AMR TECHNOLOGY POWER UP / DOWNAnisotropic magnetoresistance (AMR) makes use of a common An under-voltage lockout circuit monitors the voltage on the material, Permalloy, to act as a magnetometer. Permalloy is an VCC pin. If the VCC voltage is less than the under-voltage alloy containing roughly 80% nickel and 20% iron. The alloy’s threshold the MCR1101 is in an inactive state. Vout and Vcap resistance depends on the angle between the magnetization both drive to ground. If the VCC voltage exceeds the under- and the direction of current flow. In a magnetic field, voltage threshold Vout and Vcap are released and will drive to magnetization rotates toward the direction of the magnetic field approximately half the VCC supply voltage and an initial and the rotation angle depends on the external field’s magnitude. calibration will commence. Once the initial calibration has Permal oy’s resistance decreases as the direction of completed the MCR1101 becomes active. Vout will slew to magnetization rotates away from the direction in which current indicate the value of current flowing in the IP+/- conductor. flows, and is lowest when the magnetization is perpendicular to Current flow in the IP+/- conductor with a VCC voltage less than the direction of current flow. The resistance changes roughly as the under-voltage threshold will not cause damage to the the square of the cosine of the angle between the magnetization sensor. and the direction of current flow. Permalloy is deposited on a silicon wafer and patterned as a resistive strip. The film’s properties cause it to change resistance in the presence of a
OVERCURRENT DETECTION (OCD)magnetic field. In a current sensor application, two of these resistors are connected in a Wheatstone bridge configuration to The MCR1101 have fast and accurate overcurrent fault permit the measurement of the magnitude of the magnetic field detection circuitry. The overcurrent fault threshold (I FAULTB ) is produced by the current.
user-configurable via an external resistor divider and supports a
range of 120% to 200% of the full-scale primary input (IP). AMR properties are well behaved when the film’s magnetic The overcurrent fault threshold (I FAULTB ) is set via a resistor domains are aligned in the same direction. This configuration divider from VCC to ground on the VOC pin. The voltage on the ensures high sensitivity, good repeatability, and minimal VOC pin (VVOC), may range from 0 ×VCC to 0.5 ×VCC. hysteresis. During fabrication, the film is deposited in a strong magnetic field that sets the preferred orientation, or “easy” axis, For +/-5A parts of the magnetization vector in the Permalloy resistors. AMR has For VVOC between 0 ×VCC and 0.225 ×VCC the I FAULTB better sensitivity than other methods and reasonably good threshold level is 1.2×IP. temperature stability. The AMR sensor has sensitivity which is For VVOC between 0.225 ×VCC and 0.35 ×VCC the I FAULTB approximately a linear function of temperature. threshold level is 1.5×IP.
For VVOC between 0.35 ×VCC and 0.5×VCC the I FAULTB
FUNCTIONAL DESCRIPTIONthreshold level is 2×IP. Figure 2 provide block diagrams of the ratiometric gain. The For +/-20A parts AMR sensor monitors the magnetic field generated by the For VVOC between 0 ×VCC and 0.225 ×VCC the I FAULTB current flowing through the U shaped IP+/IP- package lead threshold level is 1.2×IP. frame. The AMR sensor produces a voltage proportional to the For V FAULTB magnetic field created by the positive or negative current in the VOC between 0.225 ×VCC and 0.5 ×VCC the I IP+/IP- current loop while rejecting external magnetic threshold level is 1.5×IP. interference. The sensor voltage is fed into a differential amplifier whose gain is temperature compensated. This is For +/-50A parts followed by an instrumentation amplifier output stage that For VVOC between 0 ×VCC and 0.5 ×VCC the I FAULTB threshold provides a voltage that indicates the current passing through the level is 1.2×IP. IP+/IP- pins. To provide both positive and negative current data the Vout output pin is referenced to the ground. With no current If the input current exceeds the OCD threshold value I FAULTB flowing in the IP+/IP- pins, the voltage on the Vout output will the output pin FAULTB will transition low and stay low, even if typically equal the voltage of VCC/2. Positive IP+/IP- current input current drops below the threshold. In order to reset the causes the voltage on Vout to increase relative to VCC/2 while negative IP+/IP- current will cause it to decrease. FAULTB output the user needs to bring VOC pin to VCC and hold it there for at least THvoc. Once the OCD function is reset
the VOC voltage should return back to its normal operating voltage Vvoc. A switch SW1 on Figure 1 can be used for this.
GAINOther methods are available as well. The sensor resistors are biased to the VCC supply voltage and produce a differential voltage that is ratiometric to VCC. This If OCD function is used, an OCD reset must be applied to the configuration is suited to applications where the A-to-D or other VOC pin after system power up, to put the OCD function and circuitry receiving the current sensor output signals are biased FAULTB pin in a known state. by and ratiometric to the same supply voltage as the current sensor. The ratiometric configuration provides increased gain and resolution compared to fixed gain. The user can also provide a well-regulated 3.3V supply or monitor the VCC voltage The FAULTB output is active low open drain. A pull-up resistor and factor it into the current measurement to take advantage of should be connected between FAULTB and VCC. The VCC the ratiometric configuration. voltage will determine the high level of FAULTB signal. The Vout pin drives to 90% of VCC at full positive current and FAULTB low output voltage is below 200mV. The value of pull- 10% of VCC at full negative current. up resistor is 2-10kOhm. Phone: 978.965.3200 Fax: 978.965.3201 E-mail:
[email protected] www.aceinna.com Document: 6020-1103-01 Rev C Page 10 of 13
