High-Side Current Sense Amplifier LT6107

0 Introduction
LT6107 is a simple, compact, multifunctional high-side current sensing amplifier launched by Linear Technology Corporation. It is a simple and easy-to-use general-purpose device with high input voltage range, high accuracy, wide operating temperature range, low offset voltage, low supply current and other characteristics. It is an MP-level device that can be applied to products such as automatic devices, industrial facilities and power management, and can meet the application requirements of automobiles, military and industry and other harsh environments. The
maximum input bias current of LT6107 is only 40nA, and the offset voltage is only 250μV. Its gain can be adjusted by two external resistors with an accuracy of more than 1%. The input voltage range is 2.7V~44V, which can adapt to the working environment of -55℃~+150℃, and the working current is only 65μA.
LT6107 has a very low detection voltage. When VSENSE=0V, the output voltage is always positive, and the static input offset voltage and the small total current flowing through the output device are only 0.7μA~1.2μA.

1 Pin Functions
Figure 1 shows the appearance of the LT6107, and its pin functions are as follows:

OUT: Current output terminal, the current is proportional to the voltage on the external detection resistor.
V-: Usually connected to GND.
-IN: The internal detection amplifier makes the voltage on the -IN terminal the same as that on the +IN terminal. Connecting a resistor RIN between the V+ and -IN terminals can set the output current IOUT, IOUT=VSENSE/RIN, where VSENSE is the detection voltage on RSENSE.
+IN: Connected to the system load terminal through a resistor.
V+: The positive voltage supply terminal of the IC, directly connected to the detection resistor, the supply current drives the IC to work through this terminal. If only the system load current is monitored, V+ can be connected to the positive terminal of the detection resistor; if all operating currents including LT6107 are monitored, V+ needs to be connected to the negative terminal of the detection resistor.

2 Basic Principles
LT6107 monitors current by detecting the voltage on the external detection resistor. The internal circuit first converts the voltage on the external detection resistor into output current, and then gives a small detection signal on a high common-mode voltage, which is used as a reference ground. Its low DC offset can monitor very small detection voltages, and only a small resistor needs to be connected in the circuit to minimize power consumption. Its internal block circuit is shown in Figure 2.

The internal current amplifier loop pulls -IN and +IN to the same potential. An external resistor RIN is connected between -IN and V+. When the potential of RIN is the same as the potential on RSENSE, the corresponding current VSENSE/RIN will flow through RIN. The high input impedance of the detection amplifier will not conduct this current, but it will flow to the IOUT terminal through the internal PNP tube. After the output current is transmitted through the OUT terminal to the resistor of V-, it is converted into a voltage signal to the IC. The voltage is: VO=V-+IOUTXROUT, and its gain value is listed in Table 1.

3 Basic Applications
LT6107 monitors current by setting an adjustable detection current. Its detection voltage is obtained by adjustable gain amplification and changes linearly from the positive power supply voltage to the reference ground. By adding an output filter, its output signal can also be used as an analog signal. Figure 3 shows a basic application circuit diagram of LT6107.

4 Hardware Design
4.1 Selection of External Current Sense Resistor
The external sense resistor RSENSE has a significant impact on the functionality of the current sense system and must be selected with care. First, the power dissipation of the resistor must be considered. The system load current causes heating and voltage loss on RSENSE, so the sense resistor should be as small as possible to measure the required dynamic range of the device. The input dynamic range is limited by the DC input offset voltage on the primary side of the LT6107, which is different at the maximum input signal and the minimum measured signal. RSENSE must also be small enough that VSENSE does not exceed the maximum input voltage of the LT6107. For example, if the maximum sense voltage is 100mV and the external peak load current is 2A, RSENSE cannot exceed 50mΩ.
Once the maximum RSENSE value is determined, the minimum RSENSE value can be determined by its accuracy or the required dynamic range, and its minimum signal is also limited by the input offset voltage. For example, if the offset voltage of LT6107 is 150μV, and the minimum current is 20mA, the detection resistor is 7.5mΩ, and its VSENSE is 150μv, that is, input offset. A large detection resistor will reduce the error caused by offset. When RSENSE is 50mΩ, its dynamic range is the largest, with a detection voltage of 100mV and an input offset load current error of 3mA at peak load (2A), and its power consumption is 200mW; when the detection resistor is 5mΩ, its effective error is 30mA, and the peak detection voltage (2A load) is 10mV at this time, and the power consumption is only 20mW.
When the offset voltage of LT6107 is a typical value: 150μV, it can provide a dynamic range of 60dB for a maximum detection voltage of 150 mV, and a dynamic range of more than 70dB for a maximum detection voltage of 0.5V. This allows the LT6107 with low offset and relatively large dynamic range characteristics to adapt to a wider range of applications.
To reduce power consumption, the sense resistor between -IN and +IN needs to use the Kelvin connection method. When large currents flow, solder connections and PC board interconnections can cause large measurement errors. A 10mmx 10mm square track has an error of about 0.5mΩ, and an error of 1mV is equivalent to a 2A current flowing through it. Therefore, the sense line needs to be isolated from the high current path to reduce the error. Using a Kelvin connection with an integrated sense resistor is also an effective method. Figure 4 is a recommended connection method.

4.2 Selection of external input resistor RIN
When the output current required by the system is 1mA, a suitable RIN must be selected. The maximum value of RIN is 500Ω. RIN depends on the maximum detection voltage corresponding to IOUT=1mA, from which the maximum output dynamic range can be derived. The output dynamic range depends on the maximum allowable output current and the maximum allowable output voltage, which can be used as the minimum actual output signal. If this value is lower than the required dynamic range, the maximum output current and power consumption can be reduced by increasing RIN. If the system has a very wide dynamic range and requires a small detection current, a small RIN can bring a larger current, and the circuit also needs another connection method, see Figure 5. Using a Schottky diode in parallel with RSENSE will reduce the accuracy of large current devices, thereby increasing the accuracy of small currents.

It is worth noting that when designing RIN, especially with a small RIN value, all series resistors on the PCB will increase the value of RIN, causing greater errors.
4.3 Selection of external output resistor ROUT
When selecting the output resistor, the maximum output voltage must be considered first. If there is an input limit in the subsequent circuit, such as a buffer or ADC, ROUT must satisfy IOUT(MAX)·ROUT below the maximum input range of the circuit.
In addition, the output impedance is determined by ROUT. If the driven circuit has sufficient input impedance, the output impedance is not limited. If the input impedance of the driven circuit is low, or the ADC has current spikes, ROUT needs to be low to ensure its output accuracy. For example, if the input impedance is 100 times that of ROUT, the accuracy of VOUT will be reduced by 1%, as follows:

4.4 Error considerations
1) Error sources
The current detection system uses an amplifier and several resistors to adjust the gain and level shift. The output depends on the characteristics of the amplifier, the gain and input offset are adjusted by the resistors, and the circuit output is:

The error in this case comes only from the mismatch of the resistors, and there is only an error in the gain. Of course, offset voltage and bias current will also cause other errors.
2) Output error
The output error caused by the amplifier DC offset voltage VOS is:

Bias current flows into the positive input of the internal op amp and flows into the negative input of the op amp. The output error caused by the bias current and is:

For example, when IBIAS is 60nA, RIN is 1K, and the input reference error is 60μV. Note that when RSENSE=RIN, a voltage offset will be generated on RSENSE to eliminate the error caused by, and EOUT(IBIAS)=0mV. In most applications, RSENSE< , and its bias current error will be minimized, see the connection method in Figure 6.

If the offset current IOS of the LT6107 amplifier is 6nA, the 60μV error will be reduced to 6μV.
The above mentioned will maximize the dynamic range of the circuit. If you want to simplify the design, you can also ignore it .
3) Output error caused by gain difference
When the output current of LT6107 is 1mA, its typical gain error is 0.25%. The gain error mainly comes from the limited gain of the PNP output transistor, which makes it impossible to see a small ratio current on the output load ROUT like on RIN.

4.5 Power consumption considerations
The power consumption of LT6107 will increase the chip temperature. The junction temperature can be used to reflect the power supply current and output current flowing through the device.


When the operating voltage reaches a maximum of 36V and the maximum output current reaches 1mA, the total power consumption is 41mW, and its junction temperature rises by 10℃ compared to the ambient temperature.
Note that there are many factors that affect the output. The LT6107 must have at least 1mA output and must also limit the maximum output current. Therefore, the selected detection resistor and must be appropriate. Once the input fails, the external will be clamped.
4.6 Output filtering
To simplify the filter design, the output voltage VOUT is simplified to IOUTXZOUT here, so only the appropriate ZOUT is needed to design the required filter for any circuit. For example, a capacitor in parallel with ROUT can produce a low-pass response, reduce unnecessary output noise, and provide a stable output for charging equipment to drive switching circuits such as MUX or ADC. Its output capacitor is connected in parallel with the output resistor, which will generate f-3dB at the output response point, that is:

4.7 The power supply connection
V+ terminal should be connected to one side of the detection resistor and meet the following conditions: +IN≤V+ and -IN≤V+. Under normal operating conditions, VSENSE must not exceed 500mV. In addition, V+-(+IN)≤500mV must be considered. Refer to Figure 7. Its feedback will make the voltage at the -IN and +IN terminals equal to VS-VSENSE.
In actual applications, LT6107 cannot be reversed. In order to prevent damage to the device, a Schottky diode can be connected in series in the loop, or a resistor can be connected in series at the output to protect the output. The circuits are shown in Figures 8 and 9 respectively.