Powering DDR memory and SSTL logic

VTT
While VDDQ and VTTREF are relatively straightforward power supplies, the tracking termination voltage, VTT, is very different.  Most power converters are intended to source current to their output in order to maintain a regulated voltage or current at their output.  This is even true of a termination voltage for conventional logic, which is equal to the logic’s I/O voltage.

The mid-rail VTT termination voltage used in SSTL logic and DDR memory devices is different.  When the SSTL logic circuit generates a 0, an active pull-down device sinks current from the termination rail and the termination supply acts like a conventional supply voltage, sourcing the required current to maintain the desired termination voltage.  However, when the logic circuit generates a 1, a pull-up device sources current into the termination rail and the termination supply must suddenly become a load, sinking current sourced by the memory output.  This sink and source requirement increases the complexity of the VTT design significantly.

While this sink and source requirement complicates the memory design, it does provide a very valuable feature to the memory device.  Each logic “1” is sourcing current into the termination and each logic “0” is sinking a similar current out of the termination; therefore, the termination supply needs only to support the differential current (0’s - 1’s), normalizing the load currents and improving signal integrity over rail terminated logic.

Further increasing its complexity is the fact that it must track VTTREF very closely.  SSTL logic voltages are so low, that small variations between VTTREF and VTT could quickly erode the noise margin and degrade signal integrity.  Unlike a conventional regulator, with an output voltage set by comparing a divided version of the output voltage to a high-precision reference voltage, VTT solutions must compare an output voltage to the VTTREF voltage to ensure the widest possible noise margin, largest eye windows and most accurate data transfer.  This “tracking” requirement dramatically reduces the range of available devices.

VTT experiences more sever transients than most power supplies designed to support similar current.  VDQQ might transition from 10 percent to 90 percent nominal load in a few micro-seconds; however if the data and address lines of a DDR memory device switch from all ”0’s” to all ”1’s” the load seen by the termination supply rapidly changes from sourcing its maximum load, to sinking its maximum load.  This 200 percent load step makes transient performance critical for DDR memory power.

For simplicity and load balancing, VTT is generally generated using a tracking sink-source linear device, such as a high current op amp or dedicated sink-source low dropout regulator.  In such applications, VTT is commonly generated from VDDQ.  In addition to minimizing the power loss in a linear device by providing a low source voltage, this has the effect of normalizing the load current on VDDQ. The reason for this effect is that any address line not drawing current from the I/O function of VDDQ by generating a ”1” is sinking current from VTT, which is drawing the same current from VDDQ.  While not the most efficient solution, it does provide a consistent load equal to ½ - 1x the VTT current (ITT) on the VDDQ supply.

In much larger systems that terminate hundreds of lines, such as the long recording arrays used in digitizing test equipment, or in extremely power sensitive systems, such as battery powered solutions that need to operate for extended periods without recharging, a tracking synchronous buck switcher (see Figure 2), such as the TPS40042 can be employed.



A synchronous buck converter can draw current from its output, returning the recovered energy to its input voltage much like a boost converter.  This sink/source capability along with the efficiency of a synchronous buck makes it an ideal choice for high current or high efficiency termination.

When using a tracking synchronous buck converter to realize the termination voltage, it is critical to keep in mind  the source voltage for the termination converter.  While it is often ideal to operate the VTT regulator from VDDQ like before, the low VDDQ voltages might make this impractical or the cascade effect of double conversion can sacrifice some efficiency benefits that can be realized with alternate schemes.  

When VDDQ is not used as the source for an active, switcher-based termination regulator, the termination regulator should share a common source with the VDDQ regulator.  This ensures that when sinking current from logic “1’s”, the active switcher cannot source more energy into its supply than is being drawn by the VDDQ regulator. This eliminates the need for the VTT sourcing supply to also sink load current and prevents a dangerous over-voltage condition at the source of the VTT regulator.