The spec of Vref for AM3874

Hi,

I would like to ask one more question regarding DDR3 Vtt.

As you know, Vtt is used for the termination of address and control signal. And address and control signal is driven by AM3874.

Rtt for termination resistor value is already specified in datasheet. But "Vtt min/max" value is not specified in AM3874 datasheet.

Customer said me "Vtt min/max" is needed to take the signal matching. Could someone give me "Vtt min/max" value for AM3874? 

Customer can't design the system without this spec.

Please help !

Best regards,

Michi

Hello,

The nominal value of the VTT supply is half the DDR3 supply voltage, and the min/max values can also be derived from the min/max values for the DDR3 supply voltage.

Regards,
Marc

Hello,

The VREFSSTL_DDR(VREF) current is not specified separately, but the TPS51200 should work fine.  The DM8168 EVM schematics are a good reference point for using the TPS51200.

Regards,
Marc

Dear Marc-san,

Thank you for your reply.

Regarding DDR3Vtt, please let me know the meaning of you said that "the min/maxu values can also be derived  from the min/max values for the DDR3 supply voltage.".

Are Vtt min/max values the same as the half value of DDR3 supply voltage min/max?  Is AM3874 acceptable the fluctuation of the signal caused by Vtt value spec?

I appreciate your quick reply.

Best regards,

Michi

Michi-san,

Yes, the Vtt min/max values are the same as half the value of DDR3 supply voltage min/max.

Regards,
Marc

Dear Marc-san,

Thank you for your reply.

I talked your answer to my customer. They say there is no guarantee even though EVM works fine with TPS51200.

They can't understand the reason why VREF current is not specified. They say it should be specified if the block of the VREF(VREFSSTL_DDR) is unique than other IO pins. Pleasel let me know the VREFcurrent specification.

I appreciate your quick reply.

Best regards,

Dear Marc-san,

Thank you for your reply.

The  below is the case of using TPS51200. Could you confirm it?

DDR3 supply voltage(VDDQ)  is specified as the below. 

VDDQ = 1.5V +- 0.075V 

Based on your answer,

VTT = VDDQ / 2 = 0.75V +- 0.0375V  ---> VTT requirement of AM3874. Tolerance is +- 37.5mV.

According to the datasheet of TPS51200, VO(VTT) of TPS51200 tolerance is +-25mV .

This value(+-25mV)  satisfies AM3874's Vtt requirement.

Is this right?

Best regards,

Michi

Michi-san,

Yes, that looks correct..

Regards,
Marc

Marc-san,

Thank you for your support.

You had given an answer to me regarding Vtt(min/max) value. I appreciate your support.

Please give me other question's answer. It is VREF current spec.

The below is my last post regading this question.

>Dear Marc-san,

>Thank you for your reply.

>I talked your answer to my customer. They say there is no guarantee even though EVM works fine with TPS51200.

>They can't understand the reason why VREF current is not specified. They say it should be specified if the block of the VREF(VREFSSTL_DDR) is unique than other IO >pins. Pleasel let me know the VREFcurrent specification.

Your quick reply is helpful.

Best regards,

Michi