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TPS61185 Isolation Switch

tateo yamashiro
Guru 10890 points
Other Parts Discussed in Thread: TPS61185, TPS61195

Hi,

My customer has questions about TPS61185.
Could you tell me about questoin?

[Question]
My customer considers to use isolation switch.
They have question about Figure 19 in datasheet.

- Why do we need D2 on GND to drain of the isolation MOSFET?
  Should do we need this diode?

Best Regards,
tateo

  • 0
    TI__Prodigy 320 points

    Hi Tateo,

    Yes, it's required.

    When the isolation FET is turned off, the diode is forward biased. So it can be used as signal output for system fault report.

    Best regards,

    Milo

  • 0 in reply to Milo Kim
    Guru 10890 points

    I'm sorry for the long delay in my reply.

    Thank you for your support.
    My customer has other questions.
    Could you help us?

    - My customer considers to use isolation FET.
      Isolation FET will be OFF at fault conditions.
      What is fault conditions?

    - My customer considers to use frequency program PWM dimming mode.
      Is there any limitations between switching frequency and PWM dimming frequency?
      If switching frequency is 1.25MHz, how much is the recommended PWM dimming frequency range?

    Best Regrds,
    tateo

  • 0 in reply to tateo yamashiro
    TI__Prodigy 320 points

    Hi Tateo,

    * Fault conditions

    TPS61195 has internal protection circuits. It protects from over-current, over-voltage and others.

    In the datasheet, you can see the max condition of each event.

    * SW freq range

    600 kHz ~ 2MHz. It's configurable by the resistor value.

    * Dimming frequency range

    MODE pin is low: 100Hz ~ 20 KHz

    MODE pin is high: 100Hz ~ 5 KHz (configurable by the resistor value)

    * Recommended PWM dimming frequency in case of Fsw = 1.25MHz

    I don't have specific data but let me try to get it.

    Best regards,

    Milo

  • 0 in reply to Milo Kim
    Guru 10890 points

    Hi, Could you tell us about these parameters?
    If the device detects these parameters, what does the device do?

     - Vovp2_ifb
     - Ifb_low
     - Vol

    Best Regards,
    tateo

  • 0 in reply to tateo yamashiro
    Guru 10890 points
    Sorry for the rush, but could you give us your feedback by the end of this week?
  • 0 in reply to tateo yamashiro
    TI__Prodigy 320 points
    1) Vovp2_ifb
    I'm not clear how the 2nd level works. PWM output will stop as soon as the OVP is detected on each IFB.

    2) Ifb_low
    It is detected when the IFBx has no current. In other words, WLED(s) is(are) open or IFB pin is shorted to GND after startup. Then, TPS61185 disables IFB string(s).

    3) Vol
    TPS61185 detects overload condition on IFB1 ~ IFB8 during specific period (64 cycles of oscillator).
    After the overload is detected, the IC is shutdown and internal regulator is disabled.
  • 0 in reply to Milo Kim
    Guru 10890 points

    Thank you for your help.
    Sorry to bother you again.
    Could you tell us below questions?

    - Vol is called OVP pin overload detection in datasheet. So I guess this protection detects on OVP pin. If OVP pin detects Vout_clamp*60%(64 cycles of oscillator), the IC will shut down.
      Is it correct?

    - If all Ifbx pin detects Ifb over voltage threshold, will IC shut down?

    Best Regards,
    tateo

  • 0 in reply to tateo yamashiro
    Guru 10890 points

    Sorry for rush, but could you give us your feedback by next monday. TPS61185 is almost DIN Phase!!!

  • 0 in reply to tateo yamashiro
    TI__Prodigy 320 points
    Hi Tateo,

    Yes, all your description is correct. I'd like to add one more comment.
    The detector monitors each IFB pin, so if one of IFB pins is detected as fault condition, then the device will shutdown.

    Best regards,
    Milo
  • 0 in reply to Milo Kim
    Guru 10890 points

    I appreciate for your support.
    I measured the waveform that all of the LED strings are open. I want to confirm the waveform.

    - According to datasheet, the IC only shuts down if it detects that all of the LED strings open.
      I had imagined that the device will stop switching and the FAULT pin pulled high.
      But the device is switching and FAULT pin is pulled high only when Vout reaches Vout_clamp.
      Please see figure below.
      I think the device doesn't shuts down. Is this waveform normal?

    Best Regards,
    tateo

  • 0 in reply to tateo yamashiro
    TI__Prodigy 320 points

    Hi Tateo,

    I think you got the right point. The clamp circuit response time is not fast.

    Please refer to the OVP protection procedure below.

      Vout_clamp = Vclamp_th * (1 + R3/R4) = 1.95 * ( 1 + 680/54.9) = 26.1V

      Vout_ov = Vov_th * (1 + R3/R4) = 2.03 * (1 + 680/54.9) = 27.17V

    1) OV circuit detects the output > 26.18 V (80mV higher)

    2) OVP protection is activated

    3) Drop the output voltage below Vout_clamp (26.1V)

    4) The turn off the boost

    So the device continues the switching operation in 1) ~ 3).

    Best regards,

    Milo

  • 0 in reply to Milo Kim
    Guru 10890 points

    Thak you for your reply.
    I also measured the waveform of over load condition. Please see figure below.
    The OCP period is difference between 15ohm and 20ohm.
    Why is the OCP period difference between 15ohm and 20ohm?

    Best Regards,
    tateo

  • 0 in reply to tateo yamashiro
    Guru 10890 points

    Hi, My customer has question about behavior of LED open protection.
    When LED is open, the device continues the switching operation 1~3.

     1. When Vout reaches Vout_clamp voltage, the device stop switching.
     2. Drop the Vout below Vout_clamp.
     3. The device restart switching.

    They want to know the device how to restart switching. What is the switching restart voltage?

    Best Regards,
    tateo

  • 0 in reply to tateo yamashiro
    TI__Prodigy 670 points
    Hi Tatoe,
    First, I think your test result is correct to TPS61185 behavior. Below is the machnism for each action.
    1. If all IFB pins are open, the Boost will keep switching until clamp voltage setting by resistor divider on OVP pin.
    2. If Vout reaches clamp voltage and all IFB has no current, the TPS61185 will judge the alll IFB open issue and turn off boost switching.
    3. Normally IFB open issues happen during panel production, because the LED load isn't connected before turning on the TPS61185 or the LED cable is not solid connected. Once TPS61185 detects the all IFB open issue and latches off the device, EN toggling or POR is requried to startup the device again. In order to avoid TPS61185 latched off during production test, an auto-restart function is implemented.

    The restart is not triggerred by Vout voltage dropping to certian threshold. Actually the device restarts in 4ms after switch off. We didn't see the SW switching waveform immediately after restart, because the restarts will go through the soft start process to avoid inrush current.

    Best regards,
    Xianghao
  • 0 in reply to Xianghao Meng
    Guru 10890 points

    Thank you for your support.
    I'll check the waveform again on EVM.

    According to soft start section in datasheet, "output voltage rises step by step from the minimum output voltage in 100mV increments, over a 1 ms interval."
    What is the minimum output voltage? Is it "Min Vout regulation voltage(0.72*(1+R3/R4)"?

    Best Regards,
    tateo

  • 0 in reply to tateo yamashiro
    TI__Prodigy 670 points

    Hi Tateo,

    It's correct, the minimum regulation voltage is (0.72x(1+R3/R4)).

    Best regards,

    Xianghao