PowerLab Notes: DDR Memory is Everywhere!

Double Data Rate Synchronous Dynamic Random Access Memory. Wow! That's a mouthful. Many people may not even recognize the full name; it's often shortened to DDR memory. Figure 1 is a diagram of a DDR module used in a PC. In the diagram, I've put a red circle around one of the DDR chips. As devices get smaller, there's no room or need for these complete modules, so you may just place one or more of these chips directly on the device's main circuit board.

Most people probably don't realize how many products or devices they own that have DDR memory in them. Some examples of devices with DDR memory include: servers, PCs, tablets, smartphones, GPS, car navigation, TVs, AV receivers, e-readers, IP phones, and digital cameras. Because DDR is used in so many different ways, the applications and requirements vary widely.

DDR chips require a 2 volt supply. VDDQ and VTT are the names of the two power rails. The VTT voltage needs to track half the VDDQ voltage. VTT also needs to be able to source/sink current. VDDQ is usually a switching supply, while VTT can be either a switching supply or a linear regulator. If a linear regulator is used as VTT, it must be a special type of design that can meet the source and sink current requirements. Robert Kollman has written a very good power tips article that goes into more detail on the origin of the power supply requirements. The voltage specifications of these supplies depend on the type of DDR chip. There are four types of DDR in widespread use today (DDR4 is coming soon):

1. DDR1: VDDQ = 2.5V, VTT = 1.25V;
2. DDR2: VDDQ = 1.8V, VTT = 0.9V;
3. DDR3: VDDQ = 1.5V, VTT = 0.75V;
4. DDR3LV: VDDQ = 1.35V, VTT = 0.675V.

Because of the wide range of end products, the power supply currents vary greatly. A server's memory power supply may use a large number of modules and require 100A or more. A cell phone or tablet may require 1 or 2 chips and only require a power supply rated at 1A to 4A. PowerLab  has many designs specifically for DDR, a few of which are listed below:

PMP6807 :  3-4.2Vin, 4A 1.35Vout (VDDQ), 0.5A 0.675Vout (VTT) DDR3 LV using TPS51216   —  The PMP6807  reference design uses the TPS51216  to generate a low voltage DRR3 supply. The VDDQ supply is 1.35V and supports 4A. The VTT supply is generated using a source/sink LDO and supports 0.5A. In addition, the design takes advantage of high performance small MOSFETs ( CSD17313Q2 ) to achieve over 90% efficiency for most load ranges;

PMP6807 :  12Vin, 9A 1.5Vout (VDDQ), 3A 0.75Vout (VTT) DDR3 using TPS51116   —  The PMP5225  reference design uses  the TPS51116 to generate the DDR3 power rails. The VDDQ supply is 1.5V, outputting 9A. The VTT supply is generated using a source/sink LDO supporting up to 3A. A high efficiency MOSFET ( CSD16406Q3 ) in a 3x3mm packageis used to achieve small size and high performance;

PMP6807 :  12Vin, 80A 1.5V (VDDQ only), DDR3 Multi-Phase Synchronous Buck Converter using TPS40140   —  The PMP3054 design uses 2 x TPS40140  in a four-phase synchronous buck converter  to generate 80A 1.5V output from 12V input. The design operates at 400KHz to maintain a small size;

PMP6807 :  12Vin, 60A 1.8V (VDDQ Only), DDR2 Multi-Phase Synchronous Buck Converter using TPS40180   —  The PMP3060 design uses 3 x TPS40180  in a 3-phase synchronous buck converter  to generate 60A 1.8V output from 12V input. The design operates at 250KHz per phase to maintain high efficiency.