TPS51200: VREFOUT overshoot and VTT step

Hi Nessee,

Can you increase VREFOUT cap value?
Did you see glitch/spike on VTT during soft start?

Thanks
Qian

Hi,Qian

Thanks for reply.

1. Larger cap improves VREFOUT but cannot eliminate the overshoot as far i tried 4.7uF. Waveform shows below.  YELLOW-VDDQ/GREEN-VREFOUT

The typical value in datasheet is 0.1uF in VREFOUT pin, and the original value of our design is 0.6uF (one near TPS51200 and five each near the DDR cell).

As i see in a question raised by other people , your suggested the total amount below 0.47uF. So what is the maximum capacity and is there any possibility of malfunction due to cap increasing?

https://e2e.ti.com/support/power-management/f/196/t/626237?tisearch=e2e-sitesearch&keymatch=TPS51200%20OVERSHOOT

2. There is no glitch or spike on VDDQ/VREFIN/VTT.

I think i find why VTT didnot controlled by VTT_EN at the very begining of ramping up.

The address lines are terminated with VTT,  where may exist a sneak path between VDDQ and VTT.

Hi Nessee,

1. We suggest to keep total REFOUT cap below 1uF. The original value 0.6uF is fine.
I don't think overshoot has impact DIMM.

2. You are right. There is a leakage current path from VDDQ and VTT on DIMM side.

Thanks
Qian

Hi, Qian
I understand the overshoot doesn't seem to have any effects on normal powering up. Just curious to ask
1. What do you suppose is the root cause ? Is the powering up inrush current ? Whereas the author in link below said it's not the cause of the dip.
https://e2e.ti.com/support/power-management/f/196/t/587546?tisearch=e2e-sitesearch&keymatch=TPS51200

2. Is this a common waveform in other applications of this device in your experience?

Thx for reply.