Double data rate synchronous dynamic random access memory. Wow! That is a mouthful. Many may not even recognize the full name; it is most commonly abbreviated as DDR memory. Figure 1 shows an image of a DDR module that would be used in a PC. In this image I have placed a red circle around one of the DDR chips. As devices get smaller, there is not room or a need for these complete modules. One or more of these chips may be placed directly on the main circuit board of a device.
Most people probably don’t realize how many products or devices they own with DDR Memory. Some examples of devices that contain DDR memory: Server, PC, Tablet, Smart Phone, GPS, Car Navigation, TV, AV Receivers, E Reader, IP Phones, digital cameras, etc. With DDR being in so many locations, the applications and requirements vary widely.
DDR chips require a power supply with 2 voltages. VDDQ and VTT are the names of the two supply rails. The VTT voltage needs to track one half of the VDDQ voltage. The VTT also needs to be able to source and sink current. The VDDQ is usually a switching power supply; the VTT can be either a switching power supply or linear regulator. If a linear regulator is used for the VTT, it must be a special type of design that can meet the requirement of sourcing and sinking current. Robert Kollman has a great power tip article that explains more detail about the reasoning for the power supply requirements. The voltage specifications for these power supplies are based on the type of DDR chip. There are currently 4 types of DDR that are pretty widely used (DDR4 is on the way):
- DDR1: VDDQ = 2.5V, VTT = 1.25V
- DDR2: VDDQ = 1.8V, VTT = 0.9V
- DDR3: VDDQ = 1.5V, VTT = 0.75V
- DDR3LV: VDDQ = 1.35V, VTT = 0.675V
Because of the wide range of end products the power supply currents can vary greatly. Server power supplies for memory might use a large number of modules and require 100A or more. Cell phones or tablets could require 1 or 2 chips and only require a power supply with current rating of 1A to 4A. PowerLab has a number of designs specifically for DDR, a few of them are listed below:
PMP6807: 3 - 4.2Vin, 1.35Vout @ 4A (VDDQ), 0.675Vout @ 0.5A (VTT), DDR3 LV using the TPS51216 – The PMP6807 reference design uses the TPS51216 to generate a low voltage DRR3 power supply. The VDDQ supply is 1.35V and capable of 4A. The VTT supply is generated using a source/sink LDO that is capable of 0.5A. The design also takes advantage of high performance compact MOSFETs (CSD17313Q2) to achieve efficiency over 90% for most of the load range
PMP5225: 12Vin, 1.5Vout @ 9A (VDDQ), 0.75Vout @3A (VTT), DDR3 using the TPS51116 – The PMP5225 reference design uses the TPS51116 to generate DDR3 power supply rails. The VDDQ supply is 1.5V and outputs 9A. The VTT supply is generated using a source/sink LDO that is capable of up to 3A. High efficiency MOSFETs (CSD16406Q3) in a 3x3mm packages are used for small size and good performance.
PMP3054: 12Vin, 1.5V @ 80A (VDDQ only), DDR3 Multiphase Synchronous Buck Converter using the TPS40140 – The PMP3054 design uses 2 x TPS40140 in a four phase synchronous buck converter to generate 1.5V at 80A from a 12V input. The design operates at 400KHz to keep the size small.
PMP3060: 12Vin, 1.8V @ 60A (VDDQ only), DDR2 Multiphase Synchronous Buck Converter using the TPS40180 – The PMP3060 design uses 3 x TPS40180 in a three phase synchronous buck converter to generate 1.8V at 60A from a 12V input. The design operates at 250KHz per phase to keep the efficiency high.
