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TPS54326: The threshold to leave skip mode is at much higher current level

George Chen
Intellectual 2175 points
Part Number: TPS54326
Other Parts Discussed in Thread: TPS54335A,

The threshold to leave skip mode is at much higher current level.

But at datasheet fig.10 shows that the level is around 50mA only.

Please directly see below scopeshots in different load conditions.

no load

100mA

200mA

300mA

400mA

500mA

  • 0
    TI__Genius 9550 points
    hi George,
    Will check with EVM and fingure out if the datasheet has typo( 1A instead of 0.1A for X axis mark ), thanks for your information.
  • 0 in reply to Jason Wang (BSR-MV)
    TI__Expert 4875 points
    Hi George,

    It relates to your inductor selected. Maybe your inductor is smaller than the value in the datasheet. Please try on bigger inductor.
  • 0 in reply to Anthony Lang
    TI__Intellectual 2175 points

    Anthony,

    The application is for 12V to 2.5V, inductor is 2.2uH, Cout is 24uF.

    My questions:

    1. What is the inductor value of the datasheet fig.10 for 2.5V curve to make the threshold at around 50mA?

    2. And it will affect the stability if we use much higher inductor value compare to the typical value, right?

    3. What is your final suggestion to keep the threshold at around 50mA and also with stable operation?

  • 0
    TI__Expert 4035 points

    Hi George,

    A similar case is just for your reference, maybe it helps.

  • 0 in reply to Ryan Hu (BSR-MV)
    TI__Intellectual 2175 points
    Hi Team,

    TPS54335A is current mode control.
    I don't think they are the same/similar case and stable requirement.
    Please answer my questions posted at 2018/04/11, also listed as below, thank you.

    The application is for 12V to 2.5V, inductor is 2.2uH, Cout is 24uF.

    My questions:

    1. What is the inductor value of the datasheet fig.10 for 2.5V curve to make the threshold at around 50mA?

    2. And it will affect the stability if we use much higher inductor value compare to the typical value, right?

    3. What is your final suggestion to keep the threshold at around 50mA and also with stable operation?
  • 0 in reply to George Chen
    TI__Expert 4035 points

    hi George,

    For a particular application conditions, we don't know the current when the device transitions to ECO mode. because when the 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. your mentioned 50mA is just a value obtained at one combined Vin, Vout and L, changing one of them would change the current at the transition point. If you want to make the threshold at around 50mA, you need to adjust the input, vout or L.

    And you might find that the device could work with a much higher inductor, but it's not guaranteed by design, and please double check the load regulation performance to see if it could meet your requirements.

    BRs,

    Ryan

  • 0 in reply to Ryan Hu (BSR-MV)
    TI__Intellectual 2175 points

    Ryan,

    Let focus on TPS54326 datasheet Fig.10.

    The input is 12V, one of the curve is output 2.5V.

    The threshold to leave Eco-mode at Fig.10 curve of 2.5V output is around 50mA output current.

    Q1. What is the inductor inductance value to make the threshold at 50mA?

    Q2. Is the inductor inductance value the same as datasheet table1 for 2.5V output, 2.2uH?

    Q3. If it's not 2.2uH, since it's not the suggested inductor value, will this circuit be stable and how to confirm it?

    I post the same question at 4/11, and still haven't got the answer.

  • 0 in reply to George Chen
    TI__Expert 4035 points

    George,

    For Q1&2, We don't the original data on hands now, this is old part transfered from other team. If needed, what we can do is to do the same test on bench and then get the combine Vin, Vout and L. And also please tell us what's the poupose of keeping the current around at 50mA?

    For Q3, I quoted a answer from a similar question from Anthony. <<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. >>

  • 0 in reply to Ryan Hu (BSR-MV)
    TI__Expert 4875 points
    Hi George,

    Sorry for the late due to travelling. I could not find the original document for this device. Since the recommended value of inductor is 2.2uH for 2.5Vout, the inductor for the curve in fig 10 should also be 2.2uH. If so, I think it should be typo in datasheet. The calculated skip point for output current should be around 600mA instead of 50mA with 700kHz frequency. It is not realistic to use a bigger inductor to get the skip point that low.
  • 0 in reply to Anthony Lang
    TI__Intellectual 2175 points
    Anthony,

    Thank you for your kind reply.
    If so, could you please confirm the fig.10 threshold on the demo board for 2.5V output?
    If the result is same at around 600mA, I will need to update this to my customer.
    And the datasheet need to be updated in the near future to prevent another misunderstanding.
  • 0 in reply to George Chen
    TI__Expert 4875 points
    Hi George,

    Checked on EVM. The actual skip point threshold for 2.5Vout is around 560mA. The datasheet is not correct.