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TPS5410: DC/DC startup time

Hano Venter
Prodigy 70 points
Part Number: TPS5410
Other Parts Discussed in Thread: TPS5420

I’m having trouble understanding a symptom of the 7V DC/DC switcher on my design.

 

We want to add on/off control functionality via the ENA pin on the circuit below:

 

After some testing controlling the ENA (Pin 5) with an open collector transistor circuit I found the following:

You can see from Channel 2 (Blue) that the switcher “struggle” to switch on.

Channel 1 is the ENA control pin being released to switch on

 

The datasheet stipulates that it should be within 8ms.

 

Do you perhaps know why this is happening or seen something similar before?

I did fiddle somewhat and found that I can make it worse by touching on the PH (PIN8)

 

Any help would be appreciated and I will gladly do some testing if needed.

 

Thanks,

Hano 

  • 0
    TI__Guru**** 191445 points

    That waveform looks unusual.  It should look like the enclosed:

    Perhaps the IC is damaged?

  • 0 in reply to JohnTucker
    Prodigy 70 points
    Hi John,

    Thanks, I've tried replacing the IC but still get the same results.

    Hano.
  • 0 in reply to Hano Venter
    TI__Guru**** 191445 points

    What is the application?  There may be some external factor that is causing this start up behavior.

  • 0 in reply to JohnTucker
    Prodigy 70 points
    We use it to supply a Audio PA which draw a max current of 600mA and a keypad which draws a constant 10mA. The PA is however switched off so the effective load seeing the above behavior is only 10mA.
    The input to the device can vary from 10 - 33V.
  • 0 in reply to Hano Venter
    TI__Genius 11085 points

    Hi, from the startup waveform you've attached in the first post, it seems like the part might be in HICCUP mode. Are there other capacitors at the input port of the load device in addition to the Cout shown in the schematic? If there are too much capacitance to charge, the part needs to provide a large amount of current and it might trigger hiccup. But I didn't understand why it could start properly when Vout reached about 2.5V. Anything changed at the load when Vout reached 2.5V?

    I'd suggest removing L9 (100nH output filter) inductor on the board, and try to start the TPS5410 again. This is to isolate the TPS5410 behavior without the loading device for debugging.

    -Yang

  • 0 in reply to YangZhang
    Prodigy 70 points

    Hi, I've removed L9 and now the device startup is correct (+- 8ms).

    There is a 470uF electrolytic capacitor further down on the output which is required by the audio amplifier.

    Can the hiccup mode be prevented,  and what would the maximum capacitance be to avoid the hiccup mode?

    Hano.

  • 0 in reply to Hano Venter
    TI__Guru**** 191445 points
    If the time scale on your original plot is in fact 80 msec/div, then I think it is hiccup mode over current protection. In theory, you should be able to start up into C = Iclmin * tss / Vout = 1.2 * 0.008 / 7 = 1371 uF if there is no additional load. You could possibly simulate your start up condition using the pspice transient model found here:
    www.ti.com/.../toolssoftware
  • 0 in reply to Hano Venter
    TI__Genius 11085 points

    Hi Venter,

    The datasheet did not specify the exact condition when the hiccup behavior is triggered. It is hard to get around it. Would you try to use TPS5420 instead? TPS5420 is pin to pin compatible with TPS5410 and has twice current rating.

    You can also find a part with soft start rate control. If the soft start rate is slowed down, it needs less current to charge the capacitors.

    Other methods can be adding a load switch between the output of the TPS5410 and the load device input port. The charge current to the large capacitor can be controled by the turn-on speed of the load switch.

    Hope this helps,

    Yang