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Original question:

TPS63806: TPS63806, BQ25302, BQ27427 Schematic Review

TPS61023: TPS61023 Schematic Review

Dennis Liu
Prodigy 40 points
Part Number: TPS61023
Other Parts Discussed in Thread: BQ25302, TPS63806, TPS22995

Hi team,

This issue extends from previous issue.

Description

I have decided to add a new power supply IC(TPS61023) dedicated to supplying power to piezoelectric devices that periodically draw a significant amount of current. I would like to invite you to assist in reviewing the following schematic of the TPS61023 (outputting 5V) to see if there are any issues. This switch-mode power supply will be connected to a BOS1901, a driver IC for piezoelectric actuators, which will draw a periodic current (expected Ipeak is about 2400 mA, @300 Hz sine wave). There are two possible sources for Vin: one is VBUS 5V3A, and the other is a Lipo battery. Could you advise if the design of this schematic is reasonable?

Question

1. Additionally, as stated in chapter 9 of the TPS61023 datasheet, a larger electrolytic capacitor (e.g., 100 uF) may be required to maintain stability when the power source is located further away. However, I understand that the capacitance for USB devices should typically not exceed 10 uF. Thus, my question is, if the USB cable is longer than a few inches, would adding a bulk capacitor affect the USB bus power and potentially disrupt the enumeration of other devices on the bus? Is there an alternative solution?

2. Another issue is that if I am using other ICs, and the total capacitance on the usb vbus could amount to 30 ~ 40uF (e.g., BQ25302 PMID 10uF, Vin 2.2uF; TPS63806 Vin 10uF, TPS61023 Vin 10uF, assuming bulk capacitor is not included; totaling 32.2 uF), which is much higher than the USB standard of 10uF. What can be done to resolve the capacitance issue from so many ICs to prevent the other devices on the USB bus from being reset?

I am very grateful for any suggestions and apologize for not fully considering the previous issue, which has led to the need to ask a follow-up question

overview_sch.pdf

Regards,

Dennis

  • 0
    TI__Expert 5295 points

    Hi Dennis,

    About schematic:

    • C4 is too big, we recommend reducing it to below 10pF. 
    • R1 will generate about 2.5mW loss. Please check whether you can accept this
    • Other parts looks fine to me.

    About  Questions:

    1. If you need this long wire and cannot accept the bulk cap, you can try removing the bulk cap first. D1 may  be able to generate enough damping. If the Vin cannot stay stable without the bulk cap. then you can try design an input filter: Input Filter Design for Switching Power Supplies (ti.com)
    2. It depends on the current capability of the USB 5V and is different case by case. If the USB reset itself, you can use an e-fuse to limit the current.

    Best Regards,

    Travis

  • 0 in reply to Travis Wang
    Prodigy 40 points

    Hi Travis,

    Thank you for your assistance; here are my comments:

    1. According to the datasheet, if we want to improve the stability of the output, we can do so by appropriately adding a feedforward capacitor, C3, to increase the phase margin. The recommended value for C3, with a zero frequency of 1 kHz and R1 at 732k, is calculated using the formula C3 = 1/(2pi*1000*732000), which results in 220 pF. Could you please point out where I might have gone wrong?

    2. Based on your suggestion, I have attempted to design a new input filter. Incidentally, do you think using a load switch like the TPS22995 is an effective solution for limiting the effective capacitance on the USB VBUS when the USB is plugged in?

    3. Regarding the 10k resistor, its sole function is to ensure that Vbus does not remain floating. If achieving the same goal with less power consumption is possible, what would you suggest? Should I use a higher resistance pull-down resistor?

    Thank you again for your advice and assistance; they have been very helpful.

    Best regards,
    Dennis

  • 0 in reply to Travis Wang
    Prodigy 40 points

    Hi Travis,

    Would you mind answering the question I asked you last week when you have some free time this week? Thank you.

    Regards,
    Dennis

  • +1 in reply to Dennis Liu
    TI__Expert 5295 points

    Hi Dennis,

    Sorry for the late reply. About your questions:

    1:The 1kHz fFFz is designed for large Cout with electrolytic capacitor on the output. I don't know your part number on Cout, but taking Murata GRM188R61A226ME15D as an example, 22uf x 2 means 11.1uF effective capacitance on Cout. Actually the 1kHz fFFZ is a little radical for even 40uF. For applications without electrolytic capacitors on Cout, we recommend putting the fFFZ 6-10db outside band width(50-100kHz for TPS61023).

    2.It depends on the wire length and parasite inductance on wire. You can test it on bench and see if the Vin is ringing.

    3. I understand that you are using the 10k to discharge Vbus when usb is pulled off so that the PMOS can be turned on. I'm trying to say that the 10k mignt be too small and you can apply an bigger resistance to reduce loss. The PMOS might be opened more slowly with a bigger resistance but maybe it don't matter because the PMOS is going to conduct through its body diode anyway.

    Best Regards,

    Travis

  • 0 in reply to Travis Wang
    Prodigy 40 points

    Hi Travis,

    Thank you very much for your detailed response; it has been really helpful. Lastly, I just want to confirm that, according to your recommendation, C3 should be in the range of 2pF to 5pF. Is this correct?

    Regards,
    Dennis

  • +1 in reply to Dennis Liu
    TI__Expert 5295 points

    Hi Dennis,

    Yes, 2pf-5pf is a safe range. If you are not satisfied with its transient performance, you can try increasing the capacitance. But over 10pF can be risky.

    Best Regards,

    Travis