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TPS562200: lowering the current level for transitioning into ECO mode

Lenio Cacula
Genius 16720 points
Part Number: TPS562200

My customer is experiencing issues with ripple when the TPS562200 is operating in ECO mode. We have already optimized the ripple with a feedforward capacitor. The intent is to reduce the current in which the device transitions to ECO mode to below 70mA or so. This can be achieved - and already experimented - by increasing the inductor. For 70mA, a 33uH is the calculated value, but the customer ran tests with 15uH and saw significant improvement.

What are the risks of using a 15uH or 33uH inductor in the design?

These are the parameters:

Max Vin = 15V

Vout = 5V

Iout = from 50mA to 1.8A

Max Temperature = 85°C

I appreciate the help!

  • 0
    TI__Genius 9550 points

    hi, I am looking into this question and answer on Monday.

  • 0
    TI__Expert 6425 points

    Hello Lenio,

    I'm a litter confused. what's your intent? which current do you want to reduce?

    best regards,

    edwin.

  • 0
    TI__Expert 4035 points

    Hi,

    Once Vin, Vout and L are selceted, that means the minimum on time is determinded. when the HSF works with a minimum on time, that's to say the peak current is constant. the mentioned 70mA is just a value obtained at one combined Vin, Vout and L, changing one of them would change the current at the transition point.
    As you can see in the datasheet, the output filter has a double pole, this pole is limited by the high frequency zero generated by interan circuit to get adequate phase margin. a high inductance might result in stability issue and a poor load regulation performance.
     
    BRs,
    Ryan

  • 0 in reply to Ryan Hu (BSR-MV)
    TI__Genius 16720 points
    Ryan,

    thanks for the reply. I am afraid I need more details. I understand stability MIGHT be an issue, which is my own concern all along, but the tests and simulations do not corroborate to that. Our Webench simulation shows results that are perfectly fine. My calculations do not reveal anything abnormal and more importantly, the test with the circuit works normally. The only aspect against using a larger inductor is the datasheet, which unfortunately does not describe in detail why it limits the maximum value to 4.7uH. I would like to see the numbers to support it.

    Answering Edwin's question, we want ECO mode to operate only at a lower current level, in this case 70mA.

    Thank you!
  • 0 in reply to Lenio Cacula
    TI__Expert 4875 points
    Hi Lenio,

    Continue with Ryan's comments. For DCAP2 device, if you check on its bode plot, there is an internal zero by internal ripple injection circuit. This zero is placed at 10k-20kHz. And this zero is intended to compensate the double poles by LC. In this case, if you would like to have a good phase margin and bandwidth, you will need to place LC double pole close to internal zero. This means that if you select bigger inductor, a smaller output cap could be considered. For your application, you could try to reduce the value of output cap. But since your inductor is 15uH which is 3 tims of 4.7uH recommended, it could be fine for stability if you still use the 44uF output cap recommended. The internal zero could still compensate the double pole. Thanks!