What is pulling current and filling current?

When the output end of a logic gate is low, the current of the logic gate is called the filling current. The larger the filling current, the higher the low level of the output end. The output characteristic curve of the transistor shows that the larger the filling current, the larger the saturation pressure drop, and the larger the low level. The low level of a logic gate is limited. It has a maximum uolmax value. When a logic gate is working, this value cannot be exceeded. The TTL logic gate specification stipulates that uolmax is less than or equal to 0.4 ~ 0.5 V.
When the logic gate output end is high, the current at the logic gate output end flows out of the logic gate. This current is called the pulling current. The larger the pulling current, the lower the high level at the output end. This is because the output transistor has internal resistance, and the voltage drop on the internal resistance will reduce the output voltage. The larger the pulling current, the lower the high level. The height of a logic gate is limited. It has a minimum value of uohmin. When a logic gate is working, it cannot exceed this value. The TTL logic gate specification stipulates that uohmin is greater than or equal to 2.4 v.
Because the High-level input current is very small, it is generally not necessary to consider in the microsecurity level, the low-level current is large, in the milliampere level. Therefore, low-level irrigation current does not exceed the threshold, so there is no problem. The fan-out coefficient is used to describe the logic gate's ability to access similar doors. Fan-out coefficient No is the ratio of the maximum output current of the low level to the maximum input current of the low level.
For standard TTL gates, no ≥ 10; for low-power-consumption TTL gates, no ≥ 20.
In fact, it is more appropriate to use the filled current called "filled current ".
That is, the low-level drive capability. However, our predecessors call it "filling current". This is a poor relationship between "being" and it is always dizzy, with only a dead end, understanding is easy to confuse.
To be a real person: to say the fan exit coefficient, you have to find a standard, that is, the current required by a door of "fan.
The driving capability of high and low levels may be different, so there are two fan-out coefficients,
In fact, some materials only show the smaller of the two (the same is true for actual calculation ).
In addition, the input current of the actual input circuit is different, and the input current is large and small. Therefore, during calculation,
You have to use the original fan-out coefficient and the "fan-motion" standard to calculate the current, and then compare whether it can be driven.
Rather difficult, it is better to directly calculate the high and low-level drive current? Whether it is the driver IC,
Resistance, led or something, it's easy to calculate. All are happy.

 

An important premise: fill current and pull current are for the port.

Glossary-filling: injection, filling, from the inside out, from the virtual reality. Thirsty, come to a large cup of freshly squeezed orange juice, drink and full, this is called "irrigation ".

Sink current: for a port, if the current direction is internal to it, it is "inbound current". For example, an IO is connected to VCC through a resistor and an LED, when the IO output is logical 0, the LED cannot be lit. Check the sink current parameter in the device manual.

Glossary-pull: outflow, emptying, from the inside out, from the real and virtual. A large cup of fresh orange juice was drunk. After a while, I was panic and hurry to find the bathroom. There was a "Heavy Rain" and it was quite comfortable. This was called "pull ".

Sourcing current: for a port, if the current direction is flowing outside of it, it is "pulling current". For example, an IO is connected to Gnd through a resistor and an LED, when the IO output is logical 1, the LED cannot be lit. Check the sourcing current parameter in the device manual.

When using a digital integrated circuit, pulling current output and filling current output are very important concepts. For example, when using the reverser for output display, figure 1 shows pulling current, that is to say, when the output end is high, it meets the requirements of the forward connection of the light emitting diode, however, this type of pulling current output is unreasonable for the back-end device to output only a few milliamps of current. (the normal operating current of the light-emitting diode is 5 ~ 10mA ).