Current detection circuit - including op amp current scheme

1. Application of current detection

Circuit detection circuits are commonly used in ： High voltage short circuit protection 、 Motor control 、DC/DC Converter 、 System power management 、 Current management of secondary battery 、 Battery management, current detection and other scenarios .

For most applications , The current is measured by the voltage drop across the sensing resistor .

Generally, the voltage drop when the current passes through is tens of mV～ Hundreds of mV The resistance value of , Number of low resistors used for current detection Ω Smaller resistance values below ; Detect dozens of A It takes several times when the current is high mΩ Minimum resistance of , therefore , Metal plate type and metal foil type low resistors which are good at small resistance value are commonly used , And the small current is through hundreds of mΩ～ Count Ω The larger resistance value of .

When measuring current , The resistor is usually placed in two places in the circuit . The first position is between the power supply and the load . This measurement method is called high side sensing . Usually, the second place to place the sensing resistance is between the load and the grounding terminal . This current sensing method is called low side current sensing .

The two measurement methods have their own advantages and disadvantages , The low side resistance increases the unwanted additional impedance in the grounding path ; Circuits with high side resistance must withstand relatively large common mode signals . One of the advantages of low side current measurement is the common mode voltage , That is, the average voltage at the measurement input is close to zero . This makes it easier to design application circuits , It is also convenient to select devices suitable for this measurement . The voltage measured by the low side current sensing circuit is close to ground , When handling very high voltages 、 Or in applications where the supply voltage may be prone to spikes or surges , This method is preferred to measure current . Because the low side current sensing can resist the interference of high voltage spikes , And can monitor the current in the high-voltage system .

2. Current detection circuit

2.1 Low side detection

The main disadvantage of low side current sensing is the use of power grounding terminal and load 、 When the system grounding terminal , The voltage drop across the sensing resistor will be different . If other circuits are based on the power ground terminal , There may be problems . To minimize this problem , All circuits with interaction shall be based on the same grounding terminal , Reducing the value of current sensing resistance helps to minimize grounding drift .

Pictured above , If the OP AMP in the figure GND Pin with RSENSE The positive end of the is the reference , Then its common mode input range must cover below zero , That is to say GND - (RSENSE × ILOAD).Rsensor Will land （GND） Separated .

2.2 High side detection

With a large number of high-precision amplifiers and precision matching resistors IC Launch , It is very convenient to use differential amplifier in high side current measurement . High side detection drives current detection IC The development of , The parameter variation caused by discrete devices is reduced 、 Problems such as too many devices , Integrated circuits are convenient for us to use . The figure below shows a high side detection IC programme ：

3. Detection circuit connection mode

Detect the voltage drop when the current passes through the resistor , A pattern for detecting voltage needs to be drawn from both ends of the resistor . The voltage detection connection is shown in the figure below (2) Shown , It is recommended to lead out from the inner center of the resistor electrode pad . This is because the copper foil pattern of the circuit substrate also has a small resistance value , It is necessary to avoid the influence of voltage drop caused by the resistance value of copper foil pattern . If you follow the figure below (1) Shown , The voltage detection pattern is led out from the side of the electrode pad , The test object will be the voltage drop of the resistance value of the low resistor plus the resistance value of the copper foil pattern , The current cannot be detected correctly .

4.PCB Layout Reference resources ：

Reference to the original ： Current detection circuit