SNOSC63B February   2012  – December 2014 LMP8646

PRODUCTION DATA.  

  1. 1 Features
  2. 2 Applications
  3. 3 Description
    1.     Device Images
      1.      Typical Application
  4. 4 Revision History
  5. 5 Pin Configuration and Functions
    1.     Pin Functions
  6. 6 Specifications
    1. 6.1 Absolute Maximum Ratings
    2. 6.2 ESD Ratings
    3. 6.3 Recommended Operating Conditions
    4. 6.4 Thermal Information
    5. 6.5 Electrical Characteristics: 2.7 V
    6. 6.6 Electrical Characteristics: 5 V
    7. 6.7 Electrical Characteristics: 12 V
    8. 6.8 Typical Characteristics
  7. 7 Detailed Description
    1. 7.1 Overview
    2. 7.2 Functional Block Diagram
    3. 7.3 Feature Description
      1. 7.3.1 Theory of Operation
        1. 7.3.1.1 Maximum Output Voltage, VOUT_MAX
          1. 7.3.1.1.1 Case 1: −2 V < VCM < 1.8 V, and VS > 2.7 V
          2. 7.3.1.1.2 Case 2: 1.8 V < VCM < VS, and VS > 3.3 V
          3. 7.3.1.1.3 Case 3: VCM > VS, and VS > 2.7 V
    4. 7.4 Device Functional Modes
      1. 7.4.1 Output Accuracy
      2. 7.4.2 Selection of the Sense Resistor, RSENSE
        1. 7.4.2.1 RSENSE Consideration for System Error
  8. 8 Application and Implementation
    1. 8.1 Application Information
    2. 8.2 Typical Applications
      1. 8.2.1 Application #1: Current Limiter With a Capacitive Load
        1. 8.2.1.1 Design Requirements
        2. 8.2.1.2 Detailed Design Procedure
        3. 8.2.1.3 Application Curve
      2. 8.2.2 Application #2: Current Limiter With a Resistive Load
        1. 8.2.2.1 Design Requirements
        2. 8.2.2.2 Detailed Design Procedure
        3. 8.2.2.3 Application Curve
      3. 8.2.3 Application #3: Current Limiter With a Low-Dropout Regulator and Resistive Load
        1. 8.2.3.1 Design Requirements
        2. 8.2.3.2 Detailed Design Procedure
        3. 8.2.3.3 Application Curve
  9. 9 Power Supply Recommendations
  10. 10Layout
    1. 10.1 Layout Guidelines
    2. 10.2 Layout Example
  11. 11Device and Documentation Support
    1. 11.1 Trademarks
    2. 11.2 Electrostatic Discharge Caution
    3. 11.3 Glossary
  12. 12Mechanical, Packaging, and Orderable Information

Package Options

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

8.2.2.2 Detailed Design Procedure

Step 1: Choose the components for the Regulator.

Refer to the LMZ12003 application note (AN-2031) to select the appropriate components for the LMZ12003.

Step 2: Choose the sense resistor, RSENSE

RSENSE sets the voltage VSENSE between +IN and -IN and has the following equation:

Equation 15. RSENSE = VOUT / [(ILIMIT) * (RG / 5kOhm)]

In general, RSENSE depends on the output voltage, limit current, and gain. Refer to section Selection of the Sense Resistor, RSENSE to choose the appropriate RSENSE value; this example uses 50 mOhm.

Step 3: Choose the gain resistor, RG, for LMP8646

RG is chosen from ILIMIT. As stated, VOUT = (RSENSE * ILIMIT) * (RG / 5kOhm). Since VOUT = VFB = 0.8V, ILIMIT = 1A, and RSENSE = 50 mOhm , RG can be calculated as:

Equation 16. RG = (VOUT * 5 kOhm) / (RSENSE * ILIMIT)
Equation 17. RG = (0.8 * 5 kOhm) / (50 mOhm* 1A) = 80 kOhm

Step 4: Choose the Bandwidth Capacitance, CG.

The product of CG and RG determines the bandwidth for the LMP8646. Refer to the Typical Performance Characteristics plots to see the range for the LMP8646 bandwidth and gain. Since each application is very unique, the LMP8646 bandwidth capacitance, CG, needs to be adjusted to fit the appropriate application.

Bench data has been collected for this resistive load application with the LMZ12003 regulator, and we found that this application works best for a bandwidth of 2 kHz to 30 kHz. Operating anything less than this recommended bandwidth might prevent the LMP8646 from quickly limiting the current. We recommend choosing a bandwidth that is in the middle of this range and using the equation: CG = 1/(2*pi*RG*Bandwidth) to find CG (this example uses a CG value of 0.1nF). After this selection, capture the load current plot and adjust CG until a desired output current plot is obtained.

Step 5: Choose the output resistor, ROUT, for the LMP8646

ROUT plays a very small role in the overall system performance for the resistive load application. ROUT was important in the supercap application because it affects the initial current error. Because current is directly proportional to voltage for a resistive load, the output current is not large at start-up. The bigger the ROUT, the longer it takes for the output voltage to reach its final value. We recommend that the value for ROUT is at least 50 Ohm, which is the chosen value for this example.

Step 6: Adjusting Components

Capture the output current and output voltage plots and adjust the components as necessary. The most common component to adjust is CG for the bandwidth. An example of the output current and voltage plot can be seen in Figure 31.