWM frequency: 2kHz, Lowest duty cycle 0.01%. Roff =51k, Coff=470pF, (Boot Cap.) C8=10nF,  (Output Cap) C9 removed. 

    TPS92515 fsw=500kHz. L1=47uH;  Rsense R5//R6 = 0,22R.

    Due to audible noise C1, C2, C3 were replaced with 10uF/50V electrolyte

    Thanks!

    Conny

  • 0 in reply to Conny Do
    TI__Mastermind 28655 points

    Hello Conny,

    I calculated using the higher Viadj jumper setting (divider on Iadj is 100K, 61.9K per the EVM).  I'm not sure how you are testing but this is the closest I get to 700 mA, is this correct?

    At 2 KHz PWM dimming (not shunt FET dimming) and 0.01% or 0.0001 the on-time for the pwm pulse is 50 ns.  This may not let the boot capacitor refresh.  You would have to look at the SW node to see if it is getting to GND.  If SW is on for 50 ns the current can only ramp to 16 mA excluding parasitics.

    0.0001 or 0.01% duty cycle is very low light level is this correct?

    Calculations:

    Vin 50 V
    Vled 35 V
    Iled 0.7 A
    Fdim 2000 Hz
    dimdutymin 0.0001  
    Voff 1 V
    Roff 51000 ohms
    Coff 4.7E-10 F
    Rcs 0.22 ohms
    Lind 0.000047 H
    Fswdesired 500000 Hz
    Cboot 0.00000001 F
    Vcs 0.191167387 V
         
    Ioff 0.000676471 A
    toff 6.94783E-07 sec
    ton 1.62116E-06 sec
    Fsw 431789.7372 Hz
    dioff 0.517391304 Apkpk
    dion 0.517391304 Apkpk
    Ipeak 0.868942669 Apeak
    Iave 0.610247017 Aave
    pwmminon 0.00000005 sec
    dipwmmin 0.015957447 A

    Best Regards,