JAJS908F May   2004  – June 2016 LM2675

PRODUCTION DATA.  

  1. 特長
  2. アプリケーション
  3. 概要
  4. 改訂履歴
  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
    11. 7.11 Typical Characteristics - Fixed Output Voltage Versions
  8. Detailed Description
    1. 8.1 Overview
    2. 8.2 Functional Block Diagram
    3. 8.3 Feature Description
      1. 8.3.1 Adjustable Output Voltage
    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 Application
      1. 9.2.1 Fixed Output Voltage Application
        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)
        3. 9.2.1.3 Application Curves
      2. 9.2.2 Adjustable Output Voltage Application
        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 SeIection (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 Curve
  10. 10Power Supply Recommendations
  11. 11Layout
    1. 11.1 Layout Guidelines
      1. 11.1.1 WSON Package Devices
    2. 11.2 Layout Examples
  12. 12デバイスおよびドキュメントのサポート
    1. 12.1 ドキュメントのサポート
      1. 12.1.1 関連資料
    2. 12.2 ドキュメントの更新通知を受け取る方法
    3. 12.3 コミュニティ・リソース
    4. 12.4 商標
    5. 12.5 静電気放電に関する注意事項
    6. 12.6 Glossary
  13. 13メカニカル、パッケージ、および注文情報

パッケージ・オプション

メカニカル・データ(パッケージ|ピン)
サーマルパッド・メカニカル・データ

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 LM2675 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 1 A. The following design procedure can be used to select components for the LM2675.

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 should 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 LM2675 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. Thereafter, 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 may 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. It should be noted that 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 LM2675 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 Application

9.2.1 Fixed Output Voltage Application

LM2675 01280322.gif
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 19. Fixed Output Voltage Schematic

9.2.1.1 Design Requirements

Table 1 lists the design requirements for the fixed output voltage application.

Table 1. Design Parameters

PARAMETER VALUE
Regulated output voltage, VOUT 5 V
Maximum input voltage, VIN(max) 12 V
Maximum load current, ILOAD(max) 1 A

9.2.1.2 Detailed Design Procedure

9.2.1.2.1 Inductor Selection (L1)

Select the correct inductor value selection guide from Figure 21, Figure 22, or Figure 23 (output voltages of
3.3 V, 5 V, or 12 V respectively). For all other voltages, see Detailed Design Procedure. Use the inductor selection guide for the 5-V version shown in Figure 22.

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 22, the inductance region intersected by the 12-V horizontal line and the 1-A vertical line is 33 μH, and the inductor code is L23.

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. The inductance value required is 33 μH. From the table in Table 2, go to the L23 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)
SCHOTT RENCO PULSE ENGINEERING COILCRAFT
THROUGH HOLE SURFACE MOUNT THROUGH HOLE SURFACE MOUNT THROUGH HOLE SURFACE MOUNT SURFACE MOUNT
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
L22 47 1.17 67144080 67144460 RL-5471-6 PE-53822 PE-53822-S DO3316-473
L23 33 1.4 67144090 67144470 RL-5471-7 PE-53823 PE-53823-S DO3316-333
L24 22 1.7 67148370 67148480 RL-1283-22-43 PE-53824 PE-53824-S DO3316-223
L27 220 1 67144110 67144490 RL-5471-2 PE-53827 PE-53827-S DO5022P-224
L28 150 1.2 67144120 67144500 RL-5471-3 PE-53828 PE-53828-S DO5022P-154
L29 100 1.47 67144130 67144510 RL-5471-4 PE-53829 PE-53829-S DO5022P-104
L30 68 1.78 67144140 67144520 RL-5471-5 PE-53830 PE-53830-S DO5022P-683

9.2.1.2.2 Output Capacitor Selection (COUT)

Select an output capacitor from Table 3. Using the output voltage and the inductance value found in the inductor selection guide, step 1, locate the appropriate capacitor value and voltage rating. 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 using both the manufacturers and the manufacturer's series that are listed in the table.

Use the 5-V section in Table 3. Choose a capacitor value and voltage rating from the line that contains the inductance value of 33 μH. The capacitance and voltage rating values corresponding to the 33-μH inductor are the surface mount and through hole.

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
  • 220-μF, 35-V Sanyo MV-GX series
  • 220-μF, 35-V Nichicon PL series
  • 220-μF, 35-V Panasonic HFQ series

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)

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 LM2675. The most stressful condition for this diode is a shorted output condition (see 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 Schottky diode of higher current.

The reverse voltage rating of the diode must be at least 1.25 times the maximum input voltage. Because of their fast switching speed and low forward voltage drop, Schottky diodes provide the best performance and efficiency. This Schottky diode must be located close to the LM2675 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 needed between the input pin and ground to prevent large voltage transients from appearing at the input. This capacitor must be located 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. Figure 20 shows 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.

LM2675 01280330.gif Figure 20. RMS Current Ratings for Low ESR Electrolytic Capacitors (Typical)

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 3 shows 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.

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) would be needed. 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 1-A load, a capacitor with a RMS current rating of at least 500 mA is needed. The curves shown in Figure 20 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 500 mA.

For a through hole design, a 330-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 Table 5, and the Sprague 594D series data sheet, a Sprague 594D 15-μF, 25-V capacitor is adequate.

Table 5. 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

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.

9.2.1.3 Application Curves

LM2675 1280326.png Figure 21. LM2675, 3.3-V Output
LM2675 1280328.png Figure 23. LM2675, 12-V Output
LM2675 1280327.png Figure 22. LM2675, 5-V Output

9.2.2 Adjustable Output Voltage Application

LM2675 01280323.gif
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 24. Adjustable Output Voltage Schematic

9.2.2.1 Design Requirements

Table 1 lists the design requirements for the adjustable output voltage application.

Table 6. Design Parameters

PARAMETER VALUE
Regulated output voltage, VOUT 20 V
Maximum input voltage, VIN(max) 28 V
Maximum load current, ILOAD(max) 1 A
Switching frequency, F Fixed at a nominal 260 kHz

9.2.2.2 Detailed Design Procedure

9.2.2.2.1 Programming Output Voltage

Selecting R1 and R2, as shown in Figure 19.

Use Equation 1 to select the appropriate resistor values.

Equation 1. LM2675 01280331.gif

where

  • VREF = 1.21 V

Select R1 to be 1 kΩ, 1%. Solve for R2 using Equation 2.

Equation 2. LM2675 01280334.gif

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 with Equation 3.

Equation 3. LM2675 01280332.gif

R2 = 1k (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)

Calculate the inductor Volt × microsecond constant E × T (V × μs) from Equation 4.

Equation 4. LM2675 01280333.gif

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 5.

Equation 5. LM2675 01280335.gif

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 in Figure 25. E × T = 21.6 (V × μs).

On the horizontal axis, select the maximum load current (ILOAD(max) = 1 A).

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 25, the inductance region intersected by the 21.6 (V × μs) horizontal line and the 1-A vertical line is
68 μH, and the inductor code is L30.

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 Table 2, locate line L30, and select an inductor part number from the list of manufacturers' part numbers.

9.2.2.2.3 Output Capacitor SeIection (COUT)

Select an output capacitor from the capacitor code selection guide in Table 7. 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 7. For this example, use the
15 V to 20 V row. The capacitor code corresponding to an inductance of 68 μH is C20.

Table 7. Capacitor Code Selection Guide

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

Select an appropriate capacitor value and voltage rating, using the capacitor code, from the output capacitor selection table in Table 8. There are two solid tantalum (surface mount) capacitor manufacturers and four electrolytic (through hole) capacitor manufacturers to choose from. TI recommends using both the manufacturers and the manufacturer's series that are listed in Table 8. From Table 8, 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 the surface mount and through hole.

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. 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 See(2) 150/35 150/35 150/35
C22 33/35 22/35 See(2) 120/35 120/35 120/35
C23 See(2) See(2) See(2) 220/50 100/50 120/50
C24 See(2) See(2) See(2) 150/50 100/50 120/50
C25 See(2) See(2) See(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)

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 LM2675. The most stressful condition for this diode is a shorted output condition (see 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, TI recommends a Schottky diode of higher current (at least 2.2 A).

The reverse voltage rating of the diode must be at least 1.25 times the maximum input voltage. 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 LM2675 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 needed between the input pin and ground to prevent large voltage transients from appearing at the input. This capacitor must be located 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 20 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 9 and Table 5 show the recommended application voltage for AVX TPS and Sprague 594D tantalum capacitors. TI 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 9. AVX TPS

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

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) would be needed.

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 1-A load, a capacitor with a RMS current rating of at least 500 mA is needed. The curves shown in Figure 20 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 500 mA.

For a through hole design, a 330-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 Table 5, and 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.

9.2.2.3 Application Curve

LM2675 01280329.gif Figure 25. LM2675, Adjustable Output