Features

Suitable for automotive applications

AEC-Q100 qualified with the following results: – Device Temperature Class 1: –40°C to +125 °C Ambient Operating Temperature Range – Device HBM ESD Class 2 – Device CDM ESD Class C6

Pins optimized for external filtering

Wide Common Mode Range: –16 V to +80 V 8226 ; Accuracy: – Common Mode Rejection Ratio: 120 dB – ±2.5-mV Offset (max) – ±1% Gain Error (max) – 20-µV/°C Offset Drift (max) –55 ppm/°C Gain Drift (max)

Bandwidth: up to 130 kHz

Two gain options are available: –14V/V (INA270A-Q1) –20V/V (INA271A-Q1)

Quiescent Current: 900 μA (max)

Power: 2.7 V to 18 V Package: SOIC-8

application

Electric Power Steering (EPS) System

body control module

Braking System

Electronic Stability Control (ESC) system

illustrate

The INA270A-Q1 and INA271A-Q1 (INA27xA-Q1) family of current shunt monitors with voltage outputs sense the voltage drop across the current shunt common-mode voltage of -16 V to +80 V, independent of supply voltage. The INA27xA-Q1 pins are easy to implement filtering. The INA27xA-Q1 devices have two output voltage levels: 14 V/V and 20 V/V. The 130-kHz bandwidth simplifies the use of current control loops. The INA27xA-Q1 operates from a single 2.7V to 18V-V supply with a maximum 900µA supply current. They are available in SOIC-8 packages over an extended operating temperature range of -40°C to +125°C.

Detailed description

Overview

The INA27xA-Q1 is a family of voltage output current sense amplifiers. The INA27xA-Q1 operates over a wide common-mode voltage range (–16 V to +80 V). This package shows the output of the preamp stage (PRE-OUT) and the input of the output buffer stage (BUF-IN). This pin is easily filtered, see Second Order Filtering.

Feature description

OUTPUT VOLTAGE RANGE The output of the INA27xA-Q1 is accurate within the output voltage swing set by the power supply pins. Device Functional Mode First or Second Order Filtering The INA27xA-Q1 devices easily implement filtering between the preamplifier output and the buffer input. Due to the 96-kΩ output impedance pre-loaded on pin 3 (see Figure 15a). The INA27xA-Q1 devices are easily configured with a second-order Sallen key (see Figure 15b). When designing these configurations, consideration should be given to the pre-output 96-kΩ output impedance with an initial value added – ±30% variation at a 2200 ppm/°C temperature coefficient.

The INA27xA-Q1 can be easily connected for first or second order filtering. Remember to use the appropriate design buffer gain for all key configurations (INA270A-Q1=1.4, INA271A-Q1=2).

application information

The INA27xA-Q1 measures the voltage developed through the current sense resistor when current is passed through it. There is also a filtering function to remove unwanted transients and smooth the output voltage.

basic connection

Figure 16 illustrates the basic connections of the INA27xA-Q1. The input pins, IN+ and IN-, should be connected as close as possible to the shunt resistor to minimize any resistance in series with the shunt resistor. Power supply bypass capacitors need to be stable. Applications with noisy or high impedance power supplies may require additional decoupling capacitors to suppress power supply noise. Minimum bypass capacitors with values of 0.01µF and 0.1µF should be placed near the power supply pins. While not mandatory, an extra 10-µF electrolytic capacitors placed in parallel with other bypass capacitors can be used for particularly noisy supplies.

select RS

The value of the shunt resistor, RS, is chosen depending on the application and is a compromise between small-signal accuracy and the maximum allowable voltage loss in the measurement line. The higher the RS value, the better the accuracy at lower currents by minimizing the effect of offset, while the lower value of RS enables the supply line. For most applications, optimum performance is achieved by providing a full-scale shunt with RS values ranging from 50 mV to 100 mV. The maximum input voltage for accurate measurement is (VS – 0.2)/gain.

Variation in Accuracy Due to VSENSE and Common-Mode Voltage The accuracy of the INA27xA-Q1 current shunt is a function of two main variables: VSENSE (VIN+–VIN-) The common-mode voltage VCM is expressed relative to the supply voltage VCM as (VIN++VIN-) /2. In practice, however, VCM is seen as the voltage at VIN+ because the voltage drop across VSENSE is usually small. This section discusses the accuracy of these specific areas of operation:

Normal case 1: VSENSE≥20 mV, VCM≥VS

Normal case 2: VSENSE ≥ 20 mV, VCM

Low VSENSE case 1: VSENSE<20 mV, –16 V≤VCM<0

Low VSENSE case 2: VSENSE<20 mV, 0 V≤VCM≤VS

Low Voltage Sensing Case 3: Voltage Sensing < 20 mV, Voltage Sensing

Normal Case 1: VSENSE ≥ 20 mV, VCM ≥ VS This operating region provides the highest accuracy. Here, the input offset voltage is characterized by a two-step method. First, the gain is determined by Equation 1.

VOUT1=output voltage, VSENSE=100 mV

VOUT2 = output voltage, VSENSE = 20 mV (1)

The offset voltage is then measured at VSENSE = 100 mV and referenced to the input (RTI) monitor of the current shunt, as shown in Equation 2.

In typical characterization, the output error versus common-mode voltage curve shows the operation of this region. In this figure, VS = 12v; for VCM ≥ 12v, the output error is minimal. the case is

Also used in electrical characteristics to create an output specification of VSENSE ≥ 20 mV. Low VSENSE Case 1: VSENSE<20 mV, –16 V≤VCM<0; and Low Voltage Sensing Case 3: Voltage Sensing <20 mV, Voltage Sensing

Low VSENSE case 2: VSENSE < 20 mV, 0 V≤VCM≤VS For the INA27xA-Q1 family, this surgical field has the lowest accuracy. To achieve a wide input common-mode voltage range, these devices employ two parallel operational amplifier (op amp) front ends. One op amp front end operates over the positive input common-mode voltage range and the other operates over the negative input range. In this case, neither of the two internal amplifiers dominate and the overall loop gain is very low. In this range region, VOUT approaches the linear operating level where the voltage is close to normal case 2. In this region, the closer to 0 V, the greater the deviation from linear operation VSENSE is closer to 20 mV, and the device operates closer to normal as described in 2. Figure 18 illustrates this behavior of the INA271A-Q1. The maximum VOUT peak in this case is achieved by keeping VS constant, setting VSENSE = 0 mV, and sweeping VCM from 0 V to VS. In this case it varies from person to person. For the INA270A-Q1, the maximum peak voltage is 0.28V; for the INA271A-Q1, the maximum peak voltage is 0.4v.

Transient protection

The -16-V to 80-V common-mode range of the INA27xA-Q1 is ideal for withstanding automotive fault conditions from 12-volt battery inversion to 80-volt transients, as no additional protection components are required to reach these levels. External transient absorption by semiconductor transient absorbers (Zeners or Transzorbs) is necessary if the INA27xA-Q1 device is exposed to more than its rating at the input.

MOVs or VDRs are not recommended unless they are absorbers used outside of semiconductor transients. Select the transient absorber so that it does not allow the INA27xA-Q1 to be exposed to transients greater than 80 V (ie, taking into account transient absorber tolerances, and dynamic impedance due to transient absorption). Despite the use of internal Zener-type ESD protection, the INA27xA-Q1 devices are not suitable for using external resistors in series with the input because the internal gain resistors can vary by ±30%, but the internal resistors are closely matched. If gain accuracy is not critical, the INA27xA-Q1 can be used in series with resistor inputs, each with two equal resistors.

Design requirements

In this application, the device was configured to measure and filter triangular periodic currents at 10 kHz. This average current through the shunt is the required information. This current can be a solenoid valve current or an inductor current through which a current pulse is passed. The size of the capacitor is chosen based on the lowest frequency component to be filtered out. The frequency at which the semaphore is filtered depends on this cutoff frequency. Judging from the cut-off frequency, the degree of attention is 20db/decade.

Detailed design procedure

Without this filtering capability, an input filter must be used. The error also comes into play when adding series resistance to the input, because the resistance has to be large to produce a lower cutoff frequency. Take the average value of the 10kHz signal by using a 10nF capacitor for the unipolar filter capacitor. The cutoff frequency is set by a capacitor to a frequency of 166 Hz. This frequency is well below the cycle frequency, and the output ripple and average current are easy to measure.

Typical Applications (continued)

Apply Curve

Figure 20 shows the output waveform without filtering. The output signal tracks the input signal ripples. If this current is sampled by an ADC, many samples must be taken to average the current digitally. This process requires additional time for sampling and averaging, and is time-consuming, so there is no need for this application. Figure 21 shows the filtered output waveform. The output signal is filtered and the average value is easily measured with small ripples. If this current is sampled by an ADC, then only a few samples need to be averaged. Numerical averaging is now not required and the time required is greatly reduced.

Power Recommendations

The input circuit of the INA27xA-Q1 can accurately measure voltages that exceed its supply voltage. For example, the VS supply can be 5v, while the load supply voltage can be as high as 80v. Output However, the voltage range at the output is limited by the voltage on the power supply pins.

shutdown

The INA27xA-Q1 devices do not provide a shutdown pin; however, since they consume less than 1 mA of quiescent current, they can be powered by the output of a logic gate or a transistor switch. A low gate drive turns off the INA27xA-Q1. Use a totem-pole output buffer or gate with adequate drive and a 0.1-µF bypass capacitor, preferably a ceramic with good high frequency characteristics. The supply voltage for this gate should be 3V or higher because the INA27xA-Q1 requires a minimum supply greater than 2.7V. In addition to eliminating quiescent current, this gate also turns off the 10µA bias current present at each input.

Layout Guidelines

Connect the input pin to the sense resistor using a Kelvin or 4-wire connection. This connection technique ensures that only the current sense resistor impedance is detected between the input pins. Misrouting current sense resistors often results in additional resistance between the input pins. Given the extremely low ohmic value of the current resistor, any additional high current-carrying impedance can lead to significant measurement errors.

Place power supply bypass capacitors as close to the power and ground pins as possible. The recommended value for this bypass capacitor is 0.1µF. Additional decoupling capacitors can be added to compensate for noise or high impedance power supplies.

RFI and EMI

It is recommended to pay attention to good layout practices. Keep traces short and, if possible, use a printed circuit board (PCB) ground plane with surface mount components as close to the device pins as possible. Small ceramic capacitors placed directly at the amplifier input can reduce susceptibility to RFI and EMI. PCB Layout Amplifiers should be kept as far away as possible from sources of RF interference. The source can include the same systems as the amplifier itself, such as inductors (especially switched inductors that handle large amounts of current at high frequencies). RFI can often be identified as changes in offset voltage or DC signal levels that interfere with changes in the RF signal. Shielding may be required if the amplifier cannot be kept away from the radiation source. Twisted wire input leads make them more resistant to RF fields. Differences in the input pin placement of the INA27xA-Q1 relative to the INA193 to INA198 can provide different EMI performance.