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TPS92518: Basic calculations for PKTH and TOFF_DAC

Parth Patel
Prodigy 10 points

Part Number: TPS92518

Tool/software:

Hello, 

I am trying to understand the calculation of the PKTH_DAC and TOFF_DAC register with my custom configuration. PKTH_DAC value controls the peak current but what parameter will be controlled by TOFF_DAC?

Question 1:

In the datasheet, under section 9.2.1 - Design requirement section, the given calculation covers the selection of inductor, and calculation of registers for PKTH_DAC and TOFF DAC.

With the same parameters (Vin = 48, VLed = 9, Switching frequency = 550KHz, Iripple = 12.5%, L = 67.68uf or 68uH) but different ILED current (ILED=1 amp), which register value have to be changed?

I believe only PKTH_DAC value needs to be changed. If I am wrong, please explain to me.

Can you please provide calculations for PKTH_DAC and TOFF_DAC with ILED = 1 A??

Question 2:

if my Vin = 48V to 58V

Vout = 8V to 40V,

ILED = 100ma to 2A,

Irippple = 10% of ILED

frequency is 400KHz,

how would I be able to calculate the LEDPKTH_DAC and TOFF_DAC for Iout = 1Amp and Vout can be in between 8V to 40V.....?? Can you please provide some calculations ?

  • 0
    TI__Genius 12435 points

    Hi Parth,

    The TOFF_DAC register sets the off-time in relation to the load voltage according to the equations in section 8.3.1.3.1

    The PKTH_DAC can be modified to make slight adjustments to the output current, however it is not recommended to design the system output current by modifying the PKTH_DAC. TI recommendation is to design the system using the max PKTH_DAC setting, as this will deliver the best accuracy and performance.

    Keep in mind that the sense resistor is sized based on the peak current, and the inductor is sized based on the ripple current. The circuit will have the best performance when the hardware components are designed for the real output current.

    Regards,

    Zach