SNVS422D August   2006  – September 2015 LM2831

PRODUCTION DATA.  

  1. 1 Features
  2. 2 Applications
  3. 3 Description
  4. 4 Revision History
  5. 5 Pin Configuration and 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
    6. 6.6 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
      2. 7.3.2 Soft Start
      3. 7.3.3 Output Overvoltage Protection
      4. 7.3.4 Undervoltage Lockout
      5. 7.3.5 Current Limit
      6. 7.3.6 Thermal Shutdown
    4. 7.4 Device Functional Modes
  8. 8 Application and Implementation
    1. 8.1 Application Information
    2. 8.2 Typical Applications
      1. 8.2.1 LM2831X Design Example 1
        1. 8.2.1.1 Design Requirements
        2. 8.2.1.2 Detailed Design Procedure
          1. 8.2.1.2.1 Inductor Selection
          2. 8.2.1.2.2 Input Capacitor
          3. 8.2.1.2.3 Output Capacitor
          4. 8.2.1.2.4 Catch Diode
          5. 8.2.1.2.5 Output Voltage
        3. 8.2.1.3 Application Curves
      2. 8.2.2 LM2831X Design Example 2
      3. 8.2.3 LM2831X Design Example 3
      4. 8.2.4 LM2831Y Design Example 4
      5. 8.2.5 LM2831Y Design Example 5
      6. 8.2.6 LM2831Z Design Example 6
      7. 8.2.7 LM2831Z Design Example 7
      8. 8.2.8 LM2831X Dual Converters with Delayed Enabled Design Example 8
      9. 8.2.9 LM2831X Buck Converter and Voltage Double Circuit With LDO Follower Design Example 9
  9. 9 Power Supply Recommendations
  10. 10Layout
    1. 10.1 Layout Guidelines
      1. 10.1.1 Calculating Efficiency and Junction Temperature
      2. 10.1.2 Thermal Definitions
        1. 10.1.2.1 Silicon Junction Temperature Determination Method 1
      3. 10.1.3 WSON Package
    2. 10.2 Layout Example
  11. 11Device and Documentation Support
    1. 11.1 Device Support
      1. 11.1.1 Third-Party Products Disclaimer
    2. 11.2 Documentation Support
      1. 11.2.1 Related Documentation
    3. 11.3 Community Resources
    4. 11.4 Trademarks
    5. 11.5 Electrostatic Discharge Caution
    6. 11.6 Glossary
  12. 12Mechanical, Packaging, and Orderable Information

Package Options

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

7 Detailed Description

7.1 Overview

The LM2831 device is a constant-frequency PWM buck regulator IC that delivers a 1.5-A load current. The regulator has a preset switching frequency of 550 kHz, 1.6 MHz, or 3 MHz. This high-frequency allows the LM2831 to operate with small surface mount capacitors and inductors, resulting in a DC-DC converter that requires a minimum amount of board space. The LM2831 is internally compensated, so the device is simple to use and requires few external components.

7.2 Functional Block Diagram

LM2831 20174804.gif

7.3 Feature Description

7.3.1 Theory of Operation

The LM2831 uses current-mode control to regulate the output voltage. The following operating description of the LM2831 will refer to Functional Block Diagram and to the waveforms in Figure 18. The LM2831 supplies a regulated output voltage by switching the internal PMOS control switch at constant-frequency and variable duty cycle. A switching cycle begins at the falling edge of the reset pulse generated by the internal oscillator. When this pulse goes low, the output control logic turns on the internal PMOS control switch. During this on-time, the SW pin voltage (VSW) swings up to approximately VIN, and the inductor current (IL) increases with a linear slope. IL is measured by the current sense amplifier, which generates an output proportional to the switch current. The sense signal is summed with the regulator’s corrective ramp and compared to the error amplifier’s output, which is proportional to the difference between the feedback voltage and VREF. When the PWM comparator output goes high, the output switch turns off until the next switching cycle begins. During the switch off-time, inductor current discharges through the Schottky catch diode, which forces the SW pin to swing below ground by the forward voltage (VD) of the Schottky catch diode. The regulator loop adjusts the duty cycle (D) to maintain a constant output voltage.

LM2831 20174866.gif Figure 18. Typical Waveforms

7.3.2 Soft Start

This function forces VOUT to increase at a controlled rate during start up. During soft start, the error amplifier’s reference voltage ramps from 0 V to its nominal value of 0.6 V in approximately 600 µs. This forces the regulator output to ramp up in a controlled fashion, which helps reduce inrush current.

7.3.3 Output Overvoltage Protection

The overvoltage comparator compares the FB pin voltage to a voltage that is 15% higher than the internal reference VREF. Once the FB pin voltage goes 15% above the internal reference, the internal PMOS control switch is turned off, which allows the output voltage to decrease toward regulation.

7.3.4 Undervoltage Lockout

Undervoltage lockout (UVLO) prevents the LM2831 from operating until the input voltage exceeds 2.73 V (typical). The UVLO threshold has approximately 430 mV of hysteresis, so the part will operate until VIN drops below 2.3 V (typical). Hysteresis prevents the part from turning off during power up if VIN is non-monotonic.

7.3.5 Current Limit

The LM2831 uses cycle-by-cycle current limiting to protect the output switch. During each switching cycle, a current limit comparator detects if the output switch current exceeds 2.5 A (typical), and turns off the switch until the next switching cycle begins.

7.3.6 Thermal Shutdown

Thermal shutdown limits total power dissipation by turning off the output switch when the IC junction temperature exceeds 165°C. After thermal shutdown occurs, the output switch doesn’t turn on until the junction temperature drops to approximately 150°C.

7.4 Device Functional Modes

The LM2831 has an enable pin (EN) control Input. A logic high enables device operation. Do not float this pin or let this pin be greater than VIN + 0.3 V for the SOT package option, or VINA + 0.3 V for the WSON package option.