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

8 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.

8.1 Application Information

The LM2831 device will operate with input voltage range from 3 V to 5.5 V and provide a regulated output voltage. This device is optimized for high-efficiency operation with minimum number of external components. For component selection, see Detailed Design Procedure.

8.2 Typical Applications

8.2.1 LM2831X Design Example 1

LM2831 20174807.gif Figure 19. LM2831X (1.6 MHz): VIN = 5 V, VO = 1.2 V at 1.5 A

8.2.1.1 Design Requirements

The device must be able to operate at any voltage within the recommended operating range. Load current must be defined to properly size the inductor, input, and output capacitors. Inductor should be able to handle full expected load current as well as the peak current generated during load transients and start up. Inrush current at start-up will depend on the output capacitor selection. More details are provided in Detailed Design Procedure.

8.2.1.2 Detailed Design Procedure

Table 1. Bill of Materials

PART ID PART VALUE MANUFACTURER PART NUMBER
U1 1.5-A Buck Regulator TI LM2831X
C1, Input Cap 22 µF, 6.3 V, X5R TDK C3216X5ROJ226M
C2, Output Cap 2x22 µF, 6.3 V, X5R TDK C3216X5ROJ226M
D1, Catch Diode 0.3 Vf Schottky 1.5 A, 30 VR TOSHIBA CRS08
L1 3.3 µH, 2.2 A TDK VLCF5020T-3R3N2R0-1
R2 15.0 kΩ, 1% Vishay CRCW08051502F
R1 15.0 kΩ, 1% Vishay CRCW08051502F
R3 100 kΩ, 1% Vishay CRCW08051003F

8.2.1.2.1 Inductor Selection

The duty cycle (D) can be approximated quickly using the ratio of output voltage (VO) to input voltage (VIN):

Equation 1. LM2831 20174809.gif

The catch diode (D1) forward voltage drop and the voltage drop across the internal PMOS must be included to calculate a more accurate duty cycle. Calculate D by using the following formula:

Equation 2. LM2831 20174810.gif

VSW can be approximated by:

Equation 3. VSW = IOUT × RDSON

The diode forward drop (VD) can range from 0.3 V to 0.7 V depending on the quality of the diode. The lower the VD, the higher the operating efficiency of the converter. The inductor value determines the output ripple current. Lower inductor values decrease the size of the inductor, but increase the output ripple current. An increase in the inductor value will decrease the output ripple current.

One must ensure that the minimum current limit (1.8 A) is not exceeded, so the peak current in the inductor must be calculated. The peak current (ILPK) in the inductor is calculated by:

Equation 4. ILPK = IOUT + ΔiL
LM2831 20174805.gif Figure 20. Inductor Current
Equation 5. LM2831 20174813.gif

In general,

Equation 6. ΔiL = 0.1 × (IOUT) → 0.2 × (IOUT)

If ΔiL = 20% of 1.50 A, the peak current in the inductor will be 1.8 A. The minimum ensured current limit over all operating conditions is 1.8 A. One can either reduce ΔiL, or make the engineering judgment that zero margin will be safe enough. The typical current limit is 2.5 A.

The LM2831 operates at frequencies allowing the use of ceramic output capacitors without compromising transient response. Ceramic capacitors allow higher inductor ripple without significantly increasing output ripple. See the Output Capacitor section for more details on calculating output voltage ripple. Now that the ripple current is determined, the inductance is calculated by:

Equation 7. LM2831 20174811.gif

Where:

Equation 8. LM2831 20174812.gif

When selecting an inductor, make sure that it is capable of supporting the peak output current without saturating. Inductor saturation will result in a sudden reduction in inductance and prevent the regulator from operating correctly. Because of the speed of the internal current limit, the peak current of the inductor need only be specified for the required maximum output current. For example, if the designed maximum output current is 1 A and the peak current is 1.25 A, then the inductor should be specified with a saturation current limit of > 1.25 A. There is no need to specify the saturation or peak current of the inductor at the 2.5-A typical switch current limit. The difference in inductor size is a factor of 5. Because of the operating frequency of the LM2831, ferrite based inductors are preferred to minimize core losses. This presents little restriction since the variety of ferrite-based inductors is huge. Lastly, inductors with lower series resistance (RDCR) will provide better operating efficiency. For recommended inductors, see LM2831X Design Example 2 through LM2831X Buck Converter and Voltage Double Circuit With LDO Follower Design Example 9.

8.2.1.2.2 Input Capacitor

An input capacitor is necessary to ensure that VIN does not drop excessively during switching transients. The primary specifications of the input capacitor are capacitance, voltage, RMS current rating, and ESL (Equivalent Series Inductance). The recommended input capacitance is 22 µF. The input voltage rating is specifically stated by the capacitor manufacturer. Make sure to check any recommended deratings and also verify if there is any significant change in capacitance at the operating input voltage and the operating temperature. The input capacitor maximum RMS input current rating (IRMS-IN) must be greater than:

Equation 9. LM2831 20174817.gif

Neglecting inductor ripple simplifies the above equation to:

Equation 10. LM2831 20174816.gif

It can be shown from the above equation that maximum RMS capacitor current occurs when D = 0.5. Always calculate the RMS at the point where the duty cycle D is closest to 0.5. The ESL of an input capacitor is usually determined by the effective cross sectional area of the current path. A large leaded capacitor will have high ESL and a 0805 ceramic chip capacitor will have very low ESL. At the operating frequencies of the LM2831, leaded capacitors may have an ESL so large that the resulting impedance (2πfL) will be higher than that required to provide stable operation. As a result, surface mount capacitors are strongly recommended.

Sanyo POSCAP, Tantalum or Niobium, Panasonic SP, and multilayer ceramic capacitors (MLCC) are all good choices for both input and output capacitors and have very low ESL. For MLCCs it is recommended to use X7R or X5R type capacitors due to their tolerance and temperature characteristics. Consult capacitor manufacturer data sheets to see how rated capacitance varies over operating conditions.

8.2.1.2.3 Output Capacitor

The output capacitor is selected based upon the desired output ripple and transient response. The initial current of a load transient is provided mainly by the output capacitor. The output ripple of the converter is:

Equation 11. LM2831 20174818.gif

When using MLCCs, the ESR is typically so low that the capacitive ripple may dominate. When this occurs, the output ripple will be approximately sinusoidal and 90° phase shifted from the switching action. Given the availability and quality of MLCCs and the expected output voltage of designs using the LM2831, there is really no need to review any other capacitor technologies. Another benefit of ceramic capacitors is their ability to bypass high frequency noise. A certain amount of switching edge noise will couple through parasitic capacitances in the inductor to the output. A ceramic capacitor will bypass this noise while a tantalum will not. Since the output capacitor is one of the two external components that control the stability of the regulator control loop, most applications will require a minimum of 22 µF of output capacitance. Capacitance often, but not always, can be increased significantly with little detriment to the regulator stability. Like the input capacitor, recommended multilayer ceramic capacitors are X7R or X5R types.

8.2.1.2.4 Catch Diode

The catch diode (D1) conducts during the switch off-time. A Schottky diode is recommended for its fast switching times and low forward voltage drop. The catch diode should be chosen so that its current rating is greater than:

Equation 12. ID1 = IOUT × (1-D)

The reverse breakdown rating of the diode must be at least the maximum input voltage plus appropriate margin. To improve efficiency, choose a Schottky diode with a low forward voltage drop.

8.2.1.2.5 Output Voltage

The output voltage is set using the following equation where R2 is connected between the FB pin and GND, and R1 is connected between VO and the FB pin. A good value for R2 is 10 kΩ. When designing a unity gain converter (Vo = 0.6 V), R1 should be from 0 Ω to 100 Ω, and R2 should be equal or greater than 10 kΩ.

Equation 13. LM2831 20174819.gif
Equation 14. VREF = 0.60 V

8.2.1.3 Application Curves

See Typical Characteristics.

LM2831 20174839.png
VIN = 5 V VO = 1.8 V and 3.3 V
Figure 21. η vs Load – X Option
LM2831 20174842.png
VIN = 5 V VO = 1.8 V and 3.3 V
Figure 23. η vs Load – Z Option
LM2831 20174886.png
VIN = 5 V VO = 1.8 V and 3.3 V
Figure 22. η vs Load – Y Option

8.2.2 LM2831X Design Example 2

LM2831 20174860.gif Figure 24. LM2831X (1.6 MHz): VIN = 5 V, VO = 0.6 V at 1.5 A

Table 2. Bill of Materials

PART ID PART VALUE MANUFACTURER PART NUMBER
U1 1.5-A Buck Regulator TI LM2831X
C1, Input Capacitor 22 µF, 6.3 V, X5R TDK C3216X5ROJ226M
C2, Output Capacitor 2x22 µF, 6.3 V, X5R TDK C3216X5ROJ226M
D1, Catch Diode 0.3 Vf Schottky 1.5 A, 30 VR TOSHIBA CRS08
L1 3.3 µH, 2.2 A TDK VLCF5020T- 3R3N2R0-1
R2 10.0 kΩ, 1% Vishay CRCW08051000F
R1 0 Ω
R3 100 kΩ, 1% Vishay CRCW08051003F

8.2.3 LM2831X Design Example 3

LM2831 20174808.gif Figure 25. LM2831X (1.6 MHz): VIN = 5 V, VO = 3.3 V at 1.5 A

Table 3. Bill of Materials

PART ID PART VALUE MANUFACTURER PART NUMBER
U1 1.5-A Buck Regulator TI LM2831X
C1, Input Cap 22 µF, 6.3 V, X5R TDK C3216X5ROJ226M
C2, Output Cap 2x22 µF, 6.3 V, X5R TDK C3216X5ROJ226M
D1, Catch Diode 0.3 Vf Schottky 1.5 A, 30 VR TOSHIBA CRS08
L1 2.7 µH 2.3 A TDK VLCF5020T-2R7N2R2-1
R2 10.0 kΩ, 1% Vishay CRCW08051002F
R1 45.3 kΩ, 1% Vishay CRCW08054532F
R3 100 kΩ, 1% Vishay CRCW08051003F

8.2.4 LM2831Y Design Example 4

LM2831 20174808.gif Figure 26. LM2831Y (550 kHz): VIN = 5 V, VOUT = 3.3 V at 1.5 A

Table 4. Bill of Materials

PART ID PART VALUE MANUFACTURER PART NUMBER
U1 1.5-A Buck Regulator TI LM2831Y
C1, Input Cap 22 µF, 6.3 V, X5R TDK C3216X5ROJ226M
C2, Output Cap 2x22 µF, 6.3 V, X5R TDK C3216X5ROJ226M
D1, Catch Diode 0.3 Vf Schottky 1.5 A, 30 VR TOSHIBA CRS08
L1 4.7 µH 2.1 A TDK SLF7045T-4R7M2R0-PF
R1 45.3 kΩ, 1% Vishay CRCW080545K3FKEA
R2 10.0 kΩ, 1% Vishay CRCW08051002F

8.2.5 LM2831Y Design Example 5

LM2831 20174807.gif Figure 27. LM2831Y (550 kHz): VIN = 5 V, VOUT = 1.2 V at 1.5 A

Table 5. Bill of Materials

PART ID PART VALUE MANUFACTURER PART NUMBER
U1 1.5-A Buck Regulator TI LM2831Y
C1, Input Cap 22 µF, 6.3 V, X5R TDK C3216X5ROJ226M
C2, Output Cap 2x22 µF, 6.3 V, X5R TDK C3216X5ROJ226M
D1, Catch Diode 0.3 Vf Schottky 1.5 A, 30 VR TOSHIBA CRS08
L1 6.8 µH 1.8 A TDK SLF7045T-6R8M1R7
R1 10.0 kΩ, 1% Vishay CRCW08051002F
R2 10.0 kΩ, 1% Vishay CRCW08051002F

8.2.6 LM2831Z Design Example 6

LM2831 20174808.gif Figure 28. LM2831Z (3 MHz): VIN = 5 V, VO = 3.3 V at 1.5 A

Table 6. Bill of Materials

PART ID PART VALUE MANUFACTURER PART NUMBER
U1 1.5-A Buck Regulator TI LM2831Z
C1, Input Cap 22 µF, 6.3 V, X5R TDK C3216X5ROJ226M
C2, Output Cap 2x22 µF, 6.3 V, X5R TDK C3216X5ROJ226M
D1, Catch Diode 0.3 Vf Schottky 1.5 A, 30 VR TOSHIBA CRS08
L1 1.6 µH 2.0 A TDK VLCF4018T-1R6N1R7-2
R2 10.0 kΩ, 1% Vishay CRCW08051002F
R1 45.3 kΩ, 1% Vishay CRCW08054532F
R3 100 kΩ, 1% Vishay CRCW08051003F

8.2.7 LM2831Z Design Example 7

LM2831 20174807.gif Figure 29. LM2831Z (3 MHz): VIN = 5 V, VO = 1.2 V at 1.5 A

Table 7. Bill of Materials

PART ID PART VALUE MANUFACTURER PART NUMBER
U1 1.5-A Buck Regulator TI LM2831Z
C1, Input Cap 22 µF, 6.3 V, X5R TDK C3216X5ROJ226M
C2, Output Cap 2x22 µF, 6.3 V, X5R TDK C3216X5ROJ226M
D1, Catch Diode 0.3 Vf Schottky 1.5 A, 30 VR TOSHIBA CRS08
L1 1.6 µH, 2.0 A TDK VLCF4018T- 1R6N1R7-2
R2 10.0 kΩ, 1% Vishay CRCW08051002F
R1 10.0 kΩ, 1% Vishay CRCW08051002F
R3 100 kΩ, 1% Vishay CRCW08051003F

8.2.8 LM2831X Dual Converters with Delayed Enabled Design Example 8

LM2831 20174862.gif Figure 30. LM2831X (1.6 MHz): VIN = 5 V, VO = 1.2 V at 1.5 A and 3.3 V at1.5 A

Table 8. Bill of Materials

PART ID PART VALUE MANUFACTURER PART NUMBER
U1, U2 1.5-A Buck Regulator TI LM2831X
U3 Power on Reset TI LP3470M5X-3.08
C1, C3 Input Cap 22 µF, 6.3 V, X5R TDK C3216X5ROJ226M
C2, C4 Output Cap 2x22 µF, 6.3 V, X5R TDK C3216X5ROJ226M
C7 Trr delay capacitor TDK
D1, D2 Catch Diode 0.3 Vf Schottky 1.5 A, 30 VR TOSHIBA CRS08
L1, L2 3.3 µH, 2.2 A TDK VLCF5020T-3R3N2R0-1
R2, R4, R5 10.0 kΩ, 1% Vishay CRCW08051002F
R1, R6 45.3 kΩ, 1% Vishay CRCW08054532F
R3 100 kΩ, 1% Vishay CRCW08051003F

8.2.9 LM2831X Buck Converter and Voltage Double Circuit With LDO Follower Design Example 9

LM2831 20174863.gif Figure 31. LM2831X (1.6 MHz): VIN = 5 V, VO = 3.3 V at 1.5 A and LP2986-5.0 at 150 mA

Table 9. Bill of Materials

PART ID PART VALUE MANUFACTURER PART NUMBER
U1 1.5-A Buck Regulator TI LM2831X
U2 200-mA LDO TI LP2986-5.0
C1, Input Cap 22 µF, 6.3 V, X5R TDK C3216X5ROJ226M
C2, Output Cap 22 µF, 6.3 V, X5R TDK C3216X5ROJ226M
C3 – C6 2.2 µF, 6.3 V, X5R TDK C1608X5R0J225M
D1, Catch Diode 0.3 Vf Schottky 1.5 A, 30 VR TOSHIBA CRS08
D2 0.4 Vf Schottky 20 VR, 500 mA ON Semi MBR0520
L2 10 µH, 800 mA CoilCraft ME3220-103
L1 3.3 µH, 2.2 A TDK VLCF5020T-3R3N2R0-1
R2 45.3 kΩ, 1% Vishay CRCW08054532F
R1 10.0 kΩ, 1% Vishay CRCW08051002F