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TPS62748 / condition of Vin=0V

Tsukui Yusuke
Guru 21930 points
Other Parts Discussed in Thread: TPS62748

HI,

Could you tell me about TPS62748.
Our customer is going to adopt TPS62748.
I get the following questions from a customer.

1.When customer add the voltage to Vout in Vin=0V condition, is there the problem?
　Customer is concerned about destruction of  device caused by reverse currents.

2.Is Vos OPEN state in EN=Low or UVLO condition?

3.The following table is EN=High.

   However, it is '' Device not switching ''.
   What kind of state is this?

   
========

Could you give me your advice?


Best Regards,
Yusuke/Japan Disty

  • 0
    TI__Intellectual 1110 points
    Hi Tsukui,

    2. Vos is connected to GND if EN=Low and/or Vin is below the UVLO threshold.

    1. Due to that it is not possible to apply a voltage to Vout, as the output is connected to GND by the Vout discharge transistor. There is one exception, when EN is connected to Vin: As the body diode of the highside MOSFET supplies the input of the converter, the voltage at Vout needs to be larger than the UVLO threshold voltage plus the forward voltage of the body diode. Under that condition, the discharge transistor is high impedance, but once the voltage at the output falls below the mentioned before voltage, the discharge transistor is low impedance.

    3. Without load, the converter is not switching most of the time. Due to the leakage of the output capacitor, the output voltage decreases slowly. If the output voltage is below the target value, the converter starts switching, until the output voltage is at the target voltage again. As this is dependent on the used capacitor, the quiescent current while the converter is not switching is given in the specification.

    Can you give us further information about the application? Maybe we can help you furthermore.

    Best regards,
    Felix
  • 0 in reply to Felix Risch25
    Guru 21930 points

    Felix-san,

    Thank you for your kind support.
    Application is a mobile terminal powered by coin cell.
    Let me ask you one more thing.

    ・The customer writes the software to MCU connected to load.
    　Then,Voltage is added to Vout of TPS62748.
    　Is there a problem this work?

    ・Is the your answer(Vos status) listed in a datasheet?

    Best regards,
    Yusuke

  • 0 in reply to Tsukui Yusuke
    TI__Expert 7110 points
    For further information on devices' quiescent current:
    www.ti.com/.../slyt412.pdf
  • 0 in reply to Florian Feckl
    Guru 21930 points

    Florian-san,

    Thank you for your information.
    Is there the plan listed in a data sheet about the following contents?

    >2. Vos is connected to GND if EN=Low and/or Vin is below the UVLO threshold.

    >1. Due to that it is not possible to apply a voltage to Vout, as the output is connected to GND by the Vout discharge transistor.
    >There is one exception, when EN is connected to Vin: As the body diode of the highside MOSFET supplies the input of the converter,
    >the voltage at Vout needs to be larger than the UVLO threshold voltage plus the forward voltage of the body diode.
    >Under that condition, the discharge transistor is high impedance, but once the voltage at the output falls below the mentioned before voltage,
    >the discharge transistor is low impedance.

    Best regards,
    Yusuke

  • 0 in reply to Tsukui Yusuke
    TI__Intellectual 1110 points
    Hi,
    the Vos status is listed under: "9.3.5 Output Voltage Discharge of the Buck Converter"

    I hope I understood it correctly: You program the device with an external programmer, which supplies the input of the MCU and at the same time the output of the converter?

    It works, if:
    - Input of the converter is not connected
    - EN connected to Vin of the converter
    - Supply voltage of the programmer is above VTH_ UVLO+ (2.15) plus forward voltage of the body diode. With 3V supply at the output of the converter you should be safe.

    Under these conditions, the input of the converter is supplied by the output (body diode of the highside MOSFET conducts, see: 9.2 Functional Block Diagram, the MOSFET between VIN and SW). With EN=High and Vin > VTH_ UVLO+ , the discharge is not active.

    I recommend to limit the output current of the programmer. In case EN is low or something else got wrong you won't destroy the converter.

    Best regards,
    Felix
  • 0 in reply to Felix Risch25
    Guru 21930 points

    Felix-san,

    Thank you for your support.
    I apologize for the lack of clarity in my post.
    I attach the use TPS62748 environment of customer.
    TPS62748 condition.pdf

    ''2. Vos is connected to GND if EN=Low and/or Vin is below the UVLO threshold.''
    Does Vos become the short circuit state in cutomer condition?
    (Vos=OPEN?)

    ’’I recommend to limit the output current of the programmer. In case EN is low or something else got wrong you won't destroy the converter.’’
    Vout side current spreads in the Vin side passing through a body diode.
    Could you tell me the set value of the current limit programmer side.

    Best regards,
    Yusuke

  • 0 in reply to Tsukui Yusuke
    TI__Intellectual 1110 points

    Hi,

    if the supply of the programmer is 1.8V, as mentioned in the attachment, the Vos pin is shorted to ground, because the input voltage of the converter is below the UVLO threshold. Is there a possibility to increase the supply of the programmer to 3V? As I don't know the MCU type, you have to check, if the MCU supports 3V supply voltage. Then, Vos is open.

    I would recommend to limit the programmer current to maximum 50mA. As the output discharge pin has an internal resistance of nominal 30Ohms, adding another 30Ohms resistor in series to the programmers supply should be sufficient for supply voltages of 3V.

    Best regards,

    Felix