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A little knowledge of power circuits: will help you understand difference between source current and drain current.


1.DDR power supply

01, main power supply VDD and VDDQ

Main power supply requirement: VDDQ=VDD, VDDQ is I/O buffer power supply, and VDD is I/O buffer power supply. However, in general, VDDQ and VDD are combined into one power supply. supply. Some ICs also have a VDDL which supplies power to DLL and can use same power supply as VDD. When designing a power supply, it is necessary to take into account compliance of voltage and current with requirements, sequence of switching on power supply, time of switching on power supply, and monotonicity. Power supply voltage requirements are typically within ±5%.

The current must be calculated according to chips used and number of chips. Since DDR current is generally relatively large, this is most ideal state if a full power plane is laid on pins during PCB design, and power input is increased in capacitor energy storage, and a power supply is added to power supply. each pin 100nF ~ 10nF small capacitive filter.

02. Reference power supply Vref

The power reference Vref must follow VDDQ and Vref=VDDQ/2 so it can be provided by power chip or obtained by dividing resistors. Since Vref usually has a small current, on order of a few mA to tens of mA, resistive voltage division method can save money and be more flexible in layout. It is placed next to Vref pin and monitors it closely. so this method is recommended. It should be noted that resistors used for voltage division can be 100~10K, and it is necessary to use resistors with an accuracy of 1%. Each Vref pin should be filtered with a 10nF point capacitor, preferably in parallel with each voltage divider resistor.

03. Voltage VTT (Tracking Termination Voltage) to match

VTT is a power supply pulled up by a terminating resistor, VTT=VDDQ/2. In a DDR design, depending on topology, some designs do not use VTT, such as when there are fewer DDR devices in controller. If VTT is used, current draw of VTT is relatively large, so wiring must be laid with a copper sheath. And VTT requires power supply to be able to provide both current and drain current (drain current). Generally, a VTT generating power chip specially designed for DDR can be used to meet requirement. In addition, a 10Nf~100nF capacitor is usually placed next to each resistor connected to VTT, and a large uF level capacitor is required in entire VTT circuit for energy storage.

Typically, DDR data link is a one-disk-one topology, and both ddr2 and ddr3 have ODT for matching inside so you don't need to pull VTT for matching to get best signal quality. If address and control signal lines are multiloaded, then there will be more than one drive, and there is no ODT inside, and its topology structure is a T-point structure, so it is often necessary to use VTT to match quality of control signal.

From above brief introduction, we can see that VTT power supply used for termination requires SINK/SOURCE, which is related to topic I introduced. Now all of our industrial products use TI's TPS51200. The scheme is simple and practical. , and low cost. Please refer to the TPS51200 datasheet for details.

The images below are examples of similar designs and are for reference only.

Second, remove current source and current sink

01. Explanation of current sink terms

An important premise is that current sink and current source are for ports, and both are for IC output ports. Explanation of terms - filling: injection, filling, from outside to inside, from virtual to real. When you're thirsty, drink a large glass of freshly squeezed orange juice, drink it, and feel full. This is called "filling up." The drain current (drain current), for a port, if direction of current should flow into it, is "drain current", for example, I/O connected to VCC through a resistor and an LED when I/O output is a logic one. 0 Can you light LED, check drain current setting in device manual. Explanation of terms - pull: flow out, empty, inside out, from real to empty. I drank a large glass of freshly squeezed orange juice. After a while, I was so suffocated that I hurried to find a bathroom. It began to "rain heavily" and I felt comfortable. It's called "pull". The source current for port, if direction of current flows out, it is a "pull current", such as I/O connected to GND through a resistor and LED, when I/O pin is logic 1. Can you turn on LED and check source current parameter in user manual of device.

02. Concept

Source current and drain current are parameters for measuring output power of circuit (note: both draw and drain refer to output, so they refer to drive power). This term is commonly used in digital circuits. First of all, it should be clarified here that draw and drain current in chip manual is a parameter value, which is upper limit value (allowed maximum value) of output port draw and drain current allowed by chip in actual mode. scheme. The concept that will be discussed below is actual value in chain. Since output of digital circuit has only two levels of high and low (0, 1), when a high level is output, output terminal usually supplies current to load, and value of current provided is called "pull current". "; low-level output, generally, output terminal needs to absorb load current, and value of absorbed current is called "sinking (input) current".

For devices that inject current: both sink current and sink current are injected, sink current is passive and sink current is active. If an external current "flows" into chip through pins of chip, it is called inflow current (inflow); otherwise, if an internal current "flows" from chip through pins of chip, it is called a pull (pull) current. outside).

03. Why performance management can be measured

When gate's output pin is low, current flowing through gate is called sink current. The larger drain current, higher low level at output terminal. It can also be seen from output characteristic of triode that greater drain current, greater saturation voltage drop and greater low level. However, low level of gate is limited and has a maximum value of UOLMAX. When gate is working, it is not allowed to exceed this value, and specification of TTL gate states that UOLMAX ≤ 0.4 ~ 0.5V. Therefore, drain current has an upper limit.

When output terminal of gate is high, current at output terminal of gate flows out of gate, and this current is called pull current. The greater source current, lower output high level. This is because output stage transistor has internal resistance, and a voltage drop across internal resistance will cause output voltage to drop. The greater source current, lower output high level. However, gate high level is limited and has a minimum value of UOHMIN. This value must not be exceeded when gate is running, and TTL gate specification states that UOHMIN ≥ 2.4V. Therefore, draw current also has an upper limit.

It can be seen that there is an upper limit for source current and drain current of output terminal, otherwise, when output is high, source current will make output level lower than UOHMIN; when output is low, drain current will make output level higher than UOHMIN UOLMAX. Therefore, source current and drain current reflect power of output drive. (The larger value of source and drain current parameters of microcircuit, it means that microcircuit can be connected to more loads, since, for example, drain current is given by load, and larger load, more current that is injected).

Because high level input current is very small, there is usually no need to consider it at microamp level, and low level current is relatively large, at milliamp level. Therefore, there will be no problems if low drain current does not exceed standard. The fanout factor is used to illustrate ability of logic gates to drive similar gates. The fanout factor No is ratio of maximum low level output current to maximum low level input current. In integrated circuits, drain current, source output current, and drain output current are very important concepts. Pull and release, active current output is for outputting current from output port. Filling and charging, passive input current flows from output port to sink, and active sink current flows from input port to sink current, and sink current is current that flows into chip fromcircuits outside microcircuit through contacts. is that drain current is active. Yes, flow from input terminal of chip is called drain current. The drain current is passive and flow from output terminal is called drain current.

The source current is output current provided by digital circuit to drive high to load, and output low with current sinking is external input current to digital circuit. In fact, these are possibilities of input and output current. . The drain current is related to input (input flows in), while source current (output flows out) and drain current (output sinks in) are related to output.