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This comprehensive guide delves into the intricacies of DDR memory, including its double data rate technology, power specifications, voltage thresholds, and more. Learn about VDDQ, VTT, and how to calculate power needs for your DDR memory modules.
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Understanding DDR Memory • Double Data Rate Memory • Read/Write on both Rising and Falling Edge • 2x Speed without increasing CLK frequency • Faster Transitions • Low Voltage (2.5V, 1.8V, 1.5V) • Lower Threshold Voltages • Lower noise margin
Understanding DDR Memory • Conventional Logic • Fixed Input Threshold • High Impedance Termination • Terminated to I/O Supply • Stub Logic (DDR Memory) • ½ VIO Threshold • Low Impedance Termination • Terminated to ½ I/O Supply
VDDQ, VTT What does it all mean? • VDDQ • VCC – Memory Core Power • VDD – Memory Logic Power • VI/O – Input/Output Stage Power • 2.5V (DDR), 1.8V (DDR2) or 1.5V (DDR3) • 3% Accuracy, 1.5% ripple, 50mV transient • VTT_Ref – Typically less than 1mA / device • Low Current Logic Threshold Level • ½ VDDQ • 2% Accuracy, <10mV ripple • VTT • High-Current Termination Voltage • VTT_Ref • 20mV Accuracy, 1% ripple, 40mV transient
VDDQ, VTT, What does it all mean? • VDDQ • Provides Core,Logic and I/O Power • Typical low voltages • 2.5V on older DDR • 1.8V on current DDR2 • 1.5V on newest DDR3 • Generally Produced with a High-speed switcher • Single Chip can be as low as 1-2 Amps • Use Single Channel Switcher smaller banks (3 – 15A) • Use Multi-Channel Switchers for large, multi-DIMM banks • High Transient requirement can require large output capacitor bank.
VDDQ, VTT, What does it all mean? • VTT_ref • Provides Reference for Logic Input-Stage • Must be ½ VDDQ of Source device • Can be generated by Resistor Divider • Typical for up to 4 Chips or 1 DIMM • Generate with active buffer • Less load sensitive – higher accuracy • Higher divider resistance – Less dissipation • Necessary for larger multi-DIMM banks
VDDQ, VTT, What does it all mean? • VTT • High-Current Sink/Source Termination • Source Current for Logic 0 • Sink Current for Logic 1 • High Dynamic Currents • -IMAX to +IMAX at 5A/ms not uncommon • 40mV transient response • Typical requires large bulk capacitance • Typically generated with Tracking Switcher OR Sink/Source LDO • Tracking Switchers may require external resistor divider • Standard LDO’s can-not sink current
VTT Termination – Passive • Simple Resistor Termination • 90mA Termination • 162mW Dissipation • Inexpensive • Poor Performance • Lower Through Put • Higher Bit-Error
How Much Power do I need? • VTT_REF • Low Current Requirement • Typically less than 1mA / device • VTT_REF leakage current typical given in DDR IC or DIMM datasheet
How Much Power do I need? • VTT • Typically NOT given in datasheet • Depends on Address & Datalines • DDR • Can be as high as 20mA / data/address bit channel • DDR2 • Can be as high as 18mA / data/address bit • DDR3 • Can be as high as 15mA / data/address bit • Can have upto 2x start-up current due to Active Termination / Capacitive Load
How Much Power do I need? • VDDQ • In Datasheet • Datasheets list IDDQ for every mode • Not all IC/DIMMS in highest current mode at once • Typical estimate highest IDDQ mode for 1 IC/DIMM + 50% for each additional IC/DIMM • Given in “No Load” condition • Does not include VTT current • Must Source VTT current to output Logic 1s • Increase by VTT current for I/O when Output All 1s. • IDDQMAX = IDDQ(datasheet) + VTTMAX
Integrated Memory Example • Micron 2GB Memory IC • 84pin IC • IDDQ(max) = 350mA • ITT(max) = 13.5mA * 34lines = 460mA • 16 data, 14 address, 4 command • VDDQ = 1.8V @ 810mA • VTT = 0.9V @ 460mA
