SNVS405G December   2005  – April 2015 LM3674

PRODUCTION DATA.  

  1. Features
  2. Applications
  3. Description
  4. Revision History
  5. Pin Configuration and Functions
  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 Dissipation Ratings
    6. 6.6 Electrical Characteristics
    7. 6.7 Typical Characteristics
  7. Detailed Description
    1. 7.1 Overview
    2. 7.2 Functional Block Diagram
    3. 7.3 Feature Description
      1. 7.3.1 Circuit Operation
      2. 7.3.2 PWM Operation
        1. 7.3.2.1 Internal Synchronous Rectification
        2. 7.3.2.2 Current Limiting
    4. 7.4 Device Functional Modes
  8. Application and Implementation
    1. 8.1 Application Information
      1. 8.1.1 Soft-Start
      2. 8.1.2 Low-Dropout (LDO) Operation
    2. 8.2 Typical Applications
      1. 8.2.1 Typical Application for Fixed Voltage Configuration
        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 Selection
          3. 8.2.1.2.3 Output Capacitor Selection
        3. 8.2.1.3 Application Curves
      2. 8.2.2 Typical Application Circuit for Adjustable Voltage Option
        1. 8.2.2.1 Design Requirements
        2. 8.2.2.2 Detailed Design Procedure
          1. 8.2.2.2.1 Output Voltage Selection for Adjustable (LM3674-ADJ)
        3. 8.2.2.3 Application Curves
  9. Power Supply Recommendations
  10. 10Layout
    1. 10.1 Layout Guidelines
    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 Trademarks
    3. 11.3 Electrostatic Discharge Caution
    4. 11.4 Glossary
  12. 12Mechanical, Packaging, and Orderable Information

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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

8.1.1 Soft-Start

The LM3674 has a soft-start circuit that limits in-rush current during start-up. During start-up the switch current limit is increased in steps. Soft-start is activated only if EN goes from logic low to logic high after VIN reaches 2.7 V. Soft-start is implemented by increasing switch current limit in steps of 70 mA, 140 mA, 280 mA, and 1020 mA (typical switch current limit). The start-up time thereby depends on the output capacitor and load current demanded at start-up. Typical start-up times with 10-µF output capacitor and a 300-mA load current is 350 µs and with a 10-mA load current is 240 µs.

8.1.2 Low-Dropout (LDO) Operation

The LM3674-ADJ can operate at 100% duty-cycle (no switching, PMOS switch completely on) for low-dropout support of the output voltage. In this way the output voltage will be controlled down to the lowest possible input voltage. When the device operates near 100% duty-cycle, the output voltage supply ripple is slightly higher, approximately 25 mV.

The minimum input voltage needed to support the output voltage is:

Equation 3. VIN,MIN = ILOAD × (RDSON (P) + RINDUCTOR) + VOUT

where

  • ILOAD is load current
  • RDSON (P) is drain-to-source resistance of PFET switch in the triode region
  • RINDUCTOR is inductor resistance

8.2 Typical Applications

8.2.1 Typical Application for Fixed Voltage Configuration

LM3674 20167201.gifFigure 19. Fixed-Voltage Typical Application Circuit

8.2.1.1 Design Requirements

DESIGN PARAMETER EXAMPLE VALUE
Input voltage 3.6 V
Output voltage 1.5 V
Output current 300 mA

8.2.1.2 Detailed Design Procedure

8.2.1.2.1 Inductor Selection

There are two main considerations when choosing an inductor:

  • The inductor should not saturate.
  • The inductor current ripple should 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 the 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 should be preferred.

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

Method 1:

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

Equation 4. LM3674 20167241.gif
Equation 5. LM3674 20167242.gif

where

  • IRIPPLE is average-to-peak inductor current
  • IOUTMAX is maximum load current (600 mA)
  • VIN is maximum input voltage in application
  • L is minimum inductor value including worst case tolerances (30% drop can be considered for method 1
  • f is minimum switching frequency (1.6 MHz)
  • VOUT is output voltage

Method 2:

A more conservative and recommended approach is to choose an inductor that has saturation current rating greater than the maximum current limit of 1200 mA.

A 2.2-µH inductor with a saturation current rating of at least 1200 mA is recommended for most applications. The resistance of the inductor should be less than 0.3 Ω for good efficiency. Table 1 lists suggested inductors and suppliers. For low-cost applications, an unshielded bobbin inductor is suggested. For noise critical applications, a toroidal or shielded-bobbin inductor should 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 toroidal inductor in the event that noise from low-cost bobbin models is unacceptable.

Table 1. Suggested Inductors and Their Suppliers

MODEL VENDOR DIMENSIONS L×W×H (mm) D.C.R (maximum) (mΩ)
DO3314-222MX Coilcraft 3.3 x 3.3 x 1.4 200
LPO3310-222MX Coilcraft 3.3 x 3.3 x 1.0 150
ELL5GM2R2N Panasonic 5.2 x 5.2 x 1.5 53
CDRH2D14NP-2R2NC Sumida 3.2 x 3.2 x 1.55 94

8.2.1.2.2 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 LM3674 in the first half of each cycle and reduces voltage ripple imposed on the input power source. The low equivalent series resistance (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 6. LM3674 20167226.gif

8.2.1.2.3 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:

Equation 7. LM3674 20167227.gif

Voltage peak-to-peak ripple due to ESR:

Equation 8. LM3674 20167243.gif

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.

Voltage peak-to-peak ripple, rms:

Equation 9. LM3674 20167228.gif

Note that the output ripple is dependent on the 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 part.

Table 2. Suggested Capacitors and Their Suppliers

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

8.2.1.3 Application Curves

Table 3. Related Plots

PLOT TITLE FIGURE
Output Voltage vs Supply Voltage Figure 4
Output Voltage vs Temperature Figure 5
Output Voltage vs Output Current Figure 6
Efficiency vs Output Current Figure 9
Efficiency vs Output Current Figure 10
Efficiency vs Output Current Figure 11
Efficiency vs Output Current Figure 12
Line Transient Response Figure 14
Load Transient Figure 15
Start-Up Figure 16
Start-Up Figure 17

8.2.2 Typical Application Circuit for Adjustable Voltage Option

LM3674 20167230.gifFigure 20. Typical Application Circuit for Adjustable Voltage Option Schematic

8.2.2.1 Design Requirements

DESIGN PARAMETER EXAMPLE VALUE
Output voltage 1.5 V

8.2.2.2 Detailed Design Procedure

8.2.2.2.1 Output Voltage Selection for Adjustable (LM3674-ADJ)

The output voltage of the adjustable parts can be programmed through the resistor network connected from VOUT to FB then to GND. VOUT will be adjusted to make FB equal to 0.5 V. The resistor from FB to GND (R2) should be 200 kΩ to keep the current drawn through this network small but large enough that it is not susceptible to noise. If R2 is 200 kΩ, and given the VFB is 0.5 V, then the current through the resistor feedback network will be 2.5 µA. The output voltage formula is:

Equation 10. LM3674 20167238.gif

where

  • VOUT = Output voltage (V)
  • VFB = Feedback voltage (0.5 V typical)
  • R1 = Resistor from VOUT to FB (Ω)
  • R2 = Resistor from FB to GND (Ω)

For any output voltage greater than or equal to 1.0 V, a frequency zero must be added at 45 kHz for stability. The formula is:

Equation 11. LM3674 20167239.gif

For output voltages greater than or equal to 2.5 V, a pole must also 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 12. LM3674 20167240.gif

The formula for location of zero and pole frequency created by adding C1,C2 are given below. It can be seen that by adding C1, a zero as well as a higher frequency pole is introduced.

Equation 13. LM3674 20167245.gifLM3674 20167246.gif

See Table 4.

Table 4. Adjustable LM3674 Configurations for Various VOUT

VOUT (V) R1 (kΩ) R2 (kΩ) C1 (pF) C2 (pF) L (µH) CIN (µF) COUT (µF)
1.0 200 200 18 None 2.2 4.7 10
1.1 191 158 18 None 2.2 4.7 10
1.2 280 200 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

8.2.2.3 Application Curves

Table 5. Related Plots

PLOT TITLE FIGURE
Output Voltage vs Supply Voltage Figure 4
Output Voltage vs Temperature Figure 5
Output Voltage vs Output Current Figure 6
Efficiency vs Output Current Figure 9
Efficiency vs Output Current Figure 10
Efficiency vs Output Current Figure 11
Efficiency vs Output Current Figure 12
Line Transient Response Figure 14
Load Transient Figure 15
Start-Up Figure 16
Start-Up Figure 17