Reference: Remember Cheng Electronic Design
1. Pull-up resistor
Pull up, literally means to pull up. What does "up" in digital circuits mean? Refers to logic 1, in circuit design, logic 1 represents a high level. What to pull up, of course it will not be the power supply, nor the ground, it can only be the signal.
Naturally, the concept will be obtained. Connecting a signal to a high level through a resistor is called a pull-up. The role of this resistor is a pull-up resistor.
2. Pull-down resistor
Connecting a signal to a low level (ground) through a resistor is called a pull-down. This resistor acts as a pull-down resistor.
3. Use scenarios of pull-up resistors
3.1 TTL drive CMOS
The standard TTL level V OHmax (high level output maximum value) is 2.4 V, while the CMOS V IHmin (high level input minimum value) is 3.5V. When TTL drives CMOS, when TTL outputs high level, CMOS may It cannot be effectively recognized as a high level, so a pull-up resistor needs to be added to the TTL output to increase the TTL output high level to 5V, so that CMOS can recognize it effectively.
When TTL low-level drive CMOS, TTL's V OLmin (low-level output minimum) is 0.5V, which is less than CMOS's V ILmin (low-level input maximum) 1.5V, so TTL low level can be normally detected by CMOS Recognition.
R & lt U is, the greater the ability to pull, but will increase the saturation voltage TTL end, resulting in low high TTL output. If R u is too large, it will delay the rising edge of TTL output.
3.2 OC and OD gate
Both OC and OD gate structures need to add pull-up resistors. The following figure I2C is OD structure. Both SDA and SCL signals need to add pull-up resistors. Without pull-up resistors, OC and OD cannot output high levels. .
3.3 Low-level interrupt detection
For the low-level interrupt trigger circuit, a pull-up resistor is generally added to the detection end of the MCU. When the INT low level arrives, the MCU_INT_DET will change to a low level to trigger the interrupt.
If R1 is too large, the rising edge of MCU_INT_DET will be slower. If R1 is too small, it may cause excessive sink current and cause the MCU pin to burn out.
3.4 Fixed level
For example, in the LDO circuit, when the high level is enabled, a pull-up resistor is generally added to the enable pin CE to VIN to achieve the effect of outputting when the LDO is powered on.
For R1, the SPEC of the general chip will be given, the most common ones are 10K and 100K, then you say 47K will work, of course, it depends on the sink current capability of the LDO CE pin, that is, the current that 5V is applied to R1 It needs to be less than the maximum sink current of the CE pin. If it is too large, the CE pin may be burned.
4. Use scenarios of pull-down resistors
4.1 Fixed level
For example, in the control circuit of NMOS, a pull-down resistor is generally added to the G pole, and the low level is fixed. The GS impedance of the MOS tube is very large, and it is easy to be interfered by static electricity, resulting in a higher voltage between the GS and the switching state of the MOS tube. .
For R2, the MOS tube is in the off state, and the current consumption flowing through R2 is zero. When the MOS tube is on, the current flowing through R2 is I = V CTRL /R2. If you want to reduce current consumption, increase the resistance of R2 as much as possible, generally 200K, 1M, etc.
4.2 As a discharge resistor
In some LDO circuits, a pull-down resistor of R4 is also added, which is called a dummy load. When the LDO is turned off, it is used to quickly discharge the voltage on C6, which is related to the usage scenario of the circuit. The disadvantage of adding R4 is that it will increase the current consumption of I = 3.3/R4 during normal operation. Besides, there are also LDOs with self-discharge function, with self-discharge and without self-discharge, there are advantages and disadvantages.
For R4, the smaller the resistance, the faster the discharge, but during normal operation, the increased current consumption will be greater.
