Source (current source) Sink (current sink) detailed explanation

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Source (current source) Sink (current sink) detailed explanation [Copy link]

An important premise: sink current and pull current are for ports, and both are for IC output ports. Term explanation - Sink: inject, fill, from outside to inside, from virtual to real. When you are thirsty, have a large glass of freshly squeezed orange juice, drink it all, and you are full, this is called "sinking". For a port, if the current flows into it, it is "sinking current". For example, if an IO is connected to VCC through a resistor and an LED, when the IO output is logic 0, can the LED light up? Check the sink current parameter in the device manual. Term explanation - Pull: flow out, empty, from inside to outside, from real to virtual. I drank a large glass of fresh orange juice. After a while, I felt desperate and hurried to the bathroom. A heavy rain came and I felt relieved. This is called "sourcing current". For a port, if the current flows outward, it is "sourcing current". For example, if an IO is connected to GND through a resistor and an LED, can the LED be lit when the IO output is logic 1? Check the sourcing current parameter in the device manual. Source current and sink current 1. Concept Source current and sink current are parameters that measure the output drive capability of the circuit (note: both source current and sink current refer to the output terminal, so they are drive capability). This term is generally used in digital circuits. First of all, it should be explained that the source current and sink current in the chip manual are parameter values, which are the upper limit values (maximum value allowed) of the source current and sink current allowed by the chip in the actual circuit. The concept to be discussed below is the actual value in the circuit. Since the output of digital circuits has only two level values, high and low (0, 1), when the output is high, the output end generally provides current to the load, and the value of the current provided is called "source current"; when the output is low, the output end generally absorbs the current of the load, and the value of the absorbed current is called "injection current". For devices that input current: both sinking current and sinking current are inputs, sinking current is passive, and sinking current is active. If the external current "flows" into the chip through the chip pin, it is called sink current (being poured in); On the contrary, if the internal current "flows out" from the chip through the chip pin, it is called pull current (being pulled out) 2. Why can it measure the output drive capability When the output of the logic gate is at a low level, the current poured into the logic gate is called sink current. The larger the sink current, the higher the low level at the output. It can also be seen from the output characteristic curve of the transistor that the larger the sink current, the larger the saturation voltage drop and the larger the low level. However, the low level of the logic gate is limited to a certain extent, and it has a maximum value UOLMAX. When the logic gate is working, it is not allowed to exceed this value. The specification of the TTL logic gate stipulates that UOLMAX ≤0.4～0.5 V. Therefore, there is an upper limit to the sink current. When the output of the logic gate is at a high level, the current at the output of the logic gate flows out of the logic gate. This current is called the pull current. The larger the pull current, the lower the high level of the output. This is because The output stage transistor has an internal resistance, and the voltage drop on the internal resistance will cause the output voltage to drop. The larger the pull current, the lower the high level of the output. However, the high level of the logic gate is limited to a certain extent. It has a minimum value UOHMIN. When the logic gate is working, it is not allowed to exceed this value. The specification of TTL logic gate stipulates that UOHMIN ≥2.4V. Therefore, the pull current also has an upper limit. It can be seen that the pull current and the sink current at the output end have an upper limit, otherwise when the high level output is applied, the pull current will make the output level lower than UOHMIN; when the low level output is applied, the sink current will make the output level higher than UOLMAX. Therefore, the pull current and the sink current reflect the output drive capability. (The larger the pull and sink current parameter values of the chip, the more loads the chip can connect, because, for example, the sink current is negatively charged. The more loads there are, the greater the current that is charged.) Since the high-level input current is very small, at the microampere level, it is generally not necessary to consider it. The low-level current is larger, at the milliampere level. Therefore, there is often no problem if the low-level sink current does not exceed the standard. The fan-out coefficient is used to illustrate the ability of a logic gate to drive a similar gate. The fan-out coefficient No is the ratio of the low-level maximum output current to the low-level maximum input current. In integrated circuits, current absorption, current pull output and current sinking are very important concepts. Pulling means draining, active output current, and outputting current from the output port. Sinking means charging, passive input current, and flowing into the output port. ... Source current is the output current provided by the digital circuit to the load when it outputs a high level, and the output low level when it sinks current is the external input current to the digital circuit. They are actually the input and output current capabilities. Absorption current refers to the input end (the input end absorbs current); while source current (flows out of the output end) and sink current (the output end is poured into) are relative to the output end.

lzz8031

Thanks for your advice