SNOS976M November   2001  – September 2016 LMV981-N , LMV982-N

PRODUCTION DATA.  

  1. 1 Features
  2. 2 Applications
  3. 3 Description
  4. 4 Revision History
  5. 5 Description (continued)
  6. 6 Pin Configuration and Functions
  7. 7 Specifications
    1. 7.1  Absolute Maximum Ratings
    2. 7.2  ESD Ratings
    3. 7.3  Recommended Operating Conditions
    4. 7.4  Thermal Information
    5. 7.5  Electrical Characteristics - DC, 1.8 V
    6. 7.6  Electrical Characteristics - AC, 1.8 V
    7. 7.7  Electrical Characteristics - DC, 2.7 V
    8. 7.8  Electrical Characteristics - AC, 2.7 V
    9. 7.9  Electrical Characteristics - DC, 5 V
    10. 7.10 Electrical Characteristics - AC, 5 V
    11. 7.11 Typical Characteristics
  8. 8 Detailed Description
    1. 8.1 Overview
    2. 8.2 Functional Block Diagram
    3. 8.3 Feature Description
    4. 8.4 Device Functional Modes
      1. 8.4.1 Input and Output Stage
      2. 8.4.2 Shutdown Mode
      3. 8.4.3 Input Bias Current Consideration
  9. 9 Application and Implementation
    1. 9.1 Application Information
    2. 9.2 Typical Applications
      1. 9.2.1 High-Side Current-Sensing Application
        1. 9.2.1.1 Design Requirements
        2. 9.2.1.2 Detailed Design Procedure
        3. 9.2.1.3 Application Curve
      2. 9.2.2 Half-Wave Rectifier Applications
        1. 9.2.2.1 Design Requirements
        2. 9.2.2.2 Detailed Design Procedure
        3. 9.2.2.3 Application Curves
      3. 9.2.3 Instrumentation Amplifier With Rail-to-Rail Input and Output Application
        1. 9.2.3.1 Design Requirements
        2. 9.2.3.2 Detailed Design Procedure
        3. 9.2.3.3 Application Curve
    3. 9.3 Do's and Don'ts
  10. 10Power Supply Recommendations
  11. 11Layout
    1. 11.1 Layout Guidelines
    2. 11.2 Layout Example
  12. 12Device and Documentation Support
    1. 12.1 Documentation Support
      1. 12.1.1 Related Documentation
    2. 12.2 Related Links
    3. 12.3 Receiving Notification of Documentation Updates
    4. 12.4 Community Resources
    5. 12.5 Trademarks
    6. 12.6 Electrostatic Discharge Caution
    7. 12.7 Glossary
  13. 13Mechanical, Packaging, and Orderable Information

Package Options

Mechanical Data (Package|Pins)
Thermal pad, mechanical data (Package|Pins)
Orderable Information

9 Application and Implementation

NOTE

Information in the following applications sections is not part of the TI component specification, and TI does not warrant its accuracy or completeness. TI’s customers are responsible for determining suitability of components for their purposes. Customers should validate and test their design implementation to confirm system functionality.

9.1 Application Information

The LMV98x-N devices bring performance, economy, and ease-of-use to low-voltage, low-power systems. They provide rail-to-rail input and rail-to-rail output swings into heavy loads.

9.2 Typical Applications

9.2.1 High-Side Current-Sensing Application

LMV981-N LMV982-N Curr_Sense_Ckt.gif Figure 30. High-Side Current Sensing

9.2.1.1 Design Requirements

The high-side current-sensing circuit (Figure 30) is commonly used in a battery charger to monitor charging current to prevent overcharging. A sense resistor RSENSE is connected to the battery directly. This system requires an op amp with rail-to-rail input. The LMV98x-N are ideal for this application because its common-mode input range extends up to the positive supply.

9.2.1.2 Detailed Design Procedure

As seen in Figure 30, the ICHARGE current flowing through sense resistor RSENSE develops a voltage drop equal to VSENSE. The voltage at the negative sense point is now less than the positive sense point by an amount proportional to the VSENSE voltage.

The low-bias currents of the LMV98x cause little voltage drop through R2, so the negative input of the LMV98x amplifier is at essentially the same potential as the negative sense input.

The LMV98x detects this voltage error between its inputs and servo the transistor base to conduct more current through Q1, increasing the voltage drop across R1 until the LMV98x inverting input matches the noninverting input. At this point, the voltage drop across R1 now matches VSENSE.

IG, a current proportional to ICHARGE, flows according to Equation 2.

Equation 2. IG = VRSENSE / R1 = ( RSENSE × ICHARGE ) / R1

IG also flows through the gain resistor R3 developing a voltage drop equal to Equation 3 and Equation 4.

Equation 3. V3 = IG × R3 = ( VRSENSE / R1 ) × R3 = ( ( RSENSE × ICHARGE ) / R2 ) × R3
Equation 4. VOUT = (RSENSE × ICHARGE ) × G

where

  • G = R3 / R1

The other channel of the LMV98x may be used to buffer the voltage across R3 to drive the following stages.

9.2.1.3 Application Curve

Figure 31 shows the results of the example current sense circuit. After 4 V, there is an error where transistor Q1 runs out of headroom and saturates, limiting the upper output swing.

LMV981-N LMV982-N C001_LMV931-N_SNOS993.png Figure 31. Current Sense Amplifier Results

9.2.2 Half-Wave Rectifier Applications

LMV981-N LMV982-N app_half-wave_rail-to-ground_snos993.gif Figure 32. Half-Wave Rectifier With Rail-To-Ground Output Swing Referenced to Ground
LMV981-N LMV982-N app_half-wave_neg-going_snos993.gif Figure 33. Half-Wave Rectifier With Negative-Going Output Referenced to VCC

9.2.2.1 Design Requirements

Because the LMV98x-N input common-mode range includes both positive and negative supply rails and the output can also swing to either supply, achieving half-wave rectifier functions in either direction is an easy task. All that is required are two external resistors; there is no requirement for diodes or matched resistors. The half-wave rectifier can have either positive or negative going outputs, depending on the way the circuit is arranged.

9.2.2.2 Detailed Design Procedure

In Figure 32 the circuit is referenced to ground, while in Figure 33 the circuit is biased to the positive supply. These configurations implement the half-wave rectifier because the LMV98x-N can not respond to one-half of the incoming waveform. It can not respond to one-half of the incoming because the amplifier cannot swing the output beyond either rail; therefore, the output disengages during this half cycle. During the other half cycle, however, the amplifier achieves a half wave that can have a peak equal to the total supply voltage. RI must be large enough not to load the LMV98x-N.

9.2.2.3 Application Curves

LMV981-N LMV982-N 200326c4.png Figure 34. Output of Ground-to-Rail Circuit
LMV981-N LMV982-N 200326c1.png Figure 35. Output of Rail-to-Ground Circuit

9.2.3 Instrumentation Amplifier With Rail-to-Rail Input and Output Application

LMV981-N LMV982-N 200326g4.gif Figure 36. Rail-to-Rail Instrumentation Amplifier

9.2.3.1 Design Requirements

Using three of the LMV98x-N amplifiers, an instrumentation amplifier with rail-to-rail inputs and outputs can be made as shown in Figure 36.

9.2.3.2 Detailed Design Procedure

In this example, amplifiers on the left side act as buffers to the differential stage. These buffers assure that the input impedance is high. They also assure that the difference amp is driven from a voltage source. This is necessary to maintain the CMRR set by the matching R1 to R2 with R3 to R4. The gain is set by the ratio of R2/R1 and R3 must equal R1 and R4 equal R2. With both rail-to-rail input and output ranges, the input and output are only limited by the supply voltages. Remember that even with rail-to-rail outputs, the output can not swing past the supplies so the combined common-mode voltages plus the signal must not be greater that the supplies or limiting occurs.

9.2.3.3 Application Curve

Figure 37 shows the results of the instrumentation amplifier with R1 and R3 = 1 K, and R2 and R4 = 100 kΩ, for a gain of 100, running on a single 5-V supply with a input of VCM = VS/2. The combined effects of the individual offset voltages can be seen as a shift in the offset of the curve.

LMV981-N LMV982-N C002_SNOS993_LMV931-N.png Figure 37. Instrumentation Amplifier Output Results

9.3 Do's and Don'ts

Do properly bypass the power supplies.

Do add series resistence to the output when driving capacitive loads, particularly cables, Muxes and ADC inputs.

Do add series current limiting resistors and external schottky clamp diodes if input voltage is expected to exceed the supplies. Limit the current to 1 mA or less (1 kΩ per volt).