SNVS506J May   2008  – December 2015 LM3691

PRODUCTION DATA.  

  1. Features
  2. Applications
  3. Description
  4. Revision History
  5. Voltage Options
  6. Pin Configuration and Functions
  7. Specifications
    1. 7.1 Absolute Maximum Ratings
    2. 7.2 ESD Ratings
    3. 7.3 Recommended Operating Conditions
    4. 7.4 Thermal Information
    5. 7.5 Electrical Characteristics
    6. 7.6 Typical Characteristics
  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
        1. 8.3.2.1 Internal Synchronous Rectification
        2. 8.3.2.2 Current Limiting
      3. 8.3.3 ECO Operation
      4. 8.3.4 Soft-Start
      5. 8.3.5 Thermal Shutdown Protection
      6. 8.3.6 Overtemperature Maximum Load
    4. 8.4 Device Functional Modes
      1. 8.4.1 Forced PWM Mode
      2. 8.4.2 Shutdown Mode
  9. Application and Implementation
    1. 9.1 Application Information
    2. 9.2 Typical Application
      1. 9.2.1 Design Requirements
      2. 9.2.2 Detailed Design Procedure
        1. 9.2.2.1 Inductor Selection
        2. 9.2.2.2 Input Capacitor Selection
        3. 9.2.2.3 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 Community Resources
    4. 12.4 Trademarks
    5. 12.5 Electrostatic Discharge Caution
    6. 12.6 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 LM3691 step-down DC-DC converter is optimized for powering ultralow-voltage circuits from a single Li-Ion cell (2.7 V to 5.5 V) or 3-cell NiMH/NiCd (2.4 V to 4.5 V) batteries. It provides up to 1-A load current over an input voltage range from 2.3 V to 5.5 V. Seven different fixed voltage output options are available to cover all commonly used voltage rails (0.75 V, 1 V, 1.2 V, 1.5 V, 1.8 V, 2.5 V, 3.3 V).

9.2 Typical Application

LM3691 typapp2_snvs506.gif Figure 44. LM3691 Typical Application

9.2.1 Design Requirements

For typical step-down DC-DC applications, use the parameters listed in Table 2.

Table 2. Design Parameters

DESIGN PARAMETER EXAMPLE VALUE
Minimum input voltage 2.5 V
Minimum output voltage 1.8 V
Output current 150 mA

9.2.2 Detailed Design Procedure

9.2.2.1 Inductor Selection

DC bias current characteristics of inductors must be considered. Different manufacturers follow different saturation current rating specifications, so attention must be given to details. DC bias curves should be requested from the manufacturer as part of the inductor selection process.

Minimum value of inductance to specify good performance is 0.5 µH at 1.5 A (ILIM typical) bias current over the ambient temp range. DC resistance of the inductor must be less than 0.1 Ω for good efficiency at high-current condition. The inductor AC loss (resistance) also affects conversion efficiency. Higher Q factor at switching frequency usually gives better efficiency at light load to middle load.

Table 3 lists suggested inductors and suppliers.

Table 3. Suggested Inductors and Their Suppliers

MODEL VENDOR DIMENSIONS L x W x H (mm) DCR (mΩ)
LQM2HPN1R0MG0 Murata 2.5 × 2.0 × 1.0 55
MLP2520S1R0L TDK 2.5 × 2.0 × 1.0 60
KSLI252010BG1R0 HItachi Metals 2.5 × 2.0 × 1.0 80
MIPSZ2012D1R0 FDK 2.0 × 1.25 × 1.0 90

9.2.2.2 Input Capacitor Selection

A ceramic input capacitor of 4.7 µF, 6.3 V/10 V is sufficient for most applications. Place the input capacitor as close as possible to the VIN pin and GND pin of the device. A larger value or higher voltage rating may be used to improve input voltage filtering. Use X7R, X5R or B types; do not use Y5V or F. DC bias characteristics of ceramic capacitors must be considered when selecting case sizes like 0402. Minimum input capacitance to ensure good performance is 2.2 µF at maximum input voltage DC bias including tolerances and over ambient temperature range.

The input filter capacitor supplies current to the PFET (high-side) switch in the first half of each cycle and reduces voltage ripple imposed on the input power source. A ceramic capacitor's low ESR provides the best noise filtering of the input voltage spikes due to this rapidly changing current. Select an input filter capacitor with sufficient ripple current rating. The input current ripple can be calculated as:

Equation 1. LM3691 30013426.gif

9.2.2.3 Output Capacitor Selection

Use a 4.7-μF, 6.3-V ceramic capacitor, X7R, X5R or B types; do not use Y5V or F. DC bias voltage characteristics of ceramic capacitors must be considered. DC bias characteristics vary from manufacturer to manufacturer, and DC bias curves should be requested from the manufacturer as part of the capacitor selection process. The output filter capacitor smooths 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 equivalent series resistance (ESR) to perform these functions. Minimum output capacitance to specify good performance is 2.2 µF at the output voltage DC bias including tolerances and over ambient temperature range.

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

Voltage peak-to-peak ripple due to capacitance is shown in Equation 2:

Equation 2. LM3691 30013427.gif

Voltage peak-to-peak ripple due to ESR Equation 3:

Equation 3. VPP-ESR = (2 × IRIPPLE) × RESR

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

Voltage peak-to-peak ripple, root mean squared equals:

Equation 4. LM3691 30013428.gif

Note that the output voltage ripple is dependent on the current ripple and the ESR 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 part.

Table 4 lists suggested capacitors and suppliers.

Table 4. Suggested Capacitors and Their Suppliers

MODEL TYPE VENDOR VOLTAGE RATING (V) CASE SIZE
INCH (mm)
4.7 µF for CIN and COUT
C1608X5R0J475K Ceramic TDK 6.3 0603 (1608)
C1608X5R1A475K Ceramic TDK 10.0 0603 (1608)

9.2.3 Application Curves

LM3691 30013478.png
VOUT = 1.8 V
Figure 45. Line Transient Reponse, PWM Mode
LM3691 30013482.png
VOUT = 1.8 V ECO Mode 25 mA to 1 mA
Figure 47. Load Transient Reponse
LM3691 30013481.png
VOUT = 1.8 V ECO Mode 1 mA to 25 mA
Figure 46. Load Transient Reponse
LM3691 30013485.png
VOUT = 1.8 V ECO Mode to PWM Mode
Figure 48. Load Transient Reponse
LM3691 30013490.png
VOUT = 1.8 V FPWM Mode
Figure 49. Load Transient Reponse
LM3691 30013493.png
VOUT = 1.8 V ROUT = 1.8 kΩ
Figure 51. Start-Up Into ECO Mode
LM3691 30013490.png
VOUT = 1.8 V FPWM Mode
Figure 50. Load Transient Reponse
LM3691 30013494.png
A.
VOUT = 1.8 V ROUT = 6 Ω
Figure 52. Start-Up Into PWM Mode