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TPS92515HV: R_OFF Calculation

Evan Cornell
Expert 1410 points
Part Number: TPS92515HV

I am working on the TPS92515HV calculations and have a few questions. 

I will be implementing FET shunt dimming, so I require R_OFF2 as described in section 8.3.4 of the datasheet. 

  • For R_OFF1, I can calculate the resistor value at VIN_MIN and VIN_MAX. Which should I use? 

Thanks!

  • 0
    TI__Mastermind 28655 points
    Hello Evan,

    VIN_MIN or VIN_MAX shouldn't matter you are using VCC for the R_OFF1 calculation. Also, the peak to peak ripple is determined by the off-time, LED stack forward voltage and inductor value. The input voltage only affects the on-time and switching frequency.

    Best Regards,
  • 0 in reply to Irwin Nederbragt
    Expert 1410 points

    Irwin,

    For ROFF2 (equation 9), I agree, it's only a function of VCC, not VIN. 

    But R_OFF (equation 17) is function of t_off, which is a function of D, which is a function of VIN and VLED. So you can get two values of R_OFF. 

    Thanks,
    Evan

  • 0 in reply to Evan Cornell
    TI__Mastermind 28655 points
    Hello Evan,

    It is not a function of 'D' if you look how toff is generated it is from the output voltage. If you calculate both conditions using 'D' you will get the same toff. The switching frequency will be different but toff is determined by Vout, Roff and Coff as well as the threshold in the IC. Equation 9 does not have 'D' in it. It is also missing the freewheel diode Vf which should be added to Roff shunt. Equation 17 also does not have 'D' in it. t_off is not a function of 'D'. It is a function of Vled, V(off-time threshold), and Roff, if you are trying to calculate toff. For shunt fet dimming the important part isn't toff but keeping the current ripple the same when shunted. If the shunt FET dimming frequeny is fairly high it's not even that important.

    Best Regards,
  • 0 in reply to Irwin Nederbragt
    Expert 1410 points
    Irwin,

    Okay, that makes more sense, given that the switching frequency isn't constant over the input voltage range. It seems like I should do all my design calculations starting from VIN_Max, which would be the highest F_sw, thereby ensuring thermal design is within acceptable limits.

    In Equation 8 in the TPS92515HV datasheet, there is a 0.7 term in the denominator - is this accounting for the freewheeling diode placed between V_LED and R_OFF1 (Figure 14)? Or are you referring to the free-wheeling diode on the SW node?
  • 0 in reply to Evan Cornell
    TI__Mastermind 28655 points
    Hello Evan,

    So freewheel diode means it conducts when the MOSFET is off, it's the recirculating diode for the buck (the switch node). The figure 14 diode is just a signal diode. The 0.7V in the datasheet equation should really be the freewheel diode voltage at whatever the the LED current regulation is (note, this will vary with temperature). For the Roff diode it will have a voltage drop across it to take into account so VCC - Vfdiode is what is applied to the added Roff, at low current this could be anywhere from 0.3V to 0.7V depending on the diode you chose.

    Best Regards,