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Original question:

TPS92515: Review schematic and some inquiry

TPS92515: Check the schematics for ILED 1.2A

Henry Wang3
Genius 3075 points
Part Number: TPS92515

Hi Sirs,

My customer is using 2 pcs of TPS92515 to drive 2 series LEDs w/ Iout@1.2A for each channel.

The product spec as below.
Vin normal:27.5V
Vin min: 10V
Vin max: 36V
IrLED Vf: 3.15V(max. 3.5V)
IrLED If max: 1.5A

Q1: They only got about 0.8A current on each channel, could you help to review the schematics?

Q2: When they used the Vin=28V, could get the Iled=0.8A; used the Vin=12V, only got the Iled=0.5A. Why the Vin voltage will make a difference in Iled current?

Please let me know if the schematic is necessary to do or not any changes. Thanks!

IrLEd Driver Board_20210104.pdf

  • 0
    TI__Mastermind 28655 points

    Hello Henry,

    What are R2, R9, R11, R12 and the PWM voltage level?  What is the power source to the circuit?

    Best Regards,

  • 0 in reply to Irwin Nederbragt
    Genius 3075 points

    Hi Sir,

    R3: 0.2 Ohm; R9: 0.2 Ohm; R11:0 Ohm; R12: 100k Ohm.
    The PWM voltage is 3.3V.
    The power source is the automotive system power 12V or 24V.

  • 0 in reply to Henry Wang3
    TI__Mastermind 28655 points

    Hello Henry,

    Using Rcs = 0.2 ohms I get approximately 0.9A average LED current.  Since Vled is low it will actually be a little lower since the Vf of the freewheel diode isn't included in the calculation so this is close the your 28V input measurement.  As for why 12V input is lower I would need to see some oscilloscope pictures of circuit operation, the switch node at both input voltages and the inductor current (LED current without an output capacitor works also).

    To get closer to 1.2A average Rcs would need to be approximately 0.157 ohms.

    Best Regards,

  • 0 in reply to Irwin Nederbragt
    Genius 3075 points

    Hi Sir,

    The customer would like to know why they get the low output current on the low Vin system?

    Please let me know if the schematic is necessary to do any changes. Thanks!

    CH1(Yellow)：Vin pin (by Power supply)

    CH2(Green)：D5 (L1 front)

    CH3(Orange)：C6 (L1 rear)

    For DC_12V system

    For DC_24V system

    Inductor spec: CSHF-0603T-150M (NEW).pdf

  • 0 in reply to Henry Wang3
    TI__Mastermind 28655 points

    Hello Henry,

    This is expected, 0.8A and 0.5A would not be expected.  There is a turn-off delay after the peak current threshold is met.  Since the di/dt of the indutor is high when Vin is high, 0.56A in 344 ns the actual inductor peak current is a little higher compared to when Vin is low.  The TPS92515 regulates peak current and the turn-off delay will cause the peak to be higher when Vin is higher making the average current higher.  The difference is 0.045A, 1.112A - 1.067A.  If you want to reduce this the inductor value needs to increase.

    If you can look at the inductor current you may be able to see the difference.

    Best Regards,