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TPS92641: 20A max led driver pulsed at 20% duty cycle (2ms ON, 8 ms OFF)

martim1338
Prodigy 20 points
Part Number: TPS92641

Hi,

we need to drive a green LED capable of 20A at ~3.5V. We are implementing the design in the board linked below.

http://www.ti.com/tool/PMP8016

It seems to be perfect for our needs.

Pulse requirements is:

2ms ON at max current (20A)

8ms OFF

Duty-cycle is 20%.

Questions:

(1) is it possible to run the board at 24Vdc, or there is a limit? For sure we will considering to use higher Vds mos.

(2) is the circuit able to run at that duty cycle? I'm not sure it can run in continuous way since in the various examples we see only "high frequency PWM mode" with Ton of some tens of [us]... in my design i need 2ms.

(3) the MOS that shunts the LED must be dissipated since it sinks 20A for the 80% of the time, right? During that time, it dissipates: Rds,on * I * I = (few mohms) * 20 * 20 [W] >>> few Watts ... Is it right?

(4) why there is only 1 MOS for the upper side and 2 for the lower side? Maybe because the top side requires small current, while the bottom side requires synchronous rectification of many Amps???

Thanks in advance

  • 0
    TI__Mastermind 28655 points

    Hello,

    1)  The test data shows it running to 21V input in the test report.  It should be able to go to 24 though the voltage ratings of the components needs to be checked as well as going through the whole design process to make sure it will work correct.

    2)  You can run at lower PWM frequencies.  I would look at using UDIM then the synchronous MOSFET is not being used.  It's basically enable dimming.  It just turns on and off the gate drive.  If it is not accurate enough (inductor rise and fall time) then you can use SDIM.

    3)  When SDIM is used the MOSFET has to dissipate RDSon*I^2, you are correct.  With elevated temperature this MOSFET could be as high as 10 mohm.  This would be 4W * 0.8 = 3.2W.

    4)  duty cycle.  If the output is 4V and the input is 12V the upper MOSFET is on half as often and the lower MOSFETs so if one MOSFET was used for the upper and lower mosfet the conduction loss for the upper mosfet would be half of the lower MOSFET, 33% duty cycle versus 67% duty cycle.  At 24V the upper MOSFET is on even less.

    Best Regards,