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TPS92518: Calculated Switching Frequency is not reached

Andre Lukassen
Prodigy 40 points
Part Number: TPS92518

Tool/software:

Hello, 

I designed a circuit with the following specs: 

Vin: 24V 

VLEDx: 12V

I LED 1A

Ilpkpk: 12%

Vin Eff Est: 61%

Fsw: 800KHz

LEDx_PKTH_DAC 255 

Therefore, I chose a 22µH inductor and a 220mOhm sense resistor.

My OUtput DACs are: 

LEDxTOFF_DAC: 6

LED_MAXOFF_DAC: 255

With this Specs, I measured the switching frequency at the Gate Pin ( blue line) and I have to notice that the real switching frequency differs significantly from the calculated one.

Note: According to the data sheet, the inductor should have an inductance of 22 µH at 0Hz to 1.6 MHz.

Please, can you explain how this big difference occurs?

Best regards

Andre

  • +1
    TI__Mastermind 20485 points

    Hello Andre,

    Can you share with me your calculations.  I don't think you are setting the LEDx_TOFF_DAC correctly based on your calculations.  Also, you state the output voltage is 12V but it looks more like 15V, this affect the on time too.  

    -fhoude

  • 0 in reply to fhoude
    Prodigy 40 points

    Hello fHoude,

    thanks for the fast reply. I found a mistake in my calculations, but I think that is not the answer of my problem. (I forgot the MOSFET Uds Voltage)

    For the voltage VLED I calculated Uf = 2,85V (typ) four times, because i have four LEDs in a row and a MOSFET with a VDS of 0,1V.

    Consequently I have a VLED voltage of 11,5V. The min / max charistica of the LEDs are 2,6V(min) and 3,2V(max).   

    For the Calculation, I used the Design Tool for the EVM.

      

    I measured the VLED voltage at 14V, so I think the contact resistance to my LEDs gives a higher voltage than expected.

    Sorry, I have accidentally press the "this resolved my answer" button. 

    regards 

    Andre