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TPSM843B22: How should I control the EN pin ?

minoru.takeuchi
Prodigy 60 points
Part Number: TPSM843B22
Other Parts Discussed in Thread: SN74LVC1G07, UCD9090A

We want to start the device well after the TPSM843B22's VIN rises.
Our VIN is 12V.
If the EN pin is left floating, the device will turn on immediately after VIN rises because it is pulled up internally.
We thought of using an external GPIO to control the EN pin, but the GPIO will be initialized only after a while after applying 12V.

Therefore, the EN signal will be unstable for a while after 12V rises.

In this case, EN will be controlled by internal pull-up during the period from when 12V is applied until the EN signal is determined.
What kind of circuit should we use to control the EN signal using GPIO control ?

  • 0
    TI__Expert 6175 points

    Hello,

    David will respond to this tomorrow, as he is out of office today.

    Thank you,

    Calan

  • 0
    TI__Expert 8846 points

    Hi Minoru, 

    How long after VIN do you want for enable to be turned on? It sounds like using an external GPIO would still have enough time to initialize/stabilize and output an EN signal.

    Best,

    Ryan

  • 0 in reply to Ryan Chen
    Prodigy 60 points

    Hi Ryan,

    Thank you for your reply.

    In our circuit, 12V comes up first. TPSM843B22 uses this 12V as VIN. This is because the output current is large, so VIN must be supplied from the source power supply.

    On the other hand, the IC that controls GPIO is powered by another power supply IC, which also uses 12V for VIN. Furthermore, the IC that controls GPIO needs to be initialized after startup.

    Inevitably, the output of the GPIO that controls the EN signal of the TPSM843B22 is Hi-Z until the IC initialization is completed, and during this time the EN signal is raised inside the TPSM843B22, so it must be enabled before the GPIO is initially inserted. As a result, the output of TPSM843B22 is output sooner than expected.

    We thought of a way to avoid this by inserting an open-drain buffer (for example, SN74LVC1G07) between the GPIO and EN signals, and then pulling down the input of the open-drain buffer.  This way, a low level will be input to the EN signal until GPIO initialization is complete.


    Is this idea correct?

    Thank you,

    Minoru

  • 0 in reply to minoru.takeuchi
    TI__Expert 8846 points

    Hi Minoru, 

    I think I understand what you are saying. I believe something like this should work.


    Best,

    Ryan

  • 0 in reply to Ryan Chen
    Prodigy 60 points

    Hi Ryan,

    Thank you for your ideas.
    I have a question about that.
    Now the VIN voltage is 12V. GPIO is powered by 3.3V power with GPO settings. Since the Gate Threshold Voltage of 2N7002 is 3V, r1 and r2 can be determined so that it becomes 3.3V. (e.g. 86.6K and 32.4K)
    Now, when the GPO is Hi-z, the EN signal will be at Low level, but to enable EN, the GPO must be set to Low. In this case, won't current flow to the GPO side?

    We thought as follows.
    In this circuit, when GPIO is Hi-z or Low, the EN signal becomes Low, and when GPIO becomes Hi, the EN signal becomes Hi-z and is pulled up inside the TPSM843B22, so the output is enabled.

  • 0
    Prodigy 60 points

    We are trying to control the EN signal of TPSM843B22 with GPIO of UCD9090A.
    According to the UCD9090A documentation "UCD90xxx Family Frequently Asked Questions and Answers" (SLUA815B)
     , if the POL controller for the EN pin has an internal pull-up, it states that the EN signal should be pulled down externally.

    Is it correct to think that if we use pull-down the EN signal, we can connect it directly to the GPIO pin of UCD9090A?

  • 0 in reply to minoru.takeuchi
    TI__Mastermind 20830 points

    The doc you referred

    The TPSM843B22 has an internal pull up current of 1.5uA.   Once the voltage on enable pin is greater than 1.2V, the device is enabled.

    Pulling enable below or keeping below 1.05V, the device is  disabled.      Placing a resistor on the enable pin so that the voltage from the pullup current and pulldown resistor is lower than the enable threshold will keep the device off.   

    So yes I think a resistor to ground on TPS843B22 can work. The GPIO will need to drive the pin voltage to > 1.25V to enable the device. 

  • 0 in reply to David_Daniels
    Prodigy 60 points

    Hi David,

    Thank you for your reply.

    What is the appropriate pull-down resistance value?

    EN is pulled up inside the TPSM843B22, so I would like to know the appropriate value for the resistor.

    We are concerned that the voltage may be divided due to the internal pull-up resistor.

    Note that we use the GPIO of UCD9090A. We use 3.3V for VDD, so I think it can be controlled with a 1.2V threshold, but I don't know about drivability.

    Thank you,

    Minoru