SBOA536 December   2021 INA240

 

  1.   Trademarks
  2. 1Introduction
  3. 2Grounding in DC Circuits
  4. 3Grounding in Isolated Current Sensing Applications
  5. 4Working Principle of Non-isolated Current Sense Amplifiers
    1. 4.1 Single or Multi-stage Difference Amplifier
    2. 4.2 Current Feedback
    3. 4.3 Switched Capacitor
    4. 4.4 Input Stage and Input Bias Current
  6. 5Grounding in Non-isolated Current-Sensing Applications
  7. 6Level Shifting for High-Voltage Current-Sensing Applications
  8. 7Grounding in Motor Current-Sensing Applications
    1. 7.1 Common-Mode Voltage of Motor Current Sense Amplifiers
    2. 7.2 Directionality of Motor Current-Sense Amplifiers
    3. 7.3 PCB Design for High-Performance Motor Drive
  9. 8Summary
  10. 9References

6 Level Shifting for High-Voltage Current-Sensing Applications

Voltage level shifting is a technique sometimes used to adopt a CSA even though the input common-mode voltage exceeds its specified range. This technique essentially floats the CSA relative to the common-mode voltage. Voltage level shifting takes advantage of the common-mode voltage source and uses it as one of the supply rails (that is, ground or supply) of the CSA depending on its polarity. The other corresponding supply rail is generated by an external circuitry. As a result, the CSA is operating in an environment that is voltage compliant.

Figure 6-1 –48-V Power-Sensing Reference Design

Figure 6-1 shows a –48V power sensing reference design for telecom applications. This design can accurately measure current, voltage, and power on a –48-V bus. The INA226 device is selected to monitor the shunt voltage and the bus supply voltage. This information, together with calculated power is retrieved through the I2C interface. Because the INA226 is limited to a common-mode input voltage range of 0 V to 36 V, it cannot be used directly in the –48-V application. In this design the –48 V is used as the ground rail of INA226. A simple shunt regulator employing a Zener diode provides the necessary supply voltage relative to the –48-V ground. A voltage divider consisting of resistors R1 and R2 presents to the Vbus pin a scaled version of the –48 V. Doing so helps to limit the input voltage to within specified range.

The ISO1541 is a low-power, bidirectional I2C compatible bus isolator. The ISO1541 provides the necessary isolation between the high-voltage side and the low-voltage side.

Figure 6-2 shows a similar technique used to design an accurate current sensing solution for common-mode voltage of up to 400 V, which is limited by the PNP transistor breakdown voltage. Similar to the –48-V example, the 400 V is used as one of the power rails. Although instead of ground, it is used as the power-supply rail of the INA138. Similarly, a Zener shunt regulator is used to generate the device ground for INA138, creating the necessary working voltage for it.
Figure 6-2 400-V Current Sensing