DDR Memory – what is it?

1. DDR Memory – what is it?

2. 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

3. 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

4. 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

5. 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.

6. 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

7. 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

8. VTT Termination – Passive • Simple Resistor Termination • 90mA Termination • 162mW Dissipation • Inexpensive • Poor Performance • Lower Through Put • Higher Bit-Error

9. Passive Termination - Loading

10. 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

11. 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

12. 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

13. 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