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TPS92518: Output current problem

user4413695
Prodigy 230 points
Part Number: TPS92518
Other Parts Discussed in Thread: TPS92640

Hi all：

     I have some problems with TPS92518：

1，The maximum current of TPS92518 is 5A, which should be 2.5a per channel, right?

2，Is the current value calculated by the following formula, <LEDX_PKTH_DAC> default value is 80H, which is 128, SPI configuration is required to adjust to 255, so as to maximize the current?

     If the customer's application requires pure hardware control, is it not possible to maximize?

3，Is the maximum current per channel of TPS92518 2.5A?Does Rsense have a range of options?According to the formula below, the smaller the Rsense value is, the larger the LED current will be.In the case that SPI cannot be configured, Rsense can choose at will, and then when <LEDX_PKTH_DAC> is the default value of 128, will the output current be the maximum of 2.5A for a single channel?

  • 0
    TI__Mastermind 28655 points
    Hello,

    1) The TPS92518 is a dual controller so the limit is really the design and parts chosen. It can be above 5A or 2.5A per channel.

    2) The LEDX_PKTH_DAC set to 255 makes the Vcs 255 mV which will give the highest output current. The TPS92518 regulates peak current so the datasheet calculations need to be done to figure out off-time, ripple current and ultimately average output current. The equations you posted are for calculating peak to peak current ripple for the first equation, equation 3 above. Average output current is the peak current threshold minus half of the current ripple which is equation 4 above.

    3) Rsense is limited by the design and size of the resistor. The TPS92518 is a controller so it is not limited to 2.5A. The default operation is very limited. The peak current sense is set to about 1/2 of full range, 127, and the off-time is fixed at 127. This can result in a design with very low switching frequency. Check the toff value when using your Vled in the calculation.

    Best Regards,
  • 0 in reply to Irwin Nederbragt
    Prodigy 230 points
    Thank you for your reply.I have two more questions
    1）TPS92518 is not controlled by SPI, only by Rsense. What is the default output current of each channel?
    2）LEDX_PKTH_DAC is set to a maximum value of 255. At this time, by configuring the value of Rsense, each channel can output a maximum current of 5A?
  • 0 in reply to user4413695
    TI__Mastermind 28655 points

    Hello,

    1)  If the TPS92518 is not controlled by SPI it defaults the peak current threshold to 127 and also the off time to 127.  Rsense will set what the current will be when it hits the 127 mV peak current threshold.  If it Rsense is 1 ohm the peak current threshold will be 127 mA, if Rsense is 0.1 ohm the peak current threshold will be 1.27A for example.  The average current is a function of where the peak current threshold is and the peak to peak current ripple.  Iave = Ipeak - (1/2)*Ipkpk (peak to peak current ripple).  The peak to peak current ripple is a function of off-time (or on-time), inductor value, input voltage and LED voltage. The equations for this are in the datasheet.  Note that the off-time of 127 (default without SPI and the TPS92518 forced into this mode) will be quite long, especially with high LED voltages.  I would calculate this first to see if operating this way is even feasible for your application.  Again, these calculations are in the datasheet.

    2)  LEDX_PKTH_DAC is set to a maximum of 255 (this can only be done with SPI, if you are not using SPI the TPS92518 can be forced into a fixed operating mode where LEDX_PKTH_DAC is set to 127).  The TPS92518 is a dual controller with external power components.  There is no limitation on how much current it can control.  That is limited by the design, parts used and thermal considerations.

    If you are trying to drive 5A LED loads perhaps looking at the TPS92640/641 might make more sense.  What is you input voltage and LED stack voltage?

    Best Regards,

  • 0 in reply to Irwin Nederbragt
    TI__Mastermind 28655 points
    Hello,

    I haven’t heard back from you, I’m assuming you were able to resolve your issue.
    If not, just post a reply below (or create a new thread if the thread has locked due to time-out)

    Best Regards,