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LP2996-N Inrush Current

Other Parts Discussed in Thread: LM43601, LP2996A, LP2998

I use LP2996 as VTT in DDR-SDRAM.

VIN and SD are connected. The output capacitor capacity is 47uF.

When input voltage is ramped up very slowly in this state, a inrush current flows.

Please see attached file. CH1:VIN, CH2:VTT, CH3:VREF, CH4:Iin

I guess a inrush current to be flow inside the IC because the output current isn't increased.

2.5V is being generated by LM43601, so an over-current is detected by this Inrush current.

When toggling SD pin to High from Low, this inrush current isn't flow.

.

I have two questions.

1. When VIN is below the recommendation operation voltage(<2.2V), does this phenomenon occur?

2. Isn't there a UVLO function in LP2996?

  • Hirotsugu-san,
    This inrush current is caused mainly when VDDQ is pulled up high with a fast slew signal in presence of a VTT capacitor. This is inherit with all DDR terminators and unfortunately LP2996 doesn't have any UVLO or inrush protection built in. We have seen this situation before when a DC/DC converter powering the DDR terminator hit the current limit and sags the voltage as a result.
    The best way to minimize this problem is by using the soft start which you can implement with LM43601. If you slow down the slew rate of VDDQ line (2.5V), the over current situation will not occur.
    If you reduce the VTT cap size, this problem can be possibly reduced. Looks like you already have only 24uF cap , so slowing down the 2.5V slew rate will is a better option.
  • Hi, Kaisar

    A waveform of good pars and bad parts is attaced.

    Even if the VTT capacitor is done small, there are no changes.

    It's expected that an electric current is flowing to GND from PVIN.

    Does this electric current float down this product?

    And I have one more question.
    What is a point of difference of LP2996 and LP2996A? Only a lowest value of VDDQ?

    Best Regards,

    Hirotsugu

    LP2996 Inrush.pdf

  • Hirotsugu-san,
    The only difference is that the LP2996A version supports DDRIII and DDRIIIL application and VTT can go down to VDDQ=1.5V/2. LP2996 only supports up to DDRII. In both cases the temp rating is 0 top 125V. If you need wider temp range like automotive, use LP2998.

    What is the VTT cap value for this testing? Are they both tested on the same board? Can you share your schematic?