TPS51200 LPDDR3 Configuration - Strange Behavior

Refael Shamir

Other Parts Discussed in Thread: TPS51200

Hi,

I am using a DC-DC converter, namely the TPS51200 for DDR power application.
Chosen design configuration is as seen on page 23 of the datasheet (DDR3 Configuration with LFP).

Now, I see a strange response when turning off my board, it seems that the TPS51200 generates an output voltage, even though EN pin signal is low.

Attached here is a scope dump, triggered on powering down.

Can anyone explain possible scenarios on how it can occur?

Thanks,
Refael.

over 9 years ago

0 JohnTucker over 9 years ago

TI__Guru**** 191445 points

Can you post your schematic and tell us what the signal names on your waveform correspond to?

0 Refael Shamir over 9 years ago in reply to JohnTucker

Prodigy 250 points

Hi John,

Unfortunately, I am unable to share my schematic.
You can see from image below, probe location and voltage levels for each probe (EN pin is 3.3V), same voltage levels apply to my design.

Refael.

0 JohnTucker over 9 years ago in reply to Refael Shamir

TI__Guru**** 191445 points

You can see that when EN is initially pulled low, the output is discharged to GND by the internal discharge FET. Once VIN falls, the device is completely off and the pull down FET is no longer active. I suspect there may be an external leakage path form some other voltage rail that is pre-charging your Vtt output.

0 Refael Shamir over 9 years ago in reply to JohnTucker

Prodigy 250 points

Tested today, root cause of my problem is the DDR itself. I have disconnected TPS51200 (leaving only 1.5V for DDR core voltage), observing that DDR's 1.5V appears momentarily in 0.75V rail (a 1.5V step can be seen).

Is there a way to isolate voltage rail in question - or I can live with it?

Refael.

0 JohnTucker over 9 years ago in reply to Refael Shamir

TI__Guru**** 191445 points

i don't know about your system, so I can't say how difficult it may be to track down the source of the pre-bias. If it is not causing you any particular system related issues, I would not worry about it. Pre-bias voltage on the output of dc/dc converters is a fairly common occurrence.

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TPS51200 LPDDR3 Configuration - Strange Behavior