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TPS61087-Q1: FREQ/SS Pins Open and Inrush current releted

Chuan Gao
Prodigy 125 points
Part Number: TPS61087-Q1
Other Parts Discussed in Thread: TPS61087

Hello,

We are doing some FMEA for our design and have some questions about TPS61087-Q1.

1. What will happen when FREQ pin is open? Would TPS61087-Q1 stop working(shutdown the output?)  (We don't find the description about FREQ open things in the datasheet.)

2.What will happen when SS pin is open? In datasheet, it indicates that it will not have soft-start if this port is open. Would you please give us the inforamtion about how long the output rise time will be when SS pin is opnen by default?

3.If we do not use SS function(keep it open). Is the insruh current a problem? Do we need to take inrush current into consideration when we design the input capacitors to keep input volage stable ? In datasheet ,2 x10uF is recommanded, is that enough?

Best regards,

Chuan

  • 0
    TI__Guru* 78170 points
    Thamk you for choosing TPS61087-Q1. Our responsible engineer will answer your question early next week.
  • 0 in reply to Youhao Xi
    TI__Mastermind 40080 points

    1. if FREQ is open, the TPS61087 will not stop working. But the operation frequency will dithering between 650k and 1.2MHz.

    2. if SS pin is open, the rise time would be around 0.9ms-1ms depending on the output voltage.

    3. When SS open, the startup current will be much higher. not the inrush current, inrush current is determined by the input and the output cap. while the startup current is both determined by output cap and the startup time.

    4. For this Boost converter, 10uF input cap is enough. But the selection of the output cap should be calculated based on the output voltage ripple and dynamic performance. Bigger output cap is good  for stability. but will have higher startup current. the ss cap can help  to reduce this startup current.

  • 0 in reply to Helen chen
    Prodigy 125 points

    Thank you Helen, it's very helpful.

    For question No.3  I am sorry my question is not so clear.

    I mean without SS function, Output voltage might rise up in a very short time which could lead to a big inrush current in the output capacitor.(in our design we set output as 12V) and my question is that, in general, whether the inrush current would affect the input voltage stability for the boost regulator. Because input voltage is also used as CPU power supply, voltage drop even in a short time is not expected.   

  • 0 in reply to Chuan Gao
    TI__Mastermind 40080 points

    This depend on your input wire. If the wire is short and thick. then the voltage drop will be small.

    It also depend on your layout. I think the trace connect the 3.3V to boost converter and 3.3V to the CPU should use different trace. which is a "start" connection.

  • 0 in reply to Helen chen
    Prodigy 125 points
    Thank you very much Helen.
  • 0 in reply to Chuan Gao
    TI__Mastermind 40080 points

    you're welcome.

    Could you please share the detailed application information for us? Like in what kind of equipment? Volume?

    You can send the information to me by email, my email address is helen-chen@ti.com