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USB modem design for GSM Module

Aakash Dey
Intellectual 590 points
Other Parts Discussed in Thread: TPS82084, TPS5432

Dear All,


I want to run GSM module with USB power. While searching, I have found one of your documents (https://www.powersystemsdesign.com/articles/usb-modem-design/29/4298). It looks like a possible one for my circuit.

In the document it is mentioned that five 150uF capacitor need to be used for energy conversion. But, how the values and the quantity of this capacitors are calculated? Can you please help me for calculation?

Thanks.

  • 0
    TI__Guru*** 147240 points

    Hi Aakash,

    That article is quite old.  What peak current does your GSM device need?  For 2A, the TPS82084 power module gives a simple and small solution.

    The capacitor selection is described in the middle of the paper.  The USB switch will limit the current drawn by the DC/DC and load.  Since the load draws more power than can be delivered from the USB port, the 5V_USB voltage will fall.  The caps on the input need to store enough energy to provide 3.3V to the load for the duration of the GSM pulse.

  • 0 in reply to Chris Glaser
    Intellectual 590 points

    Dear Chris,

    Thanks for your reply.

    Actually, My system input is 5V,1A and for GSM, I require 4V,2A in brust mode. I am already using TPS54322. But, Due to the constraint in the input power (5W), I want to use the method mentioned in that document (converting USB Power 5V, 0.5A and run the GSM).

    I can able to understand the use of USB switch mentioned in the document. Also, the need of capacitor is understood. My concern is that how the value of the capacitor is calculated?

    Thanks.

     

  • 0 in reply to Aakash Dey
    TI__Guru*** 147240 points

    Thanks for explaining Aakash.  You can use the TPS82084 between the USB switch and GSM.

    You need 8W of output power and have only 5W of input power.  And if you assume the DC/DC has 90% efficiency, you actually need 8W/90% = 8.9W of power at the input.  This 3.9W of extra power must come from the bulk input caps.  As the caps supplement the required current into the DC/DC, their voltage drops from 5V to 4V (and further down).

    Assuming an average voltage of 4.5Vin, 3.9W is 867 mA of input current.  This current comes out of the caps.  From I = C dv/dt, we can calculate the required C.  The dv will be about 1V and the dt is the length of the 8W pulse.  For example, if the 8W lasts 1msec, then we need 87uF of cap.  This is a starting value; a more accurate one can be found with simulating your circuit.

  • 0 in reply to Chris Glaser
    Intellectual 590 points

    Dear Chris,

    Thanks for your all support and help. Now, it became crystal clear for me and I can proceed towards calculation. Can I use the TI's online Simulation software to simulate the circuit?

    We are currently using TPS54322 which has 80% efficiency. However, if you are suggesting TPS82084, we will definitely think about it.

    Thanks.

  • 0 in reply to Aakash Dey
    TI__Guru*** 147240 points

    Yes, the TPS82084 has a Pspice model available and your USB switch should have some model as well.

    Yes, TPS82084 is simpler, smaller, and should be more efficient than the older TPS5432.

  • 0 in reply to Chris Glaser
    Intellectual 590 points

    Hi, Chris,

    Thanks for your all help and suggestions. In future, for any help regarding this case, I will contact you and TI.

    Thanks.