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TPS92515: Output abnormal

Will Wang47
Expert 8340 points

Part Number: TPS92515

Hi Team,

Customer is using TPS92515 in there project, and find one issue.

Vin from battery, voltage is 22V, vout connect to 5 LED, voltage should be 17.8V during normal operation. measured average current ~0.76A.

The following firgure 1 is schematic. Inductor is 47uH, 1.45A saturation current.

Some board works normal, Boot voltage as figure 2 shows.

Some boards works abnormal, Boot voltage as figure 3 shows, output voltage as figure 4 shows.

Does it cause by drop-out mode? Could you help share some comments on this? 

Figure 1

Figure 2

Figure 3

Figure 4

Thanks and Best Regards,

Will

  • 0
    TI__Mastermind 28655 points

    Hello Will,

    It appears the load voltage is going to the input voltage.  Can you look at the voltage across the LEDs and Vin/Vbat?  If this is the case either there is too much resistance in series with your LED load or it requires higher voltage than 17.8V (you can check this with a lab supply to verify).  Also verify that your battery voltage is not drooping (I'm not sure what your battery is). 

    You have a capacitor on the output, you can see the duty cycle change as it charges and it appears to be going to 100% duty cycle (your Figure 3 shows this).  When it does this the MOSFET stays on and boot discharges over time.  Your last waveform shows boot high for about 5 ms then the TPS92515 goes through a refresh (that is the discharge and the boot going low until it refreshes the boot capacitor voltage and the MOSFET turns back on.

    Your schematic doesn't show a bypass capacitor on Vin/Vbat.  If there isn't one a capacitor should be added per the datasheet recommendation.

    Best Regards,

  • 0 in reply to Irwin Nederbragt
    Prodigy 10 points

    Hi Irwin,

    Nice to E-meet you, I am EE from Goertek and we are using this chipset for one project. The LED driver Vout connects to 6 LEDs whose forward voltage is 2.7~3.7V.   17.6V is good enough to drive the LEDs. And the battery for the testing is a DC power supply, the voltage is also stable enough.

    BTW, we added one bypass capacitor(4.7uF) on Vin/Vbat, but no improvement for this issue.

    My question is which conditions will cause the  TPS92515 output not stable. We did component cross-check but failed to find the root cause.

  • 0 in reply to user4938829
    TI__Mastermind 28655 points

    Hello Will,

    What is shown above (figure 4) is the normal function of the TPS92515 when the MOSFET goes to full on (cannot reach peak current threshold).  When you ask if this is dropout mode it appears to be.  The Vf of your LEDs allows Vled string to be 22.2V maximum.  Also note that the TPS92515 regulates peak current so it has to be able to reach the peak current trip threshold.  Note that the the LEDs can light without the TPS92515 switching (until the boot refresh needs to happen).

    The TPS92515 is a hysteretic controller so it really doesn't go unstable, it turns on the MOSFET until the peak current threshold is met then the MOSFET turns off for a fixed amount of time set by Vled, Roff and Coff.  Note that near dropout the LED current can increase.

    I would recommend either shorting one or two of the LEDs to see if it starts working correct or turning up the input voltage to the circuit.  You can also use the lab supply directly across the LED string to see what Vled string is when near the current regulation point.  Note that as the LEDs heat up the Vled string voltage will drop.

    I didn't think adding the capacitor would change this it should be there for high frequency bypass.

    Best Regards,