SNVS008L September   1998  – June 2016 LM2671

PRODUCTION DATA.  

  1. Features
  2. Applications
  3. Description
  4. Revision History
  5. Description (continued)
  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 - 3.3 V
    6. 7.6  Electrical Characteristics - 5 V
    7. 7.7  Electrical Characteristics - 12 V
    8. 7.8  Electrical Characteristics - Adjustable
    9. 7.9  Electrical Characteristics - All Output Voltage Versions
    10. 7.10 Typical Characteristics
  8. Detailed Description
    1. 8.1 Overview
    2. 8.2 Functional Block Diagram
    3. 8.3 Feature Description
      1. 8.3.1 Switch Output
      2. 8.3.2 Input
      3. 8.3.3 C Boost
      4. 8.3.4 Ground
      5. 8.3.5 Sync
      6. 8.3.6 Feedback
      7. 8.3.7 ON/OFF
    4. 8.4 Device Functional Modes
      1. 8.4.1 Shutdown Mode
      2. 8.4.2 Active Mode
  9. Application and Implementation
    1. 9.1 Application Information
    2. 9.2 Typical Applications
      1. 9.2.1 Fixed Output Voltage Version
        1. 9.2.1.1 Design Requirements
        2. 9.2.1.2 Detailed Design Procedure
          1. 9.2.1.2.1 Inductor Selection (L1)
          2. 9.2.1.2.2 Output Capacitor Selection (COUT)
          3. 9.2.1.2.3 Catch Diode Selection (D1)
          4. 9.2.1.2.4 Input Capacitor (CIN)
          5. 9.2.1.2.5 Boost Capacitor (CB)
          6. 9.2.1.2.6 Soft-Start Capacitor (CSS) - Optional
          7. 9.2.1.2.7 Frequency Synchronization (optional)
        3. 9.2.1.3 Application Curves
      2. 9.2.2 Adjustable Output Voltage Version
        1. 9.2.2.1 Design Requirements
        2. 9.2.2.2 Detailed Design Procedure
          1. 9.2.2.2.1 Programming Output Voltage
          2. 9.2.2.2.2 Inductor Selection (L1)
          3. 9.2.2.2.3 Output Capacitor Selection (COUT)
          4. 9.2.2.2.4 Catch Diode Selection (D1)
          5. 9.2.2.2.5 Input Capacitor (CIN)
          6. 9.2.2.2.6 Boost Capacitor (CB)
        3. 9.2.2.3 Application Curves
  10. 10Power Supply Recommendations
  11. 11Layout
    1. 11.1 Layout Guidelines
    2. 11.2 Layout Examples
  12. 12Device and Documentation Support
    1. 12.1 Documentation Support
      1. 12.1.1 Related Documentation
    2. 12.2 Receiving Notification of Documentation Updates
    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
    1. 13.1 DAP (WSON Package)

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订购信息

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 LM2671 is a step-down DC-DC regulator. The device is typically used to convert a higher DC voltage to a lower DC voltage with a maximum output current of 0.5 A. The following design procedure can be used to select components for the LM2671. Alternately, the WEBENCH® software may be used to generate complete designs. When generating a design, the WEBENCH software uses iterative design procedure and accesses comprehensive databases of components. See ti.com for more details.

When the output voltage is greater than approximately 6 V, and the duty cycle at minimum input voltage is greater than approximately 50%, the designer must exercise caution in selection of the output filter components. When an application designed to these specific operating conditions is subjected to a current limit fault condition, it may be possible to observe a large hysteresis in the current limit. This can affect the output voltage of the device until the load current is reduced sufficiently to allow the current limit protection circuit to reset itself.

Under current limiting conditions, the LM267x is designed to respond in the following manner:

  1. At the moment when the inductor current reaches the current limit threshold, the ON-pulse is immediately terminated. This happens for any application condition.
  2. However, the current limit block is also designed to momentarily reduce the duty cycle to below 50% to avoid subharmonic oscillations, which could cause the inductor to saturate.
  3. Therefore, once the inductor current falls below the current limit threshold, there is a small relaxation time during which the duty cycle progressively rises back above 50% to the value required to achieve regulation.

If the output capacitance is sufficiently large, it might be possible that as the output tries to recover, the output capacitor charging current is large enough to repeatedly re-trigger the current limit circuit before the output has fully settled. This condition is exacerbated with higher output voltage settings because the energy requirement of the output capacitor varies as the square of the output voltage (½ CV2), thus requiring an increased charging current. A simple test to determine if this condition might exist for a suspect application is to apply a short circuit across the output of the converter, and then remove the shorted output condition. In an application with properly selected external components, the output recovers smoothly. Practical values of external components that have been experimentally found to work well under these specific operating conditions are COUT = 47 µF, L = 22 µH.

NOTE

Even with these components, for a device’s current limit of ICLIM, the maximum load current under which the possibility of the large current limit hysteresis can be minimized is ICLIM/2.

For example, if the input is 24 V and the set output voltage is 18 V, then for a desired maximum current of 1.5 A, the current limit of the chosen switcher must be confirmed to be at least 3 A. Under extreme overcurrent or short-circuit conditions, the LM267X employs frequency foldback in addition to the current limit. If the cycle-by-cycle inductor current increases above the current limit threshold (due to short circuit or inductor saturation for example) the switching frequency is automatically reduced to protect the IC. Frequency below 100 kHz is typical for an extreme short-circuit condition.

9.2 Typical Applications

9.2.1 Fixed Output Voltage Version

LM2671 10004222.png
CIN = 22-μF, 50-V Tantalum, Sprague 199D Series
COUT = 47-μF, 25-V Tantalum, Sprague 595D Series
D1 = 3.3-A, 50-V Schottky Rectifier, IR 30WQ05F
L1 = 68-μH Sumida #RCR110D-680L
CB = 0.01-μF, 50-V ceramic
Figure 15. Typical Application for Fixed Output Voltage Versions

9.2.1.1 Design Requirements

Table 1 lists the design parameters for this example.

Table 1. Design Parameters

PARAMETER VALUE
Regulated output voltage (3.3 V, 5 V, or 12 V), VOUT 5 V
Maximum DC input voltage, VIN(max) 12 V
Maximum load current, ILOAD(max) 500 mA

9.2.1.2 Detailed Design Procedure

9.2.1.2.1 Inductor Selection (L1)

  1. Select the correct inductor value selection guide from Figure 17 and Figure 18 or Figure 19 (output voltages of 3.3 V, 5 V, or 12 V respectively). For all other voltages, see the design procedure for the adjustable version. Use the inductor selection guide for the 5-V version shown in Figure 18.
  2. From the inductor value selection guide, identify the inductance region intersected by the maximum input voltage line and the maximum load current line. Each region is identified by an inductance value and an inductor code (LXX). From the inductor value selection guide shown in Figure 18, the inductance region intersected by the 12-V horizontal line and the 500-mA vertical line is 47 μH, and the inductor code is L13.
  3. Select an appropriate inductor from the four manufacturer's part numbers listed in Table 2. Each manufacturer makes a different style of inductor to allow flexibility in meeting various design requirements. See the following for some of the differentiating characteristics of each manufacturer's inductors:
    • Schottky: ferrite EP core inductors; these have very low leakage magnetic fields to reduce electro-magnetic interference (EMI) and are the lowest power loss inductors
    • Renco: ferrite stick core inductors; benefits are typically lowest cost inductors and can withstand E•T and transient peak currents above rated value. Be aware that these inductors have an external magnetic field which may generate more EMI than other types of inductors.
    • Pulse: powered iron toroid core inductors; these can also be low cost and can withstand larger than normal E•T and transient peak currents. Toroid inductors have low EMI.
    • Coilcraft: ferrite drum core inductors; these are the smallest physical size inductors, available only as SMT components. Be aware that these inductors also generate EMI—but less than stick inductors.

Complete specifications for these inductors are available from the respective manufacturers.

The inductance value required is 47 μH. From the table in Table 2, go to the L13 line and choose an inductor part number from any of the four manufacturers shown. In most instances, both through hole and surface mount inductors are available.

Table 2. Inductor Manufacturers' Part Numbers

IND.
REF.
DESG.
INDUCTANCE
(μH)
CURRENT
(A)
SCHOTTKY RENCO PULSE ENGINEERING COILCRAFT
THROUGH HOLE SURFACE MOUNT THROUGH HOLE SURFACE MOUNT THROUGH HOLE SURFACE MOUNT SURFACE MOUNT
L2 150 0.21 67143920 67144290 RL-5470-4 RL1500-150 PE-53802 PE-53802-S DO1608-154
L3 100 0.26 67143930 67144300 RL-5470-5 RL1500-100 PE-53803 PE-53803-S DO1608-104
L4 68 0.32 67143940 67144310 RL-1284-68-43 RL1500-68 PE-53804 PE-53804-S DO1608-683
L5 47 0.37 67148310 67148420 RL-1284-47-43 RL1500-47 PE-53805 PE-53805-S DO1608-473
L6 33 0.44 67148320 67148430 RL-1284-33-43 RL1500-33 PE-53806 PE-53806-S DO1608-333
L7 22 0.52 67148330 67148440 RL-1284-22-43 RL1500-22 PE-53807 PE-53807-S DO1608-223
L9 220 0.32 67143960 67144330 RL-5470-3 RL1500-220 PE-53809 PE-53809-S DO3308-224
L10 150 0.39 67143970 67144340 RL-5470-4 RL1500-150 PE-53810 PE-53810-S DO3308-154
L11 100 0.48 67143980 67144350 RL-5470-5 RL1500-100 PE-53811 PE-53811-S DO3308-104
L12 68 0.58 67143990 67144360 RL-5470-6 RL1500-68 PE-53812 PE-53812-S DO3308-683
L13 47 0.7 67144000 67144380 RL-5470-7 RL1500-47 PE-53813 PE-53813-S DO3308-473
L14 33 0.83 67148340 67148450 RL-1284-33-43 RL1500-33 PE-53814 PE-53814-S DO3308-333
L15 22 0.99 67148350 67148460 RL-1284-22-43 RL1500-22 PE-53815 PE-53815-S DO3308-223
L18 220 0.55 67144040 67144420 RL-5471-2 RL1500-220 PE-53818 PE-53818-S DO3316-224
L19 150 0.66 67144050 67144430 RL-5471-3 RL1500-150 PE-53819 PE-53819-S DO3316-154
L20 100 0.82 67144060 67144440 RL-5471-4 RL1500-100 PE-53820 PE-53820-S DO3316-104
L21 68 0.99 67144070 67144450 RL-5471-5 RL1500-68 PE-53821 PE-53821-S DO3316-683

9.2.1.2.2 Output Capacitor Selection (COUT)

Select an output capacitor from the output capacitor table in Table 9. Using the output voltage and the inductance value found in the inductor selection guide, step 1, locate the appropriate capacitor value and voltage rating.

Use the 5-V section in the output capacitor table in Table 9. Choose a capacitor value and voltage rating from the line that contains the inductance value of 47 μH. The capacitance and voltage rating values corresponding to the 47-μH inductor are:

  • Surface mount:
    • 68-μF, 10-V Sprague 594D series
    • 100-μF, 10-V AVX TPS series
  • Through hole:
    • 68-μF, 10-V Sanyo OS-CON SA series
    • 150-μF, 35-V Sanyo MV-GX series
    • 150-μF, 35-V Nichicon PL series
    • 150-μF, 35-V Panasonic HFQ series

The capacitor list contains through-hole electrolytic capacitors from four different capacitor manufacturers and surface mount tantalum capacitors from two different capacitor manufacturers. TI recommends that both the manufacturers and the manufacturer's series that are listed in the table be used.

Table 3. Output Capacitor Table

OUTPUT
VOLTAGE
(V)
INDUCTANCE
(μH)
OUTPUT CAPACITOR
SURFACE MOUNT THROUGH HOLE
SPRAGUE 594D SERIES (μF/V) AVX TPS SERIES (μF/V) SANYO OS-CON SA SERIES (μF/V) SANYO MV-GX SERIES (μF/V) NICHICON PL SERIES (μF/V) PANASONIC HFQ SERIES (μF/V)
3.3 22 120/6.3 100/10 100/10 330/35 330/35 330/35
33 120/6.3 100/10 68/10 220/35 220/35 220/35
47 68/10 100/10 68/10 150/35 150/35 150/35
68 120/6.3 100/10 100/10 120/35 120/35 120/35
100 120/6.3 100/10 100/10 120/35 120/35 120/35
150 120/6.3 100/10 100/10 120/35 120/35 120/35
5 22 100/16 100/10 100/10 330/35 330/35 330/35
33 68/10 10010 68/10 220/35 220/35 220/35
47 68/10 100/10 68/10 150/35 150/35 150/35
68 100/16 100/10 100/10 120/35 120/35 120/35
100 100/16 100/10 100/10 120/35 120/35 120/35
150 100/16 100/10 100/10 120/35 120/35 120/35
12 22 120/20 (2×) 68/20 68/20 330/35 330/35 330/35
33 68/25 68/20 68/20 220/35 220/35 220/35
47 47/20 68/20 47/20 150/35 150/35 150/35
68 47/20 68/20 47/20 120/35 120/35 120/35
100 47/20 68/20 47/20 120/35 120/35 120/35
150 47/20 68/20 47/20 120/35 120/35 120/35
220 47/20 68/20 47/20 120/35 120/35 120/35

9.2.1.2.3 Catch Diode Selection (D1)

  1. In normal operation, the average current of the catch diode is the load current times the catch diode duty cycle, 1-D (D is the switch duty cycle, which is approximately the output voltage divided by the input voltage). The largest value of the catch diode average current occurs at the maximum load current and maximum input voltage (minimum D). For normal operation, the catch diode current rating must be at least 1.3 times greater than its maximum average current. However, if the power supply design must withstand a continuous output short, the diode must have a current rating equal to the maximum current limit of the LM2671. The most stressful condition for this diode is a shorted output condition (refer to Table 4). In this example, a 1-A, 20-V Schottky diode provides the best performance. If the circuit must withstand a continuous shorted output, TI recommends a higher-current Schottky diode.
  2. The reverse voltage rating of the diode must be at least 1.25 times the maximum input voltage.
  3. Because of their fast switching speed and low forward voltage drop, Schottky diodes provide the best performance and efficiency. This Schottky diode must be placed close to the LM2671 using short leads and short printed-circuit traces.

Table 4. Schottky Diode Selection Table

VR 1-A DIODES 3-A DIODES
SURFACE MOUNT THROUGH HOLE SURFACE MOUNT THROUGH HOLE
20 V SK12 1N5817 SK32 1N5820
B120 SR102 SR302
30 V SK13 1N5818 SK33 1N5821
B130 11DQ03 30WQ03F 31DQ03
MBRS130 SR103
40 V SK14 1N5819 SK34 1N5822
B140 11DQ04 30BQ040 MBR340
MBRS140 SR104 30WQ04F 31DQ04
10BQ040 MBRS340 SR304
10MQ040 MBRD340
15MQ040
50 V SK15 MBR150 SK35 MBR350
B150 11DQ05 30WQ05F 31DQ05
10BQ050 SR105 SR305

9.2.1.2.4 Input Capacitor (CIN)

A low ESR aluminum or tantalum bypass capacitor is required between the input pin and ground to prevent large voltage transients from appearing at the input. This capacitor must be placed close to the IC using short leads. In addition, the RMS current rating of the input capacitor must be selected to be at least ½ the DC load current. The capacitor manufacturer data sheet must be checked to assure that this current rating is not exceeded. The curves shown in Figure 16 show typical RMS current ratings for several different aluminum electrolytic capacitor values. A parallel connection of two or more capacitors may be required to increase the total minimum RMS current rating to suit the application requirements.

For an aluminum electrolytic capacitor, the voltage rating must be at least 1.25 times the maximum input voltage. Caution must be exercised if solid tantalum capacitors are used. The tantalum capacitor voltage rating must be twice the maximum input voltage. Table 5 and Table 6 show the recommended application voltage for AVX TPS and Sprague 594D tantalum capacitors. TI also recommends that they be surge current tested by the manufacturer. The TPS series available from AVX, and the 593D and 594D series from Sprague are all surge current tested. Another approach to minimize the surge current stresses on the input capacitor is to add a small inductor in series with the input supply line.

Table 5. AVX TPS

RECOMMENDED
APPLICATION VOLTAGE
VOLTAGE
RATING
85°C RATING
3.3 6.3
5 10
10 20
12 25
15 35

Table 6. Sprague 594D

RECOMMENDED
APPLICATION VOLTAGE
VOLTAGE
RATING
85°C RATING
2.5 4
3.3 6.3
5 10
8 16
12 20
18 25
24 35
29 50

Use caution when using ceramic capacitors for input bypassing, because it may cause severe ringing at the VIN pin. The important parameters for the input capacitor are the input voltage rating and the RMS current rating. With a maximum input voltage of 12 V, an aluminum electrolytic capacitor with a voltage rating greater than 15 V (1.25 × VIN) is required. The next higher capacitor voltage rating is 16 V.

The RMS current rating requirement for the input capacitor in a buck regulator is approximately ½ the DC load current. In this example, with a 500-mA load, a capacitor with a RMS current rating of at least 250 mA is required. The curves shown in Figure 16 can be used to select an appropriate input capacitor. From the curves, locate the 16-V line and note which capacitor values have RMS current ratings greater than 250 mA.

LM2671 10004233.png Figure 16. RMS Current Ratings for Low ESR Electrolytic Capacitors (Typical)

For a through-hole design, a 100-μF, 16-V electrolytic capacitor (Panasonic HFQ series, Nichicon PL, Sanyo MV-GX series or equivalent) would be adequate. Other types or other manufacturers' capacitors can be used provided the RMS ripple current ratings are adequate. Additionally, for a complete surface mount design, electrolytic capacitors such as the Sanyo CV-C or CV-BS and the Nichicon WF or UR and the NIC Components NACZ series could be considered.

For surface mount designs, solid tantalum capacitors can be used, but caution must be exercised with regard to the capacitor surge current rating and voltage rating. In this example, checking the Sprague 594D series datasheet, a Sprague 594D 15-μF, 25-V capacitor is adequate.

9.2.1.2.5 Boost Capacitor (CB)

This capacitor develops the necessary voltage to turn the switch gate on fully. All applications must use a
0.01-μF, 50-V ceramic capacitor. For this application, and all applications, use a 0.01-μF, 50-V ceramic capacitor.

9.2.1.2.6 Soft-Start Capacitor (CSS) – Optional

This capacitor controls the rate at which the device starts up. The formula for the soft-start capacitor CSS is Equation 1.

Equation 1. LM2671 10004226.png

where

  • ISS= soft-start current (4.5 μA typical)
  • tSS= soft-start time (selected)
  • VSSTH= soft-start threshold voltage (0.63 V typical)
  • VOUT= output voltage (selected)
  • VSCHOTTKY= schottky diode voltage drop (0.4 V typical)
  • VIN= input voltage (selected)

For this application, selecting a start-up time of 10 ms and using Equation 2 for CSS.

Equation 2. LM2671 10004228.png

If this feature is not desired, leave this pin open. With certain soft-start capacitor values and operating conditions, the LM2671 can exhibit an overshoot on the output voltage during turnon. Especially when starting up into no load or low load, the soft-start function may not be effective in preventing a larger voltage overshoot on the output. With larger loads or lower input voltages during start-up this effect is minimized. In particular, avoid using soft-start capacitors between 0.033 µF and 1 µF.

9.2.1.2.7 Frequency Synchronization (optional)

The LM2671 (oscillator) can be synchronized to run with an external oscillator, using the sync pin (pin 3). By doing so, the LM2671 can be operated at higher frequencies than the standard frequency of 260 kHz. This allows for a reduction in the size of the inductor and output capacitor.

As shown in the drawing below, a signal applied to a RC filter at the sync pin causes the device to synchronize to the frequency of that signal. For a signal with a peak-to-peak amplitude of 3 V or greater, a 1-kΩ resistor and a 100-pF capacitor are suitable values.

LM2671 10004227.png

For all applications, use a 1-kΩ resistor and a 100-pF capacitor for the RC filter.

9.2.1.3 Application Curves

for continuous mode operation

LM2671 10004229.png Figure 17. LM2671-3.3
LM2671 10004231.png Figure 19. LM2671-12
LM2671 10004230.png Figure 18. LM2671-5
LM2671 10004232.png Figure 20. LM2671-ADJ

9.2.2 Adjustable Output Voltage Version

LM2671 10004223.png
CIN = 22-μF, 50-V Tantalum, Sprague 199D Series
COUT = 47-μF, 25-V Tantalum, Sprague 595D Series
D1 = 3.3-A, 50-V Schottky Rectifier, IR 30WQ05F
L1 = 68-μH Sumida #RCR110D-680L
R1 =1.5 kΩ, 1%
CB = 0.01-μF, 50-V ceramic
Figure 21. Typical Application for Adjustable Output Voltage Versions

9.2.2.1 Design Requirements

Table 7 lists the design parameters for this example.

Table 7. Design Parameters

PARAMETER VALUE
Regulated output voltage, VOUT 20 V
Maximum input voltage, VIN(max) 28 V
Maximum load current, ILOAD(max) 500 mA
Switching frequency, F Fixed at a nominal 260 kHz

9.2.2.2 Detailed Design Procedure

9.2.2.2.1 Programming Output Voltage

Select R1 and R2, as shown in Figure 21.

Use the following formula to select the appropriate resistor values.

Equation 3. LM2671 10004234.png

where

  • VREF = 1.21 V

Select R1 to be 1 kΩ, 1%. Solve for R2.

Equation 4. LM2671 10004237.png

Select a value for R1 between 240 Ω and 1.5 kΩ. The lower resistor values minimize noise pickup in the sensitive feedback pin. For the lowest temperature coefficient and the best stability with time, use 1% metal film resistors.

Equation 5. LM2671 10004235.png

R2 = 1 kΩ (16.53 − 1) = 15.53 kΩ, closest 1% value is 15.4 kΩ.

R2 = 15.4 kΩ.

9.2.2.2.2 Inductor Selection (L1)

  1. Calculate the inductor Volt • microsecond constant E • T (V • μs) from Equation 6.
  2. Equation 6. LM2671 10004236.png

    where

    • VSAT = internal switch saturation voltage = 0.25 V
    • VD = diode forward voltage drop = 0.5 V

    Calculate the inductor Volt • microsecond constant (E • T) with Equation 7.

    Equation 7. LM2671 10004238.png
  3. Use the E • T value from the previous formula and match it with the E • T number on the vertical axis of the inductor value selection guide shown in Figure 20.
  4. Equation 8. E • T = 21.6 (V • μs)
  5. On the horizontal axis, select the maximum load current in Equation 9.
  6. Equation 9. ILOAD(max) = 500 mA
  7. Identify the inductance region intersected by the E • T value and the maximum load current value. Each region is identified by an inductance value and an inductor code (LXX). From the inductor value selection guide shown in Figure 20, the inductance region intersected by the 21.6 (V • μs) horizontal line and the 500-mA vertical line is 100 μH, and the inductor code is L20.
  8. Select an appropriate inductor from the four manufacturer's part numbers listed in Table 2. For information on the different types of inductors, see the inductor selection in the fixed output voltage design procedure. From the table in Table 2, locate line L20, and select an inductor part number from the list of manufacturers' part numbers.

9.2.2.2.3 Output Capacitor Selection (COUT)

  1. Select an output capacitor from the capacitor code selection guide in Table 8. Using the inductance value found in the inductor selection guide, step 1, locate the appropriate capacitor code corresponding to the desired output voltage. Use the appropriate row of the capacitor code selection guide, in Table 8. For this example, use the 15-V to 20-V row. The capacitor code corresponding to an inductance of 100 μH is C20.
  2. Select an appropriate capacitor value and voltage rating, using the capacitor code, from the output capacitor selection table in Table 9. There are two solid tantalum (surface mount) capacitor manufacturers and four electrolytic (through hole) capacitor manufacturers to choose from. TI recommends using the manufacturers and the manufacturer's series that are listed in the table.
  3. From the output capacitor selection table in Table 9, choose a capacitor value (and voltage rating) that intersects the capacitor code(s) selected in section A, C20.

    The capacitance and voltage rating values corresponding to the capacitor code C20 are:

    • Surface mount:
      • 33-μF, 25-V Sprague 594D series
      • 33-μF, 25-V AVX TPS series
    • Through hole:
      • 33-μF, 25-V Sanyo OS-CON SC series
      • 120-μF, 35-V Sanyo MV-GX series
      • 120-μF, 35-V Nichicon PL series
      • 120-μF, 35-V Panasonic HFQ series

Other manufacturers or other types of capacitors may also be used, provided the capacitor specifications (especially the 100-kHz ESR) closely match the characteristics of the capacitors listed in the output capacitor table. See the capacitor manufacturers' data sheet for this information.

Table 8. Capacitor Code Selection Guide

CASE
STYLE (1)
OUTPUT
VOLTAGE (V)
INDUCTANCE (μH)
22 33 47 68 100 150 220
SM and TH 1.21–2.5 C1 C2 C3
SM and TH 2.5–3.75 C1 C2 C3 C3
SM and TH 3.75–5 C4 C5 C6 C6 C6
SM and TH 5–6.25 C4 C7 C6 C6 C6 C6
SM and TH 6.25–7.5 C8 C4 C7 C6 C6 C6 C6
SM and TH 7.5–10 C9 C10 C11 C12 C13 C13 C13
SM and TH 10–12.5 C14 C11 C12 C12 C13 C13 C13
SM and TH 12.5–15 C15 C16 C17 C17 C17 C17 C17
SM and TH 15–20 C18 C19 C20 C20 C20 C20 C20
SM and TH 20–30 C21 C22 C22 C22 C22 C22 C22
TH 30–37 C23 C24 C24 C25 C25 C25 C25
(1) SM - Surface Mount, TH - Through Hole

Table 9. Output Capacitor Selection Table

OUTPUT CAPACITOR
CAP.
REF.
DESG.
#
SURFACE MOUNT THROUGH HOLE
SPRAGUE 594D SERIES (μF/V) AVX TPS SERIES (μF/V) SANYO OS-CON SA SERIES (μF/V) SANYO MV-GX SERIES (μF/V) NICHICON PL SERIES (μF/V) PANASONIC HFQ SERIES (μF/V)
C1 120/6.3 100/10 100/10 220/35 220/35 220/35
C2 120/6.3 100/10 100/10 150/35 150/35 150/35
C3 120/6.3 100/10 100/35 120/35 120/35 120/35
C4 68/10 100/10 68/10 220/35 220/35 220/35
C5 100/16 100/10 100/10 150/35 150/35 150/35
C6 100/16 100/10 100/10 120/35 120/35 120/35
C7 68/10 100/10 68/10 150/35 150/35 150/35
C8 100/16 100/10 100/10 330/35 330/35 330/35
C9 100/16 100/16 100/16 330/35 330/35 330/35
C10 100/16 100/16 68/16 220/35 220/35 220/35
C11 100/16 100/16 68/16 150/35 150/35 150/35
C12 100/16 100/16 68/16 120/35 120/35 120/35
C13 100/16 100/16 100/16 120/35 120/35 120/35
C14 100/16 100/16 100/16 220/35 220/35 220/35
C15 47/20 68/20 47/20 220/35 220/35 220/35
C16 47/20 68/20 47/20 150/35 150/35 150/35
C17 47/20 68/20 47/20 120/35 120/35 120/35
C18 68/25 (2×) 33/25 47/25 (1) 220/35 220/35 220/35
C19 33/25 33/25 33/25 (1) 150/35 150/35 150/35
C20 33/25 33/25 33/25 (1) 120/35 120/35 120/35
C21 33/35 (2×) 22/25  (2) 150/35 150/35 150/35
C22 33/35 22/35  (2) 120/35 120/35 120/35
C23  (2)  (2)  (2) 220/50 100/50 120/50
C24  (2)  (2)  (2) 150/50 100/50 120/50
C25  (2)  (2)  (2) 150/50 82/50 82/50
(1) The SC series of Os-Con capacitors (others are SA series)
(2) The voltage ratings of the surface mount tantalum chip and Os-Con capacitors are too low to work at these voltages.

9.2.2.2.4 Catch Diode Selection (D1)

  1. In normal operation, the average current of the catch diode is the load current times the catch diode duty cycle, 1-D (D is the switch duty cycle, which is approximately VOUT/VIN). The largest value of the catch diode average current occurs at the maximum input voltage (minimum D). For normal operation, the catch diode current rating must be at least 1.3 times greater than its maximum average current. However, if the power supply design must withstand a continuous output short, the diode must have a current rating greater than the maximum current limit of the LM2671. The most stressful condition for this diode is a shorted output condition.
  2. Refer to the table shown in Table 4. Schottky diodes provide the best performance, and in this example a 1-A, 40-V Schottky diode would be a good choice. If the circuit must withstand a continuous shorted output, a higher current (at least 1.2 A) Schottky diode is recommended.

  3. The reverse voltage rating of the diode must be at least 1.25 times the maximum input voltage.
  4. Because of their fast switching speed and low forward voltage drop, Schottky diodes provide the best performance and efficiency. The Schottky diode must be placed close to the LM2671 using short leads and short printed-circuit traces.

9.2.2.2.5 Input Capacitor (CIN)

A low ESR aluminum or tantalum bypass capacitor is required between the input pin and ground to prevent large voltage transients from appearing at the input. This capacitor must be placed close to the IC using short leads. In addition, the RMS current rating of the input capacitor must be selected to be at least ½ the DC load current. The capacitor manufacturer data sheet must be checked to assure that this current rating is not exceeded. The curves shown in Figure 16 show typical RMS current ratings for several different aluminum electrolytic capacitor values. A parallel connection of two or more capacitors may be required to increase the total minimum RMS current rating to suit the application requirements.

For an aluminum electrolytic capacitor, the voltage rating must be at least 1.25 times the maximum input voltage. Caution must be exercised if solid tantalum capacitors are used. The tantalum capacitor voltage rating must be twice the maximum input voltage. The Table 10 and Table 11 show the recommended application voltage for AVX TPS and Sprague 594D tantalum capacitors. TI also recommends that they be surge current tested by the manufacturer. The TPS series available from AVX, and the 593D and 594D series from Sprague are all surge current tested. Another approach to minimize the surge current stresses on the input capacitor is to add a small inductor in series with the input supply line.

Table 10. AVX TPS

RECOMMENDED
APPLICATION VOLTAGE
VOLTAGE
RATING
85°C RATING
3.3 6.3
5 10
10 20
12 25
15 35

Table 11. Sprague 594D

RECOMMENDED
APPLICATION VOLTAGE
VOLTAGE
RATING
85°C RATING
2.5 4
3.3 6.3
5 10
8 16
12 20
18 25
24 35
29 50

Use caution when using ceramic capacitors for input bypassing, because it may cause severe ringing at the VIN pin.

The important parameters for the input capacitor are the input voltage rating and the RMS current rating. With a maximum input voltage of 28 V, an aluminum electrolytic capacitor with a voltage rating of at least
35 V (1.25 × VIN) is required.

The RMS current rating requirement for the input capacitor in a buck regulator is approximately ½ the DC load current. In this example, with a 500-mA load, a capacitor with a RMS current rating of at least 250 mA is required. The curves shown in Figure 22 can be used to select an appropriate input capacitor. From the curves, locate the 35-V line and note which capacitor values have RMS current ratings greater than 250 mA.

LM2671 10004233.png Figure 22. RMS Current Ratings for Low ESR Electrolytic Capacitors (Typical)

For a through-hole design, a 68-μF, 35-V electrolytic capacitor (Panasonic HFQ series, Nichicon PL, Sanyo MV-GX series or equivalent) would be adequate. Other types or other manufacturers' capacitors can be used provided the RMS ripple current ratings are adequate. Additionally, for a complete surface mount design, electrolytic capacitors such as the Sanyo CV-C or CV-BS and the Nichicon WF or UR and the NIC Components NACZ series could be considered.

For surface mount designs, solid tantalum capacitors can be used, but caution must be exercised with regard to the capacitor surge current rating and voltage rating. In this example, checking the Sprague 594D series data sheet, a Sprague 594D 15-μF, 50-V capacitor is adequate.

9.2.2.2.6 Boost Capacitor (CB)

This capacitor develops the necessary voltage to turn the switch gate on fully. All applications must use a
0.01-μF, 50-V ceramic capacitor. For this application, and all applications, use a 0.01-μF, 50-V ceramic capacitor.

If the soft-start and frequency synchronization features are desired, look at steps 6 and 7 in Detailed Design Procedure.

9.2.2.3 Application Curves

LM2671 10004218.png
Continuous Mode Switching Waveforms, VIN = 20 V, VOUT = 5 V,
ILOAD = 500 mA, L = 100 μH, COUT = 100 μF, COUTESR = 0.1 Ω
A: VSW pin voltage, 10 V/div.
B: Inductor current, 0.2 A/div
C: Output ripple voltage, 50 mV/div ac-coupled
Figure 23. Horizontal Time Base: 1 μs/div
LM2671 10004220.png
Load Transient Response for Continuous Mode, VIN = 20 V,
VOUT = 5 V, L = 100 μH, COUT = 100 μF, COUTESR = 0.1 Ω
A: Output voltage, 100 mV/div, ac-coupled
B: Load current: 100-mA to 500-mA load pulse
Figure 25. Horizontal Time Base: 50 μs/div
LM2671 10004219.png
Discontinuous Mode Switching Waveforms, VIN = 20 V,
VOUT = 5 V, ILOAD = 300 mA, L = 15 μH, COUT = 68 μF (2×), COUTESR = 25 mΩ
A: VSW pin voltage, 10 V/div.
B: Inductor current, 0.5 A/div
C: Output ripple voltage, 20 mV/div ac-coupled
Figure 24. Horizontal Time Base: 1 μs/div
LM2671 10004221.png
Load Transient Response for Discontinuous Mode, VIN = 20 V,
VOUT = 5 V, L = 47 μH, COUT = 68 μF, COUTESR = 50 mΩ
A: Output voltage, 100 mV/div, ac-coupled
B: Load current: 100-mA to 400-mA load pulse
Figure 26. Horizontal Time Base: 200 μs/div