Allegro Microsystems Integrated Conductor Current Sensor CMTI Comparison

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CMTI Overview

Common-Mode Transient Immunity (CMTI) testing injects a high slew rate transient voltage across the isolation barrier of the current sensor. During the transient event, the output stability of the device is observed. The output signal is perturbed due to capacitive coupling across the isolation barrier between the lead frame and the signal pin. This test is important in fast switching applications with high frequency transients and indicates how well Allegro devices can withstand such events. Allegro integrates an on-chip shield that protects the circuitry by isolating transient disturbances from the IC circuitry, reducing the effects of common-mode transients. See Figure 1.

Figure 1: Diagram of the shield in the Allegro integrated current sensor package

Figure 1: Diagram of the shield for the Allegro integrated current sensor package. Courtesy of Allegro MicroSystems.

CMTI Test Methodology

For the Allegro Integrated Current Sensor, a transient voltage is applied from the input current pin (IP) to the GND of the device and VOUT is monitored. Under the recommended load conditions (listed in the Typical Application Circuits section), 1000V pulses of various slew rates were injected across the isolation barrier into the Allegro Integrated Current Sensor. The slew rate is the 1000V step divided by the rise time of the transient (V/ns).

The High Performance Parallel Interface (HPPI) Pulse Generator is a charge-cable type pulser. It charges an external cable to the desired voltage and then releases the charge on the cable to the output under program control. An internal power supply determines the pulse voltage and the length of the cable determines the pulse width. The system has a 50 ohm output impedance and the control software is written to reflect the output voltage into a larger impedance.

The output can be presented with a variety of impedances, producing different voltage and current combinations. Discharging into an effective short circuit is useful for surge testing, discharging into a 50 Ω load is preferable if the signal needs to be attenuated for reading back, or discharging into a high impedance can be used to maximize the applied voltage. Other combinations can also be used and analyzed with simple Ohm's Law calculations.

Settings [V]	Impedance [Ω]	Output voltage [V]	Output current [A]
1000	>1 Meg	1000	~0
1000	50	500	10
1000	<1	~0	20

Table 1: Pulse generator settings summary

As shown in Figure 2, the connection of an Allegro current sensor to an oscilloscope is illustrated. This allows the oscilloscope to be directly connected to the pulse. The primary attenuator feeds the oscilloscope and allows the power to dissipate. The subsequent attenuator and terminator can handle the residual power with standard BNC components. This setup can only be used for high impedance device under test (DUT) connections.

Figure 2: Pulser connections to the device and oscilloscope

Figure 2: Connecting the pulser to the device and oscilloscope. *Courtesy of Allegro MicroSystems.

The output of the DUT is monitored through an optical probe as shown in Figure 3. This allows the output to be monitored pseudo-differentially. The output monitor can be connected along the high or ground side of the pulser voltage. If the high side of the CMTI pulse is connected to the output side of the DUT board, the power source must be unreferenced, such as a battery. If the pulse is applied to the current loop side of the sensor, a bench power supply can be used to power the DUT. When testing non-sensor devices, care should be taken to provide battery power where appropriate.

Figure 3: DUT output monitor connection *Provided by Allegro MicroSystems

CMTI results

CMTI introduces a disturbance in the output that can be quantified in two ways.
　1: Output voltage deviation
　2: Output convergence time

See Figure 4. Voltage deviation is the maximum voltage overshoot or undershoot at the device output after the injection of a defined transient pulse. Convergence time is the time it takes for the device output to converge to within <100mV of its initial value.

The oscilloscope images included with this article show the voltage disturbances for positive and negative 1000V injection pulses. The slew rates used for the positive pulses are [200, 142, 108, 76, 39] ns. The slew rates used for the negative pulses are [–215, –155, –123, –86, –46] ns.

The output waveforms during and after the transient disturbance are shown at the bottom of the page.

One way to tabulate the CMTI waveforms is by the minimum slew rate that will result in an output with a particular voltage disturbance that will converge within a particular time. This method is common in the industry. Typical and minimum slew rates for convergence to within 100mV within 0.3µs are shown in Table 1. One way to think of the minimum slew rate is that a transient with a slew rate less than the minimum slew rate will cause a disturbance of less than 100mV at the output and have a convergence time of less than 0.3µs.

device	package	Minimum (V/ns)	Typical value (V/ns)
ACS733	LA	100	200
ACS733	MA	100	150
ACS37002	MA	75	115
ACS37002	MC	75	115
ACS37010	LZ	100	150
CT432/3	SOICW-16	100	200
CT4327/8	SOICW-8	100	200

Table 2: Typical and maximum slew rates

Another way to look at the CMTI waveforms is to consider the time it takes to converge to the original value within a particular voltage range. A 1000V pulse was injected into each device with a slew rate of 140V/ns, and the times to converge to within 100mV and 200mV are shown in Table 3.

device	package	Time to <100mV [ns]	Time to <200mV [ns]
ACS733	LA	150	125
ACS733	MA	300	250
ACS37002	MA	50	Never exceeded
ACS37002	MC	50	Never exceeded
ACS37010	LZ	150	125
CT432/3	SOICW-16	500	400
CT4327/8	SOICW-8	30	50