Too many DRAM choices. If you want low power DRAM, do you choose LPDDR1, LPDDR2, DDR2L, DDR3L, or what? It’s not an easy choice because of all the variables involved: the peak and average transfer rates, the power supply voltages, and the IC manufacturing process technology used—just to name three. In addition there are the DRAMs’ various operating modes and the corresponding current consumption in each mode. The modes of interest include:

• Read mode (Idd4R)

• Idle mode (Idd0)

• Power-down mode (Idd2P)

• Self-refresh mode (Idd6)

If you try to zero in on the optimal DDR choice using the multivariate equation needed to compute the exact energy consumption of any DRAM you might pick for a system design, taking all of these modes into account, you will most likely never get to the point where you can clearly identify the right DRAM choice.

Fortunately, your choice just needn’t be that complicated and Denali’s Marc Greenberg has an answer for you. He calls it the “big resistor” model of semiconductor DRAM power consumption. Now we all know that DRAMs aren’t big resistors. There are billions of tiny little transistors and capacitors inside of a DRAM and not one “big resistor” to be found inside of that package. But from a pins-out perspective, a DRAM’s power consumption is roughly proportional to the square of the power supply voltage, to a first-order approximation. To get the lowest power dissipation, you simply need to pick the DRAM with the lowest operating voltage that can meet your performance criteria. Now that’s a much easier choice.

Below is a table that shows the various choices for DDR DRAM, the associated operating voltages (including a few of Marc’s educated guesses), and a figure of merit based on the square of the operating voltage relative to DDR1 DRAM. Clearly from the table, LPDDR2 DRAM is the right choice if power and energy consumption are your only decision criteria.

Here are four bar graphs comparing the power consumption of the various DDR memory technologies in read, idle, power-down, and self-refresh modes.

From these bar graphs, LPDDR2 is the clear winner if power and energy consumption are the only considerations. However, power and energy consumption are never the only considerations in the real world of system design and manufacturing. Availability, device capacity, device organization, device speed, device cost, per-bit cost, second sourcing, and technology road map are other real considerations, just to name a few. So it’s helpful to see what other DRAM technologies might serve for a given system. Looking at the above bar graphs, it’s apparent that a low-voltage variation of DDR3, DDR3L, might give LPDDR2 DRAM a respectable run for the money except in power-down mode. If power-down mode is important because your system will put the DRAM in that mode a lot of the time, then LPDDR1 DRAM might also be a good choice.

It might also be a very good idea to design your system so that it can accommodate more than one DDR variant. That way, time and circumstance can determine which DRAM technology gets used at any given time. That idea, coincidentally, is the topic of a Denali White Paper scheduled to be released in the near future.