SNVS434M July   2006  – November 2016 LM3673

PRODUCTION DATA.  

  1. 1 Features
  2. 2 Applications
  3. 3 Description
  4. 4 Revision History
  5. 5 Voltage Options
  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 Ratings
    4. 7.4 Thermal Information
    5. 7.5 Electrical Characteristics
    6. 7.6 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 Circuit Operation
      2. 8.3.2 PWM Operation
      3. 8.3.3 Internal Synchronous Rectification
      4. 8.3.4 Current Limiting
      5. 8.3.5 Soft Start
      6. 8.3.6 Low Drop Out Operation (LDO)
    4. 8.4 Device Functional Modes
      1. 8.4.1 PFM Operation
      2. 8.4.2 Shutdown Mode
  9. 9 Application and Implementation
    1. 9.1 Application Information
    2. 9.2 Typical Applications
      1. 9.2.1 Design Requirements
      2. 9.2.2 Detailed Design Procedure
        1. 9.2.2.1 Output Voltage Selection for LM3673-ADJ
        2. 9.2.2.2 Inductor Selection
          1. 9.2.2.2.1 Method 1
          2. 9.2.2.2.2 Method 2
        3. 9.2.2.3 Input Capacitor Selection
        4. 9.2.2.4 Output Capacitor Selection
      3. 9.2.3 Application Curves
  10. 10Power Supply Recommendations
  11. 11Layout
    1. 11.1 Layout Guidelines
    2. 11.2 Layout Example
    3. 11.3 DSBGA Package Assembly and Use
  12. 12Device and Documentation Support
    1. 12.1 Device Support
      1. 12.1.1 Third-Party Products Disclaimer
    2. 12.2 Documentation Support
      1. 12.2.1 Related Documentation
    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 LM3673 is designed for powering low-voltage circuits from a single Li-Ion cell battery and input-voltage rails from 2.7 V to 5.5 V. The device is internally powered from the VIN pin, and the typical switching frequency is 2 MHz. The LM3673 is available in 1.2-V, 1.5-V, and 1.8-V options. An externally adjustable version is also available where the output voltage can be set with an external resistor divider to the FB pin.

9.2 Typical Applications

LM3673 20183301.gif Figure 17. LM3673 Typical Application Circuit
LM3673 20183331.gif Figure 18. LM3673-ADJ Typical Application Circuit

9.2.1 Design Requirements

For typical step-down DC-DC converter applications, use the parameters listed in Table 1.

Table 1. Design Parameters

DESIGN PARAMETER EXAMPLE VALUE
Minimum input voltage 2.7 V
Output voltage several fixed options; adjustable
Maximum load current 350 mA
Switching frequency 2 MHz (typical)

9.2.2 Detailed Design Procedure

9.2.2.1 Output Voltage Selection for LM3673-ADJ

The output voltage of the adjustable device can be programmed through the resistor network connected from VOUT to FB, then to GND. VOUT is adjusted to make the voltage at FB equal to 0.5 V. The resistor from FB to GND (R2) must be 200 kΩ to keep the current drawn through this network well below the 16-µA quiescent current level (PFM mode) but large enough that it is not susceptible to noise. If R2 is 200 kΩ, and VFB is 0.5 V, the current through the resistor feedback network is 2.5 µA. The output voltage of the adjustable device ranges from 1.1 V to 3.3 V.

The formula for output voltage selection is:

Equation 2. LM3673 20183335.gif

where

  • VOUT: output voltage (V)
  • VFB : feedback voltage = 0.5 V
  • R1: feedback resistor from VOUT to FB
  • R2: feedback resistor from FB to GND

For any output voltage greater than or equal to 1.1 V, a zero must be added around 45 kHz for stability. The formula for calculation of C1 is:

Equation 3. LM3673 20183336.gif

For output voltages higher than 2.5 V, a pole must be placed at 45 kHz as well. If the pole and zero are at the same frequency the formula for calculation of C2 is:

Equation 4. LM3673 20183337.gif

The formula for location of zero and pole frequency created by adding C1 and C2 is shown in Equation 5 and Equation 6. By adding C1, a zero as well as a higher frequency pole is introduced.

Equation 5. LM3673 20183338.gif
Equation 6. LM3673 20183339.gif

See Table 2.

Table 2. LM3673-ADJ Configurations For Various VOUT(1)

VOUT(V) R1(kΩ) R2 (kΩ) C1 (pF) C2 (pF) L (µH) CIN (µF) COUT (µF)
1.1 240 200 15 None 2.2 4.7 10
1.2 280 200 12 None 2.2 4.7 10
1.3 320 200 12 None 2.2 4.7 10
1.5 357 178 10 None 2.2 4.7 10
1.6 442 200 8.2 None 2.2 4.7 10
1.7 432 178 8.2 None 2.2 4.7 10
1.8 464 178 8.2 None 2.2 4.7 10
1.875 523 191 6.8 None 2.2 4.7 10
2.5 402 100 8.2 None 2.2 4.7 10
2.8 464 100 8.2 33 2.2 4.7 10
3.3 562 100 6.8 33 2.2 4.7 10

9.2.2.2 Inductor Selection

There are two main considerations when choosing an inductor; the inductor must not saturate, and the inductor current ripple must be small enough to achieve the desired output voltage ripple. Different saturation current rating specifications are followed by different manufacturers so attention must be given to details. Saturation current ratings are typically specified at 25°C. However, ratings at the maximum ambient temperature of application should be requested from the manufacturer. The minimum value of inductance to ensure good performance is 1.76 µH at ILIM (typical) DC current over the ambient temperature range. Shielded inductors radiate less noise and are preferred.

There are two methods to choose the inductor saturation current rating.

9.2.2.2.1 Method 1

The saturation current must be greater than the sum of the maximum load current and the worst case average to peak inductor current. This can be written as:

Equation 7. LM3673 20183334.gif

where

  • IRIPPLE: average to peak inductor current
  • IOUTMAX: maximum load current (350 mA)
  • VIN: maximum input voltage in application
  • L : min inductor value including worst case tolerances (30% drop can be considered for Method 1)
  • ƒ : minimum switching frequency (1.6 MHz)
  • VOUT: output voltage

9.2.2.2.2 Method 2

A more conservative and recommended approach is to choose an inductor that has a saturation current rating greater than the maximum current limit of 855 mA.

A 2.2-µH inductor with a saturation current rating of at least 855 mA is recommended for most applications. Resistance of the inductor must be less than 0.3 Ω for good efficiency. Table 3 lists suggested inductors and suppliers. For low-cost applications, an unshielded bobbin inductor could be considered. For noise critical applications, a toroidal or shielded-bobbin inductor must be used. A good practice is to lay out the board with overlapping footprints of both types for design flexibility. This allows substitution of a low-noise shielded inductor, in the event that noise from low-cost bobbin models is unacceptable.

9.2.2.3 Input Capacitor Selection

A ceramic input capacitor of 4.7 µF, 6.3 V is sufficient for most applications. Place the input capacitor as close as possible to the VIN pin of the device. A larger value may be used for improved input voltage filtering. Use X7R or X5R types; do not use Y5V. DC bias characteristics of ceramic capacitors must be considered when selecting case sizes like 0805 and 0603. The minimum input capacitance to ensure good performance is 2.2 µF at 3-V DC bias; 1.5 µF at 5-V DC bias including tolerances and over ambient temperature range. The input filter capacitor supplies current to the PFET switch of the LM3673 in the first half of each cycle and reduces voltage ripple imposed on the input power source. The low ESR of a ceramic capacitor provides the best noise filtering of the input voltage spikes due to this rapidly changing current. Select a capacitor with sufficient ripple current rating. The input current ripple can be calculated as:

Equation 8. LM3673 20183326.gif

Table 3. Suggested Inductors and Their Suppliers

MODEL VENDOR DIMENSIONS L × W × H (mm) DCR (maximum)
COIL
BRL2518T2R2M Taiyo Yuden 2.5 × 1.8 × 1.2 135 mΩ
DO3314-222MX Coilcraft 3.3 × 3.3 × 1.4 200 mΩ
LPO3310-222MX Coilcraft 3.3 × 3.3 × 1 150 mΩ
CDRH2D14-2R2 Sumida 3.2 × 3.2 × 1.55 94 mΩ
CHIP
KSLI-2520101AG2R2 Hitachi Metals 2.5 × 2 × 1.0 115 mΩ
LQM31PN2R2M00 Murata 3.2 × 1.6 × 0.95 220 mΩ
LQM2HPN2R2MJ0 Murata 2.5 × 2 × 1.2 160 mΩ

9.2.2.4 Output Capacitor Selection

A ceramic output capacitor of 10 µF, 6.3 V is sufficient for most applications. Use X7R or X5R types; do not use Y5V. DC bias characteristics of ceramic capacitors must be considered when selecting case sizes like 0805 and 0603. DC bias characteristics vary from manufacturer to manufacturer and DC bias curves should be requested from them as part of the capacitor selection process.

The minimum output capacitance to ensure good performance is 5.75 µF at 1.8-V DC bias including tolerances and over ambient temperature range. The output filter capacitor smoothes out current flow from the inductor to the load, helps maintain a steady output voltage during transient load changes and reduces output voltage ripple. These capacitors must be selected with sufficient capacitance and sufficiently low ESR to perform these functions.

The output voltage ripple is caused by the charging and discharging of the output capacitor and by the RESR and can be calculated as:

Voltage peak-to-peak ripple due to capacitance can be expressed as follows:

Equation 9. LM3673 20183327.gif

Voltage peak-to-peak ripple due to ESR can be expressed as follows:

VPP-ESR = (2 × IRIPPLE) × RESR

Because these two components are out of phase the root mean squared (RMS) value can be used to get an approximate value of peak-to-peak ripple.

The peak-to-peak ripple voltage, rms value can be expressed as follow:

Equation 10. LM3673 20183328.gif

The output voltage ripple is dependent on the inductor current ripple and the equivalent series resistance of the output capacitor (RESR).

The RESR is frequency dependent (as well as temperature dependent); make sure the value used for calculations is at the switching frequency of the device.

Table 4. Suggested Capacitors and Their Suppliers

MODEL TYPE VENDOR VOLTAGE RATING CASE SIZE inch (mm)
4.7 µF for CIN
C2012X5R0J475K Ceramic, X5R TDK 6.3 V 0805 (2012)
JMK212BJ475K Ceramic, X5R Taiyo-Yuden 6.3 V 0805 (2012)
GRM21BR60J475K Ceramic, X5R Murata 6.3 V 0805 (2012)
C1608X5R0J475K Ceramic, X5R TDK 6.3 V 0603 (1608)
10 µF for COUT
GRM21BR60J106K Ceramic, X5R Murata 6.3 V 0805 (2012)
JMK212BJ106K Ceramic, X5R Taiyo-Yuden 6.3 V 0805 (2012)
C2012X5R0J106K Ceramic, X5R TDK 6.3 V 0805 (2012)
C1608X5R0J106K Ceramic, X5R TDK 6.3 V 0603 (1608)

9.2.3 Application Curves

LM3673 20183308.gif
VOUT = 1.2 V L = 2.2 µH DCR = 200 mΩ
Figure 19. Efficiency vs. Output Current)
LM3673 20183309.gif
VOUT = 1.8 V L = 2.2 µH DCR = 200 mΩ
Figure 21. Efficiency vs. Output Current
LM3673 20183342.png
VOUT-ADJ = 3.3 V L= 2.2 µH DCR = 200 mΩ
Figure 23. Efficiency vs. Output Current
LM3673 20183319.png
VOUT = 1.5 V (PWM Mode)
Figure 25. Load Transient Response
LM3673 20183314.png
VOUT = 1.5 V PFM Mode 0.5 mA to 50 mA
Figure 27. Load Transient Response
LM3673 20183399.png
VOUT = 1.5 V L = 2.2 µH DCR = 200 mΩ
Figure 20. Efficiency vs. Output Current
LM3673 20183341.png
VOUT-ADJ = 1.1 V L= 2.2 µH DCR = 200 mΩ
Figure 22. Efficiency vs. Output Current
LM3673 20183312.png
VOUT = 1.5 V (PWM Mode)
Figure 24. Line Transient Response
LM3673 20183315.png
VOUT = 1.5 V PFM Mode 0.5 mA to 50 mA
Figure 26. Load Transient Response