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TPS92518EVM-878: Inductor calculation

Guruprasad Solapurkar
Prodigy 240 points
Part Number: TPS92518EVM-878
Other Parts Discussed in Thread: TPS92518

Hello,

We are planning to use TPS92518EVM-878 for POC. In our application we need to have 2.4A, so we shall change 100uH inductor part number on kit  by B82477R4104M100 (100 uH, Isat=2.9A). Its because the existing EVM inductor is designed for 1.6A current.

For our application VLEDx=14V, Vin=24V so Toff= 58.9 uS (for 550KHz frequency), Delta Ilpeak=300 mA (page 44 mentioned in datasheet of IC TPS92518)

With above specs, Inductor value coming as 2748 uH as per formula given in datasheet.

1) So whether 100 uH inductor EVM will work for our product specification?  In EVM specification its mentioned that 100 uH will work for input range of 6.5 to 65V.

2) For PWM dimming input whether output ILED current is DC current or its PWM current? I hope since we have LC filter so output current shall be DC even PWM input is applied.

  • 0
    TI__Mastermind 28655 points

    Hello,

    I hope Toff isn't 58.9 uS.  That would make Ton = 82.5 uS and the switching frequency = 7.1 KHz.

    Toff should be 1.06 uS to get approximately 550 KHz.

    This makes di = 0.148 Apkpk.  (inductor current ripple)

    PWM dimming turns the output on and off so the LED current will be similar to the PWM signal with the difference of the inductor current ramp rate up and down.  What PWM frequency are you going to use?

    Note that this is a constant ripple current output which makes it variable frequency depending on Vin and Vled.

    Best Regards,

  • 0 in reply to Irwin Nederbragt
    Prodigy 240 points

    Thanks for feedback.

    1) I shall be using PWM frequency of 10 KHz and switching frequency as 550 KHz. Is there any relation between switching frequency and PWM input?

    2) With 10KHz PWM input if duty cycle is varied from 10% to 90% whether output LED current shall also vary from 10% to 90%. If PWM input signal is continuous logic 0 in that case whether output current is also 0?

    3) If I am going to use PWM input, in that case also do I need to configure any registers over SPI? If yes which registers?

  • 0 in reply to Guruprasad Solapurkar
    TI__Mastermind 28655 points

    1)  No, they are not related.

    2)  If PWM is 10% to 90% the MOSFET will be switching 10% to 90%.  At 10 KHz you will find that the inductor current slew rate will affect the LED current.  The output current when PWM dimming will depend on the inductor value, input voltage, LED voltage and output current.  In your case you are applying 10V to the inductor when supplying current to the LEDs and 14V when the MOSFET is off and flowing through the recirculating or freewheeling diode.

    At 100 uH, 24V input, 14V output the current will rise at 0.1A/us and fall at 0.14A/us (this is using V = L * (di/dt).  It will take 10 us to ramp the current to 1A  and 7.14 us for the current to ramp from 1A to 0A.  10 KHz at 10% duty is 10 us on time so the current will spend that time, 10 us, ramping to 1A.

    If you have a capacitor at the output it will also affect the light output.

    For higher PWM resolution Shunt FET dimming is recommended however output capacitance needs to be very low or zero due to it being shunted by a MOSFET at 10 KHz

    3)  I believe the PWM inputs work as they are without SPI registers needing configuration.  See section 8.3.8 on page 20 of the datasheet.

    Best Regards,