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TPS92691: Design issue for the TPS92691PWPR

Zhang Liang
Genius 4095 points
Part Number: TPS92691

Hi,

We use the TPS92691PWPR for our new deisng and there are some issue as below:

1. We use the boost topologies that the input voltage is 24VDC and output voltage is 27V, and then, we use the Rcs=0.035ohm, the output current is 4.7A which is less than the theoretical value(About 5A),

    so please advise why this current is abnormal;

2. We change the design to boost the voltage to 40VDC, and the max. output current is 4.3A, the current is much more less than 4.7A;

3. We set the potentiometer to the max. resistor 50K, the output current don't output steady-state, and please advise why, thanks.

The schematic diagram please refer to below:

TPS92691.pdf

  • 0
    Genius 4095 points

    Hi,

    Please kindly review above issue and provide your comment, thanks.

  • 0 in reply to Zhang Liang
    TI__Prodigy 410 points

    Hi Zhang,

    1. The threshold of V(csp-csn) is 172mV and the amplifier has cumulative offset up to 7mV. Therefore, when Rcs=0.035ohm, the theoretical value is 4.9A and can have 0.2A offset.

    2. When output voltage is 40V and input is 24V, the input peak current can reach > 8A+ripple/2 if output is 4.7A. Pls check Ris value and make sure IS pin voltage does not exceed 497mV which may trigger cycle-by-cycle current limit.

    3. When setting max resistor 50K, VIADJ=2.66V which is higher than internal clamp voltage 2.42V. Pls do not make VIADJ higher than clamp voltage.

  • 0 in reply to Yichao Tang
    Genius 4095 points

    Hi Yichao,

    Thanks for your professonal comment and the result we test as below:

    1. I boost the voltage from 24VDC to 40VDC, the output current is 4A but not the 4,7-5.1A, so this is very different from the theoretical value;

    2. I adjust the potentiometer resistance to the limited steady, the Vpp is 3.8V and the average value is 520mV, so please the 497mV is based on average value, right? please refer to below:


    3. BTW, the LEDs is 12*12 matrix(12 in series and 12 in parallel), so please advise if TPS92691 is support this design or not?

  • 0 in reply to Zhang Liang
    TI__Prodigy 410 points

    Hi Zhang,

    1. It seems 4.7A at 27V output voltage is reasonable. The 4A at 40V output is most likely limited by the current limit threshold.

    2. The current limit threshold is between 497~550mV IC-to-IC. It is the peak current limit so make sure the IS pin voltage at the end of rising edge does not reach that value (note that it is not the 3.8V noise, but the peak of linear rising edge). Since your average value already reaches 520mV, the rising edge mostly hit the current limit threshold. I suggest to reduce the Ris value to prevent the condition.

    3. Yes, TPS92691 supports driving LED matrix, but need proper design to avoid current limit and VIADJ clamp. Pls change potentiometer to make sure VIADJ not exceeding 2.42V.