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TPS2420: About Fault timer flag

Tsukui Yusuke
Guru 21930 points
Part Number: TPS2420

Hi,

Could you tell me about Fault timer flag?
Please you check the attached waveform.

Is there any protection other than IFLT triggering CT?
Customers do not know the reasons for CT startup.
Please tell me all the protection that triggers CT.

TPS2420 VCT.pdf

Best Regards,
Yusuke

  • 0
    TI__Guru 57737 points

    Hi Yusuke,

    There is an internal Power Limit Circuitry ( 5W Typical) to protect the Mosfet during Start up. During start up there is a Voltage VDS across Mosfet (Vin-Vout). From your waveforms looks like you have a 17V Vin and you are trying to start up. This allows very less current during start up. Please go through the below section in datasheet for more understanding.

    So, you will have to reduce your load during start-up or start with No-Load.

    Regards,

    Praveen

  • 0 in reply to Praveen GD
    Guru 21930 points

    Praveen-san,

    Thank you for your response and support.
    I reported your advice to customers.

    However,
    It is not possible to change the load applying condition and input voltage condition.

    Customer increased the capacitor capacity connected to the CT pin.
    In such a case, the CT voltage did not rise.
    And it got to be able to start.
    Customers want to clarify this reason.
    Please check the attached waveform.

    TPS2420 CT 0.22uF vs 2.2uF.pdf

    Is there any protection other than IFLT triggering CT?
    Customers do not know the reasons for CT startup.
    Please tell me all the protection that triggers CT.

    Best Regards,
    Yusuke

  • 0 in reply to Tsukui Yusuke
    TI__Guru 57737 points

    Hi Yusuke,

    In the latest waveform you shared, I can see that the Vout is around 5V while Vin is close to 17V. The Vout did not ramp-up to Vin. 

    This is not expected.

    Regards,

    Praveen

  • 0 in reply to Praveen GD
    Guru 21930 points

    Praveen-san,

    Thank you for your response.
    I am sorry I didn't explain it clearly enough. 
    Attach the waveform again.
    Could you check the attached waveform?
    When CT capacitor is changed, Vout starts after 50 ms.
    However, the CT pin is not responding.

    0714.TPS2420 CT 0.22uF vs 2.2uF.pdf

    Could you tell me all the protection that triggers CT.

    Best Regards,
    Yusuke

  • 0 in reply to Tsukui Yusuke
    Guru 21930 points

    Praveen-san,

    Is there any update?
    My customer wants to know the "correct operation of the CT pin".
    Customers need to fix the shematic by November 8.
     I'm sorry to rush you.

    Best Regards,

    Yusuke/Japan Disty

  • 0 in reply to Tsukui Yusuke
    TI__Guru 57737 points

    Hi Yusuke,

    Sorry for the delay in response.

    During Start-up, the CT Capacitor has to charge.  From the waveforms shared by you, it looks like the part is in Power limit during start-up.

    Can you let us know,

    1. What is the output Capacitance ?

    2. What is the Type of Load and its value during startup and steady state ?

    3. Can you turn-off the load during start -up and use PG pin to turn it ON after the Vout has ramped up ? At Least try this  on the board to see  if this condition arises due to  the load.

    I think you are violating the below condition,

    Please share the schematic if possible.

    Regards,

    Praveen

  • 0 in reply to Praveen GD
    Guru 21930 points

    Praveen-san,

    Thank you for your response and support.

    During Start-up, the CT Capacitor has to charge.  From the waveforms shared by you, it looks like the part is in Power limit during start-up.
    →I understand. When remeasured, the CT voltage was rising.

    1. What is the output Capacitance ?
    →0.1uF has been installed at the output side of the TPS2420.
    　And on the load side to be connected, a total of 70 uF capacitor is installed.

    2. What is the Type of Load and its value during startup and steady state ?
    →The load information of connected equipment is unknown.
        The average current is 200 mA.

    3. Can you turn-off the load during start -up and use PG pin to turn it ON after the Vout has ramped up ? At Least try this  on the board to see  if this condition arises due to  the load.
    →I introduced this countermeasure method.
    　 However, the customer said that correspondence is difficult.
        Is there a way to shorten the other to the power limit time?

    Best Regards,
    Yusuke

  • 0 in reply to Tsukui Yusuke
    TI__Guru 57737 points

    Hi Yusuke,

    The power limit feature is internal to the device. We have no Control on it. It is necessary for the internal FET  to be safe.

    If you cannot Start-up with No-Load, then it is better to use another device for this application.

    Since your Voltage  is 17V and Load Current around 200mA, you can ask your customer to consider TPS26620 or its family of parts.

    Regards,

    Praveen

    Please click "This resolved my issue" button if this post answers your question.

  • 0 in reply to Praveen GD
    Guru 21930 points

    Praveen-san,

    Thank you for your response and support.
    "If you cannot Start-up with No-Load, then it is better to use another device for this application."
    →Customer was able to start the IC under no-load conditions.
    　 And After the Vout rises, the load is connected.
       (The customer installed the FET after the TPS2420 Vout.)

    Let me ask you one more thing.
    In case of installing FET, inrush current occurs.

    1.Could you tell me the allowable output current value of the TPS2420?
       And Is there a pulse time specification for inrush current sensing?

    2.IMAX is a function to limit the current to the set current value.
       ex) When the IMAX setting is set to 5 A, this device is suppressed to 5 A when a load of 6 A occurs.
       Is my understanding correct?

    Best Regards,
    Yusuke

  • 0 in reply to Tsukui Yusuke
    TI__Guru 57737 points

    Hi Yusuke,

    As expected,  your load is causing the device to shut down during start-up.

    Coming to your question on how much Current is allowed during start-up,  

    1. From formulae (7) of datasheet, the minimum resistance during start-up with 17Vin is 24 ohms. Which means ~700mA  of load can be drawn without any  output capacitors.

    Depending on your output capacitance, the current limit further reduces as the capacitance draws .

    From the waveforms you shared, it looks like your Load has an UVLO at 5V. When ever Vout reaches 5V, there is spike in Current and the controller is trying to limit to 5W power limit i.e. the current is being clamped to  ~400mA at a Vds across the FET of 12V (17Vin-5Vout) . Once you remove the load, the current respective to 5W limit is sufficient to charge the output caps before the timer expires.

    2. There are two current limits that you can set. Imax and Iflt.If over current is greater than Imax but less than (1.6X Imax) device limits current to IMAX. If the current does not drop below the IFAULT level before the timer times out then the output shuts off.

    If over Current is greater than 1.6x Iflt, the TPS2420 device immediately shuts off current to the load without waiting for the fault timer to expire.

    Regards,

    Praveen

    Please click "This resolved my issue" button if this post answers your question.