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TPS923653: Question about settings (current limit off by factor 2)

Martin Seeger
Expert 5411 points
Part Number: TPS923653
Other Parts Discussed in Thread: TPS923654

Hi team,

our customer modified the TPS92365xEVM to their needs, but somehow the current is too low, 850mA vs. the desired 1.6A. Can you help us check the settings?

(They use an e-Load in CV-modus.)

Best Regards
Martin

Vin = 24 V
Vload = 40-44V
Iload= 1.6 A
10 Leds
LED Vf = 4.3 V
Fsw = 1 MHz
Rop1 = 1.47 MΩ
Rop2 = 42 MΩ
Cin = 44.268 uF
Cout= 6.6 uF

Change on the EVM: 

Vin was 12V is now 24V
R6 was 53.6 kΩ is now 43 kΩ
R5 was 52.3 kΩ is now 22 kΩ
R1(shunt) was 100 mΩ is now 125 mΩ. (this should change the current from 2A to 1.6 A)
L1 was 22 uH is now 10 uH, with saturation current 3.3A
C8 was 22uF is now 2,67 uF

  • 0
    TI__Mastermind 23150 points

    Hi Martin,

    It looks like the thermal foldback function is reducing current to its half. Please change R7 = 0ohm to disable thermal foldback and have another try.

    Also, please change R6 = 33kohm to avoid OVP influence the output current. Please change C8 to 4.7uF or larger to sustain the sense signal during switching-on cycle. Please change to an inductor with larger saturation current (Your setting will have around 3.92A peak inductor current when the output current is 1.6A and LED string voltage = 44V) 

    BR,

    Steven

  • 0 in reply to Steven Li
    TI__Expert 5411 points

    Hello Steven,

    thanks for looking into this.

    The customer changed as suggested: 

    R7 from 61.9k to 0 Ω
    R6 from 43k to 33k
    C8 from 2,67 uF to 4,7 uF

    Results are: VOUT of 56,5V with a current of 0 to 0.35 A. If the customer increases the load current (CC mode), the voltage goes to 24V occasionally. Changing back to the original values brings the original results.

    Wondering what is off.

    Best Regards
    Martin

  • 0 in reply to Martin Seeger
    TI__Mastermind 23150 points

    Hi Martin,

    Sorry I overlooked the title. I just noticed you mentioned this thread is for TPS923653.

    From your description, it looks like the current limit or thermal shutdown of the IC is triggered.

    First, please use TPS923654/5 for this application as your setting will have around 3.92A peak inductor current.

    Second, please check what package the customer is using now. I would recommend VSON or WSON package as the output is around 70.4W, which may be too high for SOT-23-THN package.

    Thirdly, please try reducing the switching frequency (please do make sure your changed setting will never trigger the current limit of the IC) and therefore reducing the switching loss of the IC. This will make the IC generate less heat.

    Best Regards,

    Steven