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Making power supply for LTE Cat1 modem

Kostiantyn Denysenko
Prodigy 245 points
Other Parts Discussed in Thread: BQ24072, TPS54240, TPS63070, TPS63020, BQ24074

Hi fellow engineers,

I have some doubts about my solution.

My problem is : I need to power LTE Cat1 modem. There are 2 power sources: external 8 to 36V DC or internal Li–Ion battery pack with min 6800mAh.

What I expect: Normal operation is modem power from external DC source. If external pwr is lost modem should continue to work, powered by the battery. (Preferably integrated switch in the charger) When external pwr is back battery pack should be charged at 500mA–1A.

Modem requirements are: 3.8V, 1A peak in LTE mode. Model is Telit LE910EU v2

Current setup is : TPS54240 (Buck) + BQ24072 (charger)  

I still can't choose proper Buck/Boost for the modem and worry about performance of Buck+Charger combination. 

Can someone suggest good solution for this?

If I've missed some needed information –please ask and I will try to follow up.

Thank you all in advance. 

  • 0
    TI__Guru**** 191445 points

    So you are going to use the 8 to 36 V supply to power the TPS54240, then use the 3.8 V output to power your modem and also power the BQ24072 (28 V max input) which will charge your internal battery.  What is the battery voltage?  You will need to switch  automatically to have the battery power the input to the TPS54240 and The TPS54240 will need sufficient output capacitance to allow the modem to continuously operate during the transition.

  • 0 in reply to JohnTucker
    Prodigy 245 points
    I think that I would need buck–boost IC between charger/powerline and modem. This is based on the operational voltage of most chargers: 4.5–5.5V and the fact that I need stable voltage in range 3.4–4V for the modem and Li-Ion can and preferably should go to 3V to get as much power as possible. The pack has its own protection and will cut off at 2.9V
  • 0 in reply to Kostiantyn Denysenko
    TI__Guru**** 191445 points
    Webench does not show any buck boost that will operate over that full range. I think you will need two separate dc/dc converters for your design, TPS54240 can work for the external input. You might look at TPS63070 buck boost to operate from the battery pack
  • 0
    Prodigy 5 points
    Hello Kostiantyn,

    Our lower voltage buck boosts start with TPS63 xxx.
    Here is one that would work well for the 3.8V buck boost off a 1S LiIon.
    TPS63020 High Efficiency Single Inductor Buck-Boost Converter with 4A Switch
    www.ti.com/.../TPS63020

    For your input the TPS54240 would work fine to produce 5V at 2A.
    Use that for the input to bq24074 linear charger with power path.
    bq24074 charges the battery off the 5V while passing the 5V to its output. (power path)
    Then with the main Vin goes away the bq24074 passes the battery to its output.

    The bq24074 output will feed the TPS63020 to make 3.8V in either buck (off 5Vin or Vbat>3.8V) or boost (Vbat<3.8V)

    Also see this reference design for some ideas; http://www.ti.com/tool/pmp9769
    It uses the TPS63020 for the GSM module.
  • 0 in reply to Ed Walker
    Prodigy 245 points
    Thank you.
    That was what I was considering. Buck to 5V, use that to charge and buck/boost to get stable 3.8V. Reference design is a great place to start. My main worry is the switching from external to battery, but as it was mentioned before – bulk capacitance should help.
  • 0 in reply to Kostiantyn Denysenko
    Prodigy 5 points
    I am glad to help, Kostiantyn.

    The buck boost TPS63020 maintains constant Vout as Vin changes from buck to boost modes.
    See Figure 23 in the datasheet. Line Transient Response.
    It shows a 3.3Vout with vin changing from 3V to 3.7V.

    The PowerPath chargers also have seamless transition from Vin to Vbat.

    Shouldn't need all that much capacitance.

    I will close this for now but please post again if you have more questions.