Why does a ddrx power supply design require a VTT power supply

1、DDR Three power supplies of the system

For supply voltage ,DDR SDRAM The system requires three power supplies , Respectively VDDQ、VTT and VREF.

A、 Main power supply VDD and VDDQ

The requirements for the main power supply are VDDQ=VDD,VDDQ It's for IO buffer Power supply for power supply ,VDD Is to power the kernel . But in general, it is used to put VDDQ and VDD Synthesize a power supply to use .

Some chips have special VDDL, It's for DLL Powered by electricity , And also VDD Use the same power supply .

Power supply design , Voltage needs to be considered 、 Whether the current meets the requirements .

Power on sequence and power on time of power supply , Monotony, etc .

The power supply voltage is generally required in ±5% within . The current needs to depend on the different chips used , And the number of chips . because DDR The current is usually large , therefore PCB When the design , If there is a complete power plane laid on the pin , It's the ideal state , And increase the capacitance at the power inlet to store energy , Add one to each pin 100nF~10nF Small capacitance filtering .

here we are DDR5, Voltage from 1.2V It's changed to 1.1V, Declined 8.3%, These are generations DDR The lowest drop rate since the bus . Explain the development of electronic technology , The design of low power consumption is becoming more and more difficult . Such a low voltage , Its anti-interference design will be more difficult . The design requirements for power supply integrity and signal integrity are becoming more and more stringent .

B、 Reference power supply Vref

Reference power supply Vref Ask to follow VDDQ, also Vref=VDDQ/2, Therefore, the power chip can be used to provide , It can also be obtained by means of resistance voltage division . because Vref Generally, the current is small , In a few days mA~ Dozens of mA The order of magnitude , So we use the method of resistance voltage division , Cost savings , And it can be flexible in layout , Placed away from Vref The pins are close , Follow closely VDDQ voltage , So it's recommended to use this way . It should be noted that the resistance used for voltage division is 100Ω~10kΩ All possible , Need to use 1% Precision resistance .Vref Each pin of the reference voltage needs to be added 10nF Capacitance filtering of , And it is better to connect a capacitor in parallel with each voltage dividing resistor .

Vref The current here is not large , Through partial pressure , You can choose a resistance with a slightly higher resistance . So it needs to be placed close to the chip , The placement route is too long , Disturbed by other large current signals .

C、 Voltage for matching VTT(Tracking Termination Voltage)

VDDQ It is a high current power supply DDR The core of the chip 、I/O And memory logic power supply , and Vref It is a low current 、 Accurate reference voltage , It is at logic high level （1） And logic low level （0） Provide a threshold between , Adapt to I/O Changes in supply voltage . By providing a precise threshold value adapted to the supply voltage ,VREF The noise margin is larger than that under the condition of fixed threshold and normal change of terminal and drive .

VTT Is to improve signal quality , The most common specification is 0.49 To 0.51 times VDDQ,VTT Pull up the power supply to match the resistance ,VTT=VDDQ/2.

DDR In the design of , Depending on the topology , Some designs do not use VTT, Such as the controller DDR When there are few devices . If you use VTT, be VTT The current requirement is relatively large , Therefore, it is necessary to use copper sheet for wiring . also VTT It is required that the power supply can absorb current , You can also inject electric current . In general, it can be used specially for DDR The production of design VTT Power chip to meet the requirements . In many cases , The pull-down resistor is also used to realize the functions of current absorption and current filling , Thevenin circuit .

Signal integrity of jiangxiuguo

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and , Each pull to VTT A resistor is usually placed next to the resistor 10nF~100nF The capacitance of , Whole VTT The circuit needs to have uF Stage large capacitance for energy storage .

because VTT The power supply must be at 1/2 VDDQ Supply and absorb current , Therefore, if the power supply is not allowed to absorb current by shunting , Then you can't use a standard switching power supply . and , Because of the connection to VTT Each data line of has a low impedance , So the power supply must be very stable . Any noise in this power supply will directly enter the data line .

VTT Is used to get from DDR controller IC Obtain the voltage from the , Power the data bus and address bus ,VTT Not directly applied to DDR On the device , But on the system power supply （VTT And terminal resistors are integrated into DDR CONTROLLER On ）, Therefore, there is no need to mark in the circuit diagram . Its value is usually set to be approximately equal to VREF Value （ stay VREF Up and down 0.04V float ）, And with VREF Change Change through change . about DDR1 SDRAM Both the address bus control signal and the data bus signal in the application have termination resistors . A reference voltage without any noise or voltage change is required (VREF), Used as a DDR SDRAM Input receiver ,VREF Also equal to 1/2 VDDQ.VREF The change of will affect the setting and holding time of the memory .

2、 Why VTT

In order to meet DDR And ensure the best performance ,VTT and VREF Need to be under voltage 、 The temperature and noise tolerance are closely controlled for tracking 1/2 VDDQ.

In the actual circuit , about VREF The voltage is obtained by means of resistance voltage division , As shown in the figure below ：

The capacitance is decoupling capacitance .

DDR The receiving end of the particles is special , It is a differential amplifier , One of them PIN Foot connection Vref Is constant , the other one PIN Connect to DDR The sending end of the controller , The signal sent by the sender , Just compare Vref high , Above a certain threshold , The receiving end thinks 1, Just compare Vref low , Below a certain threshold , The receiving end thinks 0. We know DDR Rate ( Level switching ) It's very fast , At the same time, a controller will hang many particles , This causes the current on the bus ( Electric charge ) There is no time to release and supplement , This requires that VTT stay VOUT When you are high , Absorbing current , stay VOUT Make up current when it is low ;

With DDR2 For example , When VOUT When it's high level ,VOUT＝1V8,VTT＝0V9, electric current b To be in an increasing trend , When VOUT by 0,VTT＝0V9, electric current a To be in an increasing trend ;

commonly DDR VTT The topology is shown in the following figure ：

3、VTT Working principle of power supply

among VFB Is the voltage feedback terminal ,SW Is the voltage output terminal ;

combination DDR Look at the topology , When VOUT When it's low , because a The current in the direction is increasing , inductance L Temporary reverse electromotive force will be generated , To suppress current changes , This leads to VTT The voltage decreases , The upper tube is connected , To compensate for this current , Until the current flowing through the inductor equals the new current ;

When VOUT When you are high , because b The current in the direction is increasing , inductance L Temporary reverse electromotive force will be generated , To suppress current changes , This leads to VTT The power supply at becomes larger , Leading to Vsense Bigger , The upper pipe is closed , Lower the conduction tube , Absorbing current ;

4、 Thevenin circuit replaces VTT

In some designs , In the use of DDR In the case of particles , It has basically not been used VTT Power Supply , All of them adopt Thevenin circuit with resistance pull-down （ Replace with a pull-down resistor VTT Power Supply ）, Use only when using memory modules VTT Power Supply . Signal Integrity simulation is required , Determine the resistance of the pull-up resistor . This resistance is generally small , Although the design complexity is reduced , But it increases the power consumption of the whole system .

In general ,DDR The data lines of are all in a one-to-one topology , And DDR2 and DDR3 Inside ODT Do the matching , So there is no need to pull VTT Better signal quality can be obtained by matching .DDR2 If the address and control signal lines are multi load , There will be more than one drive , And there is no ODT, Its topological structure is walking T The structure of type , So it is often necessary to use VTT Match and control the signal quality .DDR3 May adopt Fly-by Way of wiring .

4、VTT Current estimation

for example ：VTT（0.6V） As an address line / Control line （ common 25 root ） Pull up power supply of  , Pull up resistance 39.2 ohm , Maximum current calculation formula ：（0.6V/39.2）*25 = 0.38A.

The maximum current is that all signals are high , Or when both are low , All signal lines are filled with current or drawn with current . If there are high and low , Will cancel each other out ,VTT The output current of is not so large .