From: http://forum.eepw.com.cn/thread/207297/1drawing current and filling current
1. Concepts
Pulling current and filling current are parameters used to measure the output driving capacity of the circuit (Note: pulling and filling are for the output end, so they are driving capacity). This is generally used in digital circuits.
First of all, it should be noted that the pulling and filling current in the chip manual is a parameter value, which is the upper limit (maximum allowed) of the chip that allows the pulling and filling current at the output end in the actual circuit ). The concept below is the actual value in the circuit.
Because the output of a digital circuit has only two levels: High and Low (0, 1). When the output is high, the output end usually provides the current to the load. The value of the current provided is called the "pull current "; low-level output is generally the current at the output end to absorb the load. The value of the current absorption is called "inbound (inbound) Current ".
For the input current device, both the input current and the absorption current are input, the input current is passive, and the absorption current is active. If the external current flows into the chip through the Chip Pin, it is called the filling current (injected );
Otherwise, if the internal current is 'output' from the chip via the Chip Pin, it is called the pulling current (pulled out)
2. Why can the output drive capability be measured?
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.
However, the low level of a logic gate is limited, and it has a maximum uolmax. 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. Therefore, there is an upper limit on the filling current.
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 at the output end.
However, the high level 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. Therefore, there is an upper limit on the pulling current.
It can be seen that there is an upper limit on the pulling current and filling current at the output end. Otherwise, the pulling current will make the output level lower than uohmin when the output is low. When the output is low, the filling current will make the output level higher than uolmax. Therefore, the pulling current and filling current reflect the output driving capability. (The larger the parameter value of the chip's pulling and filling current means that the chip can receive more load, because, for example, the filling current is loaded, the more load, the larger the filled current)
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. The fan-out coefficient is used to describe the logic gate's ability to drive similar doors. The 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.
In an integrated circuit, the concept of current absorption, current pulling output, and current filling output is very important. Active output current is the output current from the output port. Fill is filled, passive input current, from the output port inbound suction is active inhalation current, is from the input port inbound
The current absorption and filling are the current flowing from the chip's external circuit to the chip through the pins. The difference is that the current absorption is active, and the current absorption is called from the chip input end. The inbound current is passive, and the inbound current from the output end is called the inbound current.
The pulling current is the output current provided by the digital circuit output to the load. The output low level when the current is filled is the input current from the external to the digital circuit, which is actually the input and output current capabilities.
The absorption current is for the input end (input end suction), while the pulling current (output end outflow) and filling current (output end is filled in) are relative to the output end.
