• State TI Thinks Resolved
  • Locked Locked
  • Replies 1 reply
  • Answers 1 answer
  • Subscribers 14 subscribers
  • Views 273 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

SN6505B: Soft start questions

Ruixin Feng
Intellectual 2035 points
Part Number: SN6505B
Other Parts Discussed in Thread: SN6501, , SN6507

Hi team,

Recently my customer is evaluating SN6501 and SN6505B, some questions about protection and safety came up. Could you pls help with these questions? Thanks a lot!

SN6505B:

1. how soft start works when power up? In datasheet, the description is "Upon power up or when EN pin transitions from Low to High, the gate drive of the output power-MOSFET is gradually increased over a period of time from 0 V to VCC." So how the device control the gate voltage or gate current? when power up, a single high-current pulse may be applied on output MOSFET, can the device suppress it and protect the MOSFET? 

2. About 1.7A current limit, is it an average current limit of output current?  what will the device act when overload? When the capacitive load is heavy, what is the maximum current that the device can afford when power up?

SN6501:

1. SN6501 don't have soft start, so how the device protect itself from inrush current when power up? 

2. What'll happen when overload?

3. What's the design guide when considering safety and protection?

Thanks!

Rayna 

  • 0
    TI__Guru 53705 points

    Hi Rayna,

    Thank you for reaching out. Please see my inputs below,

    SN6505:

    1. The description from datasheet that you have highlighted is accurate.
      1. It states that the voltage to Gate of MOSFET is controlled and not the current to Gate. Since MOSFETs are voltage controlled, the Gate voltage determines if the MOSFET is fully turned ON or partially turned ON. When the voltage is intermediate, the MOSFET will be partially turned ON and the drain current into MOSFET is limited. Hence, the final output voltage ramps up gradually avoiding the need for large inrush current.
    2. This is not the average current limit rather this is the threshold where device initiates current limit protection. This means that all the peak currents in your application should be <1.7A to operate normally.
      1. When the load capacitance is high, then the demand for input current can go up. Soft-start does limit this current and help avoid hitting the input current limit.
      2. But if the load capacitance is much higher than normal, then it is possible that even with soft-start input current limit is hit. In this case, device won't be able to reliably power up.
      3. In all test conditions, the current limit stays the same at 1.7A.

    SN6501:

    1. The device doesn't any feature to limit inrush, customer will have the design system to avoid high inrush currents.
    2. If the device operated is forced with overload condition often or if device is left in overload condition for long periods, it could get damaged.
    3. SN6501 is a simple to use solution with minimal features enabling ease of use. If customer needs a simple isolated power solution they can consider SN6501 but if they require protection features then we recommend that they use SN6505 or SN6507.


    Regards,
    Koteshwar Rao