• State Resolved
  • Locked Locked
  • Replies 6 replies
  • Subscribers 63 subscribers
  • Views 777 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

TPS82084 for 2 USB rails?

HK19397
Prodigy 70 points
Other Parts Discussed in Thread: TPS82084, TPS542941, TPS63020, TPS62750

Hi,

My application uses USB 2.0 as input into my system. System loads are 3.3V @ 1A and 5V @ 2A. So I will need to use two TPS82084s . Please let me know :

Q1. If the USB 5Vin and Vout=5V  OK to do with TPS82084 ( as data sheet indicates Vout can be up to Vin).

Q2. If the USB 2.0 spec states 500mA max draw, what would be the ripple current to draw 1A@3.3V and 2A@5V.

Q3. if the TPS82084 is not a good match is the tps542941 a better device?

Thanks

  • 0
    TI__Guru**** 150950 points
    I split this thread since you have a completely different question that the original question.

    1) Yes, but what if Vin is lower than 5V? In this case, you will not get 5V out. I recommend a buck-boost converter such as TPS63020.
    2) I don't understand your question about ripple current. Clearly, you are trying to draw more power than 5V @ 500 mA.
  • 0 in reply to Chris Glaser
    Prodigy 70 points

    1. If Vin drops to below 5, then Vout should follow -- i.e. down to the min Vin=2.5V. Yes?
    2. My understanding of switched mode power regulators is that they can actually supply more power from the capability of input by using the output cap as a reservoir ? For example, if input power is 5V@500ma, using a capacitor capable of storing spike currents greater than 2A, the switcher would "sip" more current from input as needed ( assuming that there is time between spike currents for the switcher to operate and re-charge the cap). On continuous mode draw this may not work. Please let me know if my understanding is incorrect?

    Also I am looking for two outputs from one input , and integrated inductor module..if such modules are available?


    Thanks

  • 0 in reply to HK19397
    TI__Guru**** 150950 points
    1) Yes, but you don't get 5V out then.
    2) The standard capacitors are rather small and only holding energy for some usec before any load on the output appears on the input. If you need to deliver much more power for some small duty cycle time, the TPS62750 is a great device for this with its input current limit and stability which large output caps.

    No, there is no smaller, dual output buck available.
  • 0 in reply to Chris Glaser
    Prodigy 70 points
    Ok, many thanks Will look into the TPS62750
  • 0 in reply to Chris Glaser
    Prodigy 70 points

    Hi Chris,

    From my earlier query, and your reco: "I recommend a buck-boost converter such asTPS63020."

    I have attempted to use the TPS63020 with an Audio Amplifier (PM8403 3W @ 5V) . I have used the on line web bench design with one exception. The exception is the output cap: Web bench indicated 22uF and I put in a 220uF Electrolytic thinking that would be adequate. Also I am using SRN6045-1R0Y inductor rated at 4.2A .

    Vin = 5.1V, from power supply wall wart capable of 4A load, Vout = 4.9V @ 2.79A . 

    Another switcher takes this 4.9V to generate a 3.3V system supply . rest of the system uses about 250 mA.

    What I am seeing is that, when the audio amp (PM8403) outputs (play back of song)  a peak signal of audio into the (2 of 8Ohm) speakers,  the supply voltage spikes down by 1 to 1.5V and the system dies.  Current draw by the audio amp at the occurance of the peak output is  estimated to be about 2 times 300mA = 600 mA

    I was wondering if TPS63020 cannot handle such surge power draws, or if the output cap should be reduced to 22uF?

    I tried adding another 220uF in addition to the current 220uF, but there was no change in behavior of supply, i.e at the same place in the audio playback, the spike kills the system. This leads me to believe that the output cap is not an issue.

    Is there anything else I should be trying?

    Any help is appreciated,

    Thanks

  • 0 in reply to HK19397
    TI__Guru**** 150950 points
    You need a waveform of Vin , Vout, and inductor current to understand what is happening.

    But clearly a 220uF electrolytic cap is not an equivalent replacement to a 22uF ceramic. Not only the capacitance but the ESR are extremely different. We recommend ceramic output caps as they are smaller and have a much lower impedance.