ISL28006, ISL28133, ISL28134, ISL28191 High-Side, High Current Sensing Techniques
There is a need in many applications to sense currents on the high-side rail of a power bus and translate it into a voltage with respect to ground, which is proportional to this current (see Figure 1). Typically, this voltage is fed into a microcontroller or used as an analog input to a switching power controller. This application note compares several methods of high-side current sensing, including two simple techniques that can be used to sense current on a high voltage rail using low voltage amplifiers.
There are various ways to sense high-side current, but no single solution. The ISL28006 (see Figure 2), is a very compact solution for applications up to 28V. The fixed gain version requires almost no external components and the 450μV max offset over-temperature is excellent compared to similar products; however, some applications require lower offset for better precision or require sensing of a rail higher than 28V.
A high voltage op amp can be used in a standard differential amp configuration, but this circuit requires extremely tight resistor matching when amplifying low voltages on high voltage rails. For example, if sensing 10mV on a 5V rail using 1% resistors, amplifying by 50 and assuming a zero offset in the amp, the worst case error is ~37%. Figure 3 shows this numeric example of the worst case tolerance. The voltage at the non-inverting terminal of the amplifier is calculated to be 4.9137V based on the worst case values of 0.99kΩ and 50.5kΩ for the resistor divider and the voltage at the inverting terminal is the same by virtual null. The current in the 1.01kΩ resistor is (5V minus 4.9137V)/1.01kΩ = 85.4μA. The output is 4.9137V minus 85.4μA*49.5kΩ = 683mV. This is 37% higher than the nominal value of 50*10mV = 500mV that one would expect with 50kΩ and 1kΩ resistors.
One important item of note: The differential amp configuration in Figure 2 on page 1 will require the use of an amp with a voltage rating on the order of the high rail being sensed. There is much better availability of low offset op amps which operate from a 5V nominal supply rail than amps which operate at higher supply rails. Best in class 5V amps have μVs of offset. Best in class high voltage (30~40V) amps have 10μV to 100μV offset.
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Application note & Design Guide |
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Please see the document for details |
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English Chinese Chinese and English Japanese |
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May 1, 2013 |
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Rev.0.00 |
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AN1827I |
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455 KB |
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