INA20x オープン・ドレイン・コンパレータおよび基準電圧搭載のハイサイド測定、電流シャント・モニタ

7.3.1 Basic Connections

Figure 26 shows the basic connections of the INA20x devices. The input pins (V_IN+ and V_IN–) must be connected as closely as possible with Kelvin connections to the shunt resistor to minimize any resistance in series with the shunt resistance.

Power-supply bypass capacitors are required for stability. Applications with noisy or high-impedance power supplies may require additional decoupling capacitors to reject power-supply noise. Connect bypass capacitors close to the device pins.

Figure 26. INA200 Basic Connections

7.3.2 Selecting R_S

The selected value for the shunt resistor, R_S, depends on the application and is a compromise between small-signal accuracy and maximum permissible voltage loss in the measurement line. High values of R_S provide better accuracy at lower currents by minimizing the effects of offset, while low values of R_S minimize voltage loss in the supply line. For most applications, using an R_S value that provides a full-scale shunt voltage range of 50 mV to 100 mV results in the best performance. Maximum input voltage for accurate measurements is 500 mV, but output voltage is limited by supply.

7.3.3 Comparator

The INA200, INA201, and INA202 devices incorporate an open-drain comparator. This comparator typically has 2 mV of offset and a 1.3-μs (typical) response time. The output of the comparator latches and is reset through the RESET pin; see Figure 28.

When V_s and RESET are different, TI recommends adding a low-pass filter (LPF) on the RESET pin to avoid comparator behavior inconsistent with the data sheet. For instance, with a 12-V supply and a 3.3-V RESET, a rise time of 400 ns is appropriate. Similarly, with an 18-V supply and a 2.7-V RESET, a 1-µs rise time is appropriate; see Figure 31.

Figure 27. Input Filter (Gain Error: 1.5% to 2.8%)

Figure 28. Comparator Latching Capability

1. Q1 cascodes the comparator output to drive a high-side FET (the 2N3904) shown is good up to 60 V. The shunt can be located in any one of the three locations shown. The latching capability must be used in shutdown applications to prevent oscillation at the trip point.

Figure 29. High-Side Switch Overcurrent Shutdown

1. It is possible to set different limits for each direction.

Figure 30. Bidirectional Overcurrent Comparator

Figure 31. Filter on RESET Pin

7.4.1 Input Filtering

An obvious and straightforward location for filtering is at the output of the INA20x series; however, this location negates the advantage of the low output impedance of the internal buffer. The only other option for filtering is at the input pins of the INA20x devices, which is complicated by the internal 5 kΩ + 30% input impedance. This is shown in Figure 27. Using the lowest possible resistor values minimizes the initial shift in gain and effects of tolerance. The effect on initial gain is shown in Equation 1:

Equation 1.

Total effect on gain error can be calculated by replacing the 5-kΩ term with 5 kΩ – 30%, (or 3.5 kΩ) or 5 kΩ + 30% (or 6.5 kΩ). The tolerance extremes of R_FILT can be inserted into the equation. If a pair of 100-Ω 1% resistors are used on the inputs, the initial gain error equals 1.96%. Worst-case tolerance conditions always occur at the lower excursion of the internal 5-kΩ resistor (3.5 kΩ), and the higher excursion of R_FILT – 3% in this case.

The specified accuracy of the INA20x devices must then be combined in addition to these tolerances. While this discussion treated accuracy worst-case conditions by combining the extremes of the resistor values, it is appropriate to use geometric mean or root sum square calculations to total the effects of accuracy variations.

7.4.2 Accuracy Variations as a Result of V_SENSE and Common-Mode Voltage

The accuracy of the INA200, INA201, and INA202 current shunt monitors is a function of two main variables: V_SENSE (V_IN+ – V_IN–), common-mode voltage, (V_CM), relative to the supply voltage (V_S). V_CM is expressed as (V_IN+ + V_IN–) / 2; however, in practice, V_CM is seen as the voltage at V_IN+ because the voltage drop across V_SENSE is typically small.

This section addresses the accuracy of these specific operating regions:

• Normal Case 1: V_SENSE ≥ 20 mV, V_CM ≥ V_S
• Normal Case 2: V_SENSE ≥ 20 mV, V_CM < V_S
• Low V_SENSE Case 1: V_SENSE < 20 mV, –16 V ≤ V_CM < 0
• Low V_SENSE Case 2: V_SENSE < 20 mV, 0 V ≤ V_CM ≤ V_S
• Low V_SENSE Case 3: V_SENSE < 20 mV, V_S < V_CM ≤ 80 V

7.4.2.1 Normal Case 1: V_SENSE ≥ 20 mv, V_CM ≥ V_S

This region of operation provides the highest accuracy. Here, the input offset voltage is characterized and measured using a two-step method. First, the gain is determined by Equation 2.

Equation 2.

where

• V_OUT1 = output voltage with V_SENSE = 100 mV
• V_OUT2 = output voltage with V_SENSE = 20 mV

Then the offset voltage is measured at V_SENSE = 100 mV, and referred to the input (RTI) of the current shunt monitor, as shown in Electrical Characteristics: Current-Shunt Monitor.

Equation 3.

In the Typical Characteristics, Figure 7 shows the highest accuracy for the this region of operation. In this plot, V_S = 12 V. For V_CM ≥ 12 V, the output error is at the minimum value. This case creates the V_SENSE ≥ 20-mV output specifications in Electrical Characteristics: Current-Shunt Monitor .

7.4.2.3 Low V_SENSE Case 1: V_SENSE < 20 mV, –16 V ≤ V_CM < 0
and Low V_SENSE Case 3: V_SENSE < 20 mV, V_S < V_CM ≤ 80 V

Although the INA200 family of devices are not designed for accurate operation in these regions, some applications are exposed to these conditions. For example, when monitoring power supplies that are switched on and off while V_S is still applied to the INA20x devices, it is important to know what the behavior of the devices is in these regions.

As V_SENSE approaches 0 mV, in these V_CM regions, the accuracy of the device output degrades. A larger-than-normal offset can appear at the current shunt monitor output with a typical maximum value of V_OUT = 300 mV for
V_SENSE = 0 mV. As V_SENSE approaches 20 mV, V_OUT returns to the expected output value with accuracy as shown in Electrical Characteristics: Current-Shunt Monitor. Figure 32 shows this effect using the INA202 (gain = 100).

Figure 32. Example For Low V_SENSE Cases 1 and 3 (INA202, Gain = 100)

7.4.2.4 Low V_SENSE Case 2: V_SENSE < 20 mV, 0 V ≤ V_CM ≤ V_S

This region of operation is the least accurate for the INA20x family. To achieve the wide input common-mode voltage range, these devices use two op amp front ends in parallel. One op amp front end operates in the positive input common-mode voltage range, and the other in the negative input region. For this case, neither of these two internal amplifiers dominates and overall loop gain is low. Within this region, V_OUT approaches voltages close to linear operation levels for normal case 2. This deviation from linear operation becomes greatest the closer V_SENSE approaches 0 V. Within this region, as V_SENSE approaches 20 mV, device operation is closer to that is described in normal case 2. Figure 33 shows this behavior for the INA202. The V_OUT maximum peak for this case is tested by maintaining a constant V_S, setting V_SENSE equal to 0 mV and sweeping V_CM from 0 V to V_S. The exact V_CM at which V_OUT peaks during this test varies from device to device, but the V_OUT maximum peak is tested to be less than the specified V_OUT tested limit.

Figure 33. Example For Low V_SENSE Case 2 (INA202, Gain = 100)

7.4.3 Transient Protection

The –16 to 80 V common-mode range of the INA20x devices is ideal for withstanding automotive fault conditions ranging from 12-V battery reversal up to 80-V transients, since no additional protective components are required up to those levels. In the event that the INA20x devices are exposed to transients on the inputs in excess of their ratings, then external transient absorption with semiconductor transient absorbers (such as Zeners) are required. TI does not recommend using MOVs or VDRs, except when they are used in addition to a semiconductor transient absorber. Select the transient absorber so the absorber does not allow the INA20x devices to be exposed to transients greater than 80 V (that is, allow for transient absorber tolerance and additional voltage due to transient absorber dynamic impedance). Despite the use of internal Zener-type ESD protection, the INA20x devices do not lend themselves to using external resistors in series with the inputs since the internal gain resistors can vary up to ±30%. (If gain accuracy is not important, then resistors can be added in series with the INA200, INA201, and INA202 inputs with two equal resistors on each input.)

7.4.4 Output Voltage Range

The output of the INA20x devices is accurate within the output voltage swing range set by the power supply pin (V_S.) This performance is best illustrated when using the INA202 (a gain of 100 version), where a 100-mV full-scale input from the shunt resistor requires an output voltage swing of 10 V, and a power-supply voltage sufficient to achieve 10 V on the output.

8.2.1 Design Requirements

The device measures current through a resistive shunt with current flowing in one direction that enables detection of an overcurrent event only when the differential input voltage exceeds the threshold limit. When the current reaches the set limit of the divider R₁ / R₂, the output of CMP_OUT transitions high, which turns Q1 on, pulls the gate of the pass-FET low, and turns off the flow off current.

8.2.2 Detailed Design Procedure

Figure 34 shows the basic connections of the device. The input terminals (IN+ and IN –) must be connected as closely as possible to the current-sensing resistor to minimize any resistance in series with the shunt resistance. Additional resistance between the current-sensing resistor and input terminals results in errors in the measurement. When input current flows through this external input resistance, the voltage developed across the shunt resistor differs from the voltage reaching the input terminals.

Use the gain of the INA20x and shunt value to calculate the OUT voltage for the desired trip current. Configure R1 and R2 so that the current trip point is equal to the 0.6-V reference voltage.

8.2.3 Application Curve

Figure 35. Low-Side Switch Overcurrent Shutdown Response