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TPS27081A: Recommended Design procedure for using TPS27081 for controlling GSM modem

Suraj Hebbar1
Prodigy 80 points
Part Number: TPS27081A
Other Parts Discussed in Thread: BQ24072, , TPS22918, TPS22950, TPS22917, TPS22916, TPS22919

Hello,

We would want to control(On/OFF) the power to a GSM modem (Quectel M66) powered by lithium-ion battery. We want to use TPS27081 chip for the control.

Design requirements are as follows:

1. System out from BQ24072(used for charging the battery) will be given as input to the TPS27081A chip

2. At the input pin of TPS27081, will have voltages ranging from (3.3-4.2v), On the load pin of TPS27081 will have a GSM modem 

3. ON/OFF pin of TPS27081 have GPIO voltages of 0 and ~5V

GSM draws a peak current of 1.2A at the time of registration. and  200mA for normal operation. 

Could you please help me select values of R1, R2 ,C1, C2  for a smooth operation, because I am worried if the device would enter saturation and heat up at the time of operation.

Regards,

Suraj

 

  • 0
    TI__Mastermind 32650 points

    Hi Suraj,

    The operating conditions are acceptable for the device but the choice of passives depends heavily on the design intention. May I first ask, why have you chosen to use this device over other offerings in our portfolio? Usually for battery powered systems, we would recommend devices with integrated reverse current blocking. We also have devices with integrated current limit and short circuit protection that could help protect the battery from short or high current events. We also offer devices with slew rate control like that officered by TPS27081A but easier to implement with a single capacitor. 

    Please look at devices like TPS22916, TPS22917, TPS22918, TPS22919 and TPS22950 as potential options for this voltage and current level.

    As for your question on selecting the passives:

    The choice of R1, R2 and C1 can be made easier by looking at Table 2 in the datasheet. I would recommend using R2=1kOhm and R1=10kOhm as R1 >= 10*R2 for the lowest On resistance of the FET. The choice of C1 then becomes only dependent on your preferred slew rate.

  • 0 in reply to Shreyas Dmello
    Prodigy 80 points

    Hi Shreyas,

    If I chose, R1 =10K and R2 =1K, can you please suggest the value of C1.

    As the input voltage is from a battery which would vary over the range of 3.3V to 4.2V, and the load is a gsm modem. Following is the image that shows the current drawn by the GSM at the time of network registration.

    Can you please suggest an optimal value of C1?

    As of now, I have a tight deadline and the PCB has reached our workplace. Note: We have designed the PCB as the layout guidelines mentioned in the TI datasheet.

    I don't want to assume things so I would be grateful if you can help me with the value of C1 to get started.

    In addition, I'll consider the parts highlighted by you in our next project! 

    Regards,

    Suraj

  • 0 in reply to Suraj Hebbar1
    TI__Mastermind 32650 points

    Hi Suraj,

    What is the load capacitance here? With faster turn ons, you will get capacitive inrush. 

    How is the 1.6A pull configured? Since this is a GSM module I am assuming that the device is left in the ON state and the 1.6A pull occurs while the device is ON? In this operationg, the C1 has no effect since the device is already in the ON state. I do  not recommend operation where the device switches ON for every pulse.

    The optimal C1 would be the soft start time that doesn't cause a huge inrush spike. It will not affect the device after it is already in the ON state. Please look at the table in the datasheet to select the rise time needed here according to what inrush current level is acceptable for your system. I would say, if there is not load capacitance, 1000pF is sufficient. If there is significant output capacitance, then the C1 should be larger. I would use 0.18uF for a smooth, slow ramp to avoid any large inrush spikes.

  • 0 in reply to Shreyas Dmello
    Prodigy 80 points

    Hi Shreyas, 

    Please find the responses inline.

    Q1.What is the load capacitance here? With faster turn-ons, you will get a capacitive inrush

    A1: Load capacitor is a tantalum 100uF Capacitor.

    Q2. How is the 1.6A pull configured? Since this is a GSM module I am assuming that the device is left in the ON state and the 1.6A pull occurs while the device is ON?

    A2: Yes, the device first boots up around 200-300mA of current; The peak current is drawn when the GSM transmits the GSM packet

    With this configuration can I proceed with R1=10K, R2=1K and C1 = 0.18uF ?

    Regards,

    Suraj

  • 0 in reply to Suraj Hebbar1
    Prodigy 80 points

    Hi Shreyas,

    In addition to the previous response, I would like to share the sequence of operations.

    1. Enable the GPIO(5V) of TPS to Power ON the GSM module

    2.Wait for the GSM module to transmit the data

    3.Disable the GPIO(0V) of TPS to Power OFF the module(To save power).

    The above sequence is repeated every 5 minutes. Basically, we would sample data every 5 minutes and then enter power save mode.

    Regards,

    Suraj

    Regards,

    Suraj

  • 0 in reply to Suraj Hebbar1
    TI__Mastermind 32650 points

    Hi Suraj,

    With the configuration you stated, I believe R1=10K, R2=1K and C1 = 0.18uF will be sufficient. This will result in a turn on time of around 250us which would result in inrush spikes of around 200mA which is acceptable for the system.

    As long as you are okay with this slower 250us rise time every time you turn on the device, this setup should work fine even with the power save mode.

  • 0 in reply to Shreyas Dmello
    Prodigy 80 points

    Hi Shreyas,

    This would probably the last question from my side. I have posted the same question in another forum as well

    e2e.ti.com/.../3615606

    We observed an interesting behavior with this chip, as you suggested we have mounted R1=10K, R2=1K and C1 =0.18uF. We were using the pull-down resistor of 4.7K to GND at ON/OFF pin of TPS(not shown in the schematic, it was added onto the PCB. Sorry for not pointing this detail earlier) 

    Yesterday, we removed this pull-down resistor and now the chip is functioning as expected, there is no voltage drop at the Arduino side and we are able to control power on and off the GSM module smoothly. 

    Is there a design rule for adding the pull -down resistor? Because adding the pull down resistor is causing the chip to draw more current from the Arduino pin.

    Regards,

    Suraj

  • 0 in reply to Suraj Hebbar1
    TI__Mastermind 32650 points

    Hi Suraj,

    I am closing this thread. Feel free to continue the conversation in the other thread or feel free to start a new one.

    I responded on the other thread about your concerns with the pull down. The summary is that this device is not meant to operate in any state where the input to the enable pin is high-Z. A weak pull up/down (10k or 100k) is acceptable to define the high-z states as a 0 or 1.