SNOSB28G August   2010  – November 2014 LMP8640 , LMP8640-Q1 , LMP8640HV

PRODUCTION DATA.  

  1. 1 Features
  2. 2 Applications
  3. 3 Description
  4. 4 Revision History
  5. 5 Device Comparison Table
  6. 6 Pin Configuration and Functions
  7. 7 Specifications
    1. 7.1 Absolute Maximum Ratings
    2. 7.2 Handling Ratings - LMP8640, LMP8640HV
    3. 7.3 Handling Ratings - LMP8640-Q1
    4. 7.4 Recommended Operating Conditions
    5. 7.5 Thermal Information
    6. 7.6 Electrical Characteristics 2.7 V
    7. 7.7 Electrical Characteristics 5 V
    8. 7.8 Electrical Characteristics 12 V
    9. 7.9 Typical Characteristics
  8. 8 Detailed Description
    1. 8.1 Overview
    2. 8.2 Functional Block Diagram
    3. 8.3 Feature Description
      1. 8.3.1 Selection of Sense Resistor
        1. 8.3.1.1 Resistor Power Rating and Thermal Issues
        2. 8.3.1.2 Using PCB Trace as a Sense Resistor
      2. 8.3.2 Sense Line Inputs
      3. 8.3.3 Effects of Series Resistance on Sense Lines
    4. 8.4 Device Functional Modes
      1. 8.4.1 Bias Current at Low Common Mode Voltage
      2. 8.4.2 Applying Input Voltage with No Supply Voltage
      3. 8.4.3 Driving an ADC
  9. 9 Application and Implementation
    1. 9.1 Application Information
    2. 9.2 Typical Application
      1. 9.2.1 Design Requirements
      2. 9.2.2 Design Procedure
        1. 9.2.2.1 First Step - LMP8640 or LMP8640HV Selection
        2. 9.2.2.2 Second Step - Gain Option Selection
        3. 9.2.2.3 Third Step - Shunt Resistor Selection
      3. 9.2.3 Application Performance Plot
    3. 9.3 Do's and Don'ts Added Section
  10. 10Power Supply Recommendations
  11. 11Layout
    1. 11.1 Layout Guidelines
    2. 11.2 Layout Example
  12. 12Device and Documentation Support
    1. 12.1 Device Support
      1. 12.1.1 Development Support
    2. 12.2 Documentation Support
      1. 12.2.1 Related Documentation
    3. 12.3 Related Links
    4. 12.4 Trademarks
    5. 12.5 Electrostatic Discharge Caution
    6. 12.6 Glossary
  13. 13Mechanical, Packaging, and Orderable Information

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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 LMP8640x amplifies the voltage developed across a current-sensing resistor.

9.2 Typical Application

30071461.gifFigure 27. Typical Application Example

9.2.1 Design Requirements

In this example, a current monitor application is required to measure the current into a load (peak current 10 A) with a resolution of 10 mA and 0.5% of accuracy.

The 10bit analog to digital converter accepts a max input voltage of 4.1 V. In order to not burn too much power on the shunt resistor, it needs to be less than 10 mΩ. Table 1 below summarizes the other design conditions.

Table 1. Example Design Requirements

WORKING CONDITION VALUE
MIN MAX
Supply Voltage 5 V 5.5 V
Common mode Voltage 48 V 70 V
Temperature 0°C 70°C
Signal BW 50 kHz

9.2.2 Design Procedure

9.2.2.1 First Step – LMP8640 or LMP8640HV Selection

The required common mode voltage of the application implies that the right choice is the LMP8640HV (High common mode voltage up tp 76 V).

9.2.2.2 Second Step – Gain Option Selection

We can choose between three gain option (20 V/V, 50 V/V, 100 V/V). Considering the max input voltage of the ADC (4.1 V) , the max Sense voltage across the shunt resistor is evaluated according the following formula:

Equation 5. VSENSE= (MAX Vin ADC) / Gain;

hence the max VSENSE will be 205 mV, 82 mV, 41 mV respectively. The shunt resistor are then evaluated considering the maximum monitored current :

Equation 6. RS = (max VSENSE) / I_MAX

For each gain option the max shunt resistors are the following : 20.5 mΩ, 8.2 mΩ, 4.1 mΩ respectively.

One of the project constraints requires RS<10 mΩ, it means that the 20.5 mΩ will be discarded and hence the 50 V/V and 100 V/V gain options are still in play.

9.2.2.3 Third Step – Shunt Resistor Selection

At this point an error budget calculation, considering the calibration of the Gain, Offset, CMRR, and PSRR, helps in the selection of the shunt resistor. In the table below the contribution of each error source is calculated considering the values of the Electrical Characteristics table at 5 V supply.

Table 2. Resolution Calculation

ERROR SOURCE RS = 4.1 mΩ RS = 8.1 mΩ
CMRR calibrated at mid VCM range 77.9 µV 77.9 µV
PSRR calibrated at 5 V 8.9 µV 8.9 µV
Total error (squared sum of contribution) 78 µV 78 µV
Resolution (Total error / RS) 19.2 mA 9.6 mA

Table 3. Accuracy Calculation

ERROR SOURCE RS = 4.1 mΩ RS = 8.1 mΩ
Tc Vos 182 µV 182 µV
Nosie 216 µV 216 µV
Gain drift 75.2 µV 151 µV
Total error (squared sum of contribution) 293 µV 320 µV
Accuracy 100*(Max_VSENSE / Total Error) 0.7% 0.4%

From the tables above is clear that the 8.2 mΩ shunt resistor allows the respect of the project's constraints. The power burned on the Shunt is 820 mW at 10 A.

9.2.3 Application Performance Plot

C001_SNOSB28_LMP8640.pngFigure 28. Application Example Results using 8.2 mΩ Resistor

9.3 Do's and Don'ts

Do properly bypass the power supplies.

Do add series resistance to the output when driving capacitive loads, cables, long traces, muxes and ADC inputs.

Do not exceed the input common mode range.