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TPS27081A: Is it safe to connect 5V to the ON/OFF pin while the supply to Vin is 3.3V to 4.2V?

Danilo A.
Mastermind 25315 points
Part Number: TPS27081A

Hi Team,

Our customer will connect the input of TPS27081ADDCR to a lithium ion battery with voltage range of 3.3V to 4.2V to power a GSM module. He will connect the ON/OFF pin to 5V. I understand that the I/O pins normally has ESD diode going to VIN and GND. Is it safe to connect 5V to ON/OFF pin while VIN is between 3.3V to 4.2V?

Regards,

Danilo

  • 0
    TI__Mastermind 32650 points

    Hi Danilo,

    I believe that this is an acceptable method of turning on the device. 

  • 0 in reply to Shreyas Dmello
    TI__Mastermind 25315 points

    Hi Shreyas,

    Thank you for your response. 

    The customer noticed that the TPS27081ADDCR heats up when he connected it as follows

    Vin = 3.7V(fed from BQ24072RGT, Lithium ion charger with system out)

    ON/OFF pin= logic high (5V from Arduino) on GPIO

    Vout= GSM modem connected at the load.

    The values of components are R1=5.1K and R2 =1K and C1= 0.1uF.

    He also noticed that the voltage drop at the ON/OFF pin is 2.5V. The 5V is supplied by Arduino GPIO and the current of around 50mA is drawn from this pin.

     

    I suggested to insert a 1k resistor between the GPIO of Arduino and ON/OFF and after that the device didn't heat up anymore. Since the voltage from Arduion is higher than the voltage from the battery, I suspect that the current from Arduino flows to the ON/OFF pin then to the ESD diode then to VIN which supplies power to the GSM module. Can you please confirm if this is correct?

    Regards,

    Danilo

     

  • 0 in reply to Danilo A.
    TI__Mastermind 32650 points

    Hi Danilo,

    I do not believe this should be happening. 50mA should not be pulled into this pin especially since I do not think the arduino can supply that amount of current. 

    I would propose that the battery is a stronger source than the arduino and would pull the voltage of the rail to its level. Can you please ask the customer to confirm this behavior on multiple devices? 

    Is the ON pin on the device floating at any moment? Is there a pull up/down on this rail? 

  • 0 in reply to Shreyas Dmello
    TI__Mastermind 25315 points

    Hi Shreyas,

    The customer tried with another board but the same issue happened. He also shared the profile of the burst current that GSM draws. Please refer to the schematic diagram for reference. Is there an internal diode between ON/OFF and VIN?

    Regards,

    Danilo

  • 0 in reply to Danilo A.
    TI__Mastermind 32650 points

    Hi Danilo,

    That is the current pull through the GSM module during the pulses. It should be acceptable for the device with no issue. I have fielded a question like this here although I recommended different passives: https://e2e.ti.com/support/power-management/f/196/t/977937 

    There is an ESD cell on the pins but these go to GND. I cannot share other details of the internals of the chip. 

    I noted that there was no pull up/down on the control pin. Please make sure to add this as the pin cannot be left floating for any time. 

  • 0 in reply to Shreyas Dmello
    Prodigy 80 points

    Hi Shreyas,

    We observed an interesting behavior with this chip, as you suggested we have mounted R1=10K, R2=1K and C1 =0.1uF. 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,

    The enable input cannot be left floating for any period of time. This is undefined operation of the device. As such, if you have a high impedance driver (1 and high-Z) then a pull down must be used. If you have a driver that performs the opposite (high-z and 0), then a pull up must be used.

    Weak pull downs are acceptable here. Please try a 10k or a 100k pull down/up if necessary.

  • 0 in reply to Shreyas Dmello
    Prodigy 80 points

    Hi Shreyas,

    We did connect a weak pulldown as suggested by you.

    The setup is as follows:

    1.A 100K is connected from TPS(ON/OFF) pin to the ground.

    2. The voltage at the input pin of the TPS chip is 3.4V.

    TPS(ON/OFF) pin is connected to a GPIO of Arduino operating at ~5V

    Here are our observations:

    1. Without the pull-down resistor at the TPS; we observe no voltage drop on the GPIO pin of Arduino. GPIO voltage(~4.85) is almost close to the VCC(~4.95V) of the Arduino

    2.With the pull-down resistor at the TPS, we are observing a voltage drop on the GPIO pin of Arduino. GPIO voltage is (~3.2V) and the VCC of the Arduino is (~4.95V)

    We can perform the ON and OFF operations but the GATE terminal of  TPS(lower nmos) causing the voltage to drop is bothering me.

    Can you please help us in this regard, Is it safe to have a voltage drop of this order while connecting the GPIO signal to the ON/OFF pin? 

    Regards,

    Suraj

  • 0 in reply to Suraj Hebbar1
    TI__Mastermind 32650 points

    Hi Suraj,

    I'm not sure why the pull down is causing so much current draw. Usually Arduino I/Os should be able to supply a max of 5mA. The addition of a 100kOhm resistor on this rail changes the current draw by around 50uA at 5V. This should be well within the Arduino output ratings. 

    The device is built to work with 1V CPU and MCU voltage inputs on the ON pin. The voltage seen will turn on and turn off the device but it is strange that the voltage is dropping so much when a weak pull down is introduced. I would say, if possible use a 1M pull down. This should limit the draw to 5uA at 5V and is a very weak pull down. Another possibility is to use an I/O that is able to send logic high and logic low without high-z.

  • 0 in reply to Shreyas Dmello
    Prodigy 80 points

    Hi Shreyas,

    For your reference, I have attached the I/O spec of the atmega328p chip that we are using on the board(Arduino).

    As per the spec, it is evident that drop should not happen because of the addition of pull down resistor. There must be something else that's happening because the moment I remove the pull down the voltage level is back to normal.

    I would kindly request you help us in this regard because I see no provision on atmega328p to put in a default state of logic 0 or logic 1.

    At the max what I can do is initialize the GPIO to a specific level right at the start of the program(the very first command ;) ); with this approach, the corner case would be that for a small interval until the pin is initialized the TPS control will see a high impedance.

    Anticipating your response

    Regards

    Suraj

  • 0 in reply to Suraj Hebbar1
    TI__Mastermind 32650 points

    Hi Suraj,

    It is tough to give a fixed recommendation here since as you said, there is something else going on. The fact that the voltage drop was intensified by the pull down is even stranger. If there was a leakage path through the device, it would not have mattered if the pull down was used or not. You would always see a voltage drop whenever logic high was put on the control pin of the device.

    To me it seems that there is something going on with the MCU and its ability to source from this pin. Maybe a solder connection to GND? Unfortunately, I am not familiar with this MCU.

    I think having high-z on the pin is acceptable for a short period of time. The issues comes from DC operation with High-Z on the control pin.