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Power tip: How to power DDR memory

Posted: 25 Nov 2011     Print Version  Bookmark and Share

Keywords:DDR  memory clocks  sink current 

The table shows the comparison in component count, area requirements, dissipation, and cost for a switcher versus a linear regulator. This is for regulators which can output 3A of peak current. It is interesting to note that dissipation is going to be hard to handle, if the peak current is present all the time. Establishing the DC current will drive the choice. Clearly, the linear regulator is favoured from all other aspects.

One significant challenge with a DDR power supply is controlling the output voltage during wild transient loading extremes. As shown in the table, the linear approach has a much-wider control bandwidth than the switcher.

Table: Linear approach is smaller and costs less, but is not as efficient as a switcher.

Consequently, it uses much-smaller capacitors to control output impedance. For instance, to control the output to within 40 mV with a 3A load, the output impedance at the crossover frequency needs to be less than 0.013 Ω, which corresponds to about 10µF of capacitance. A switcher with a linear control loop closed at 50kHz takes over 200µF of capacitance, which results in additional cost and circuit board area.

To summarise, DDR memory enhances system speed by clocking data on both edges of the clock, resulting in increased through-put. Termination resistors are required to reduce voltage reflections because of the high frequency operation. Losses in the terminations can be minimised by connecting one end with a voltage equal to half the supply voltage.

This supply needs to be able to source or sink current and must have a high crossover frequency to minimise capacitor requirements. A linear regulator approach for the termination supply can save money and size, if the increased power dissipation is acceptable.

About the author
Kollman is a Senior Applications Manager and Distinguished Member of Technical Staff at Texas Instruments. He has more than 30 years of experience in the power electronics business and has designed magnetics for power electronics ranging from sub-watt to sub-megawatt with operating frequencies into the megahertz range. Robert earned a BSEE from Texas A&M University, and a MSEE from Southern Methodist University.

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