SNVS123D April   1998  – May 2016 LM2599

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 Version
    6. 7.6  Electrical Characteristics - 5-V Version
    7. 7.7  Electrical Characteristics - 12-V Version
    8. 7.8  Electrical Characteristics - Adjustable Voltage Version
    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 Shutdown/Soft-Start
      2. 8.3.2 Inverting Regulator
      3. 8.3.3 Undervoltage Lockout
      4. 8.3.4 Negative Voltage Charge Pump
    4. 8.4 Device Functional Modes
      1. 8.4.1 Discontinuous Mode Operation
  9. Application and Implementation
    1. 9.1 Application Information
      1. 9.1.1 Soft-Start Capacitor (CSS)
      2. 9.1.2 Delay Capacitor (CDELAY)
        1. 9.1.2.1 RPULLUP
      3. 9.1.3 Feedforward Capacitor (CFF) for Adjustable Output Voltage Version Only
      4. 9.1.4 Input Capacitor (CIN)
      5. 9.1.5 Output Capacitor (COUT)
      6. 9.1.6 Catch Diode
      7. 9.1.7 Inductor Selection
      8. 9.1.8 Output Voltage Ripple and Transients
      9. 9.1.9 Open Core Inductors
    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)
        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 Feedforward Capacitor (CFF)
          5. 9.2.2.2.5 Catch Diode Selection (D1)
          6. 9.2.2.2.6 Input Capacitor (CIN)
        3. 9.2.2.3 Application Curves
  10. 10Power Supply Recommendations
  11. 11Layout
    1. 11.1 Layout Guidelines
    2. 11.2 Layout Examples
    3. 11.3 Thermal Considerations
  12. 12Device and Documentation Support
    1. 12.1 Community Resources
    2. 12.2 Trademarks
    3. 12.3 Electrostatic Discharge Caution
    4. 12.4 Glossary
  13. 13Mechanical, Packaging, and Orderable Information

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11 Layout

11.1 Layout Guidelines

As in any switching regulator, layout is very important. Rapidly switching currents associated with wiring inductance can generate voltage transients which can cause problems. For minimal inductance and ground loops, the wires indicated by heavy lines must be wide printed-circuit traces and must be kept as short as possible. For best results, external components must be placed as close to the switcher lC as possible using ground plane construction or single point grounding.

If open core inductors are used, take special care to select the location and positioning of this type of inductor. Allowing the inductor flux to intersect sensitive feedback, lC groundpath and COUT wiring can cause problems.

When using the adjustable version, take special care to select the location of the feedback resistors and the associated wiring. Physically place both resistors near the IC, and route the wiring away from the inductor, especially an open core type of inductor (see Open Core Inductors for more information).

11.2 Layout Examples

LM2599 01258251.png
CIN = 470-μF, 50-V, aluminum electrolytic Panasonic HFQ Series
COUT = 330-μF, 35-V, aluminum electrolytic Panasonic HFQ Series
D1 = 5-A, 40-V Schottky rectifier, 1N5825
L1 = 47-μH, L39, Renco through hole
RPULL UP = 10k
CDELAY = 0.1 μF
CSD/SS = 0.1 μF
Thermalloy heat sink #7020
Figure 46. Typical Through-Hole PCB Layout, Fixed Output (1x Size), Double-Sided
LM2599 01258252.png
CIN = 470-μF, 50-V, aluminum electrolytic Panasonic, HFQ Series
COUT = 220-μF, 35-V, aluminum electrolytic Panasonic, HFQ Series
D1 = 5-A, 40-V Schottky Rectifier, 1N5825
L1 = 47-μH, L39, Renco, through-hole
R1 = 1 kΩ, 1%
R2 = Use formula in Detailed Design Procedure
CFF = See Figure 35
RFF = See Feedforward Capacitor (CFF) for Adjustable Output Voltage Version Only
RPULL UP = 10k
CDELAY = 0.1 μF
CSD/SS= 0.1 μF
Thermalloy heat sink #7020
Figure 47. Typical Through-Hole PCB Layout, Adjustable Output (1x Size), Double-Sided

11.3 Thermal Considerations

The LM2599 is available in two packages, a 7-pin TO-220 and a 7-pin surface-mount TO-263.

The TO-220 package needs a heat sink under most conditions. The size of the heat sink depends on the input voltage, the output voltage, the load current and the ambient temperature. The curves in Figure 48 show the LM2599T junction temperature rises above ambient temperature for a 3-A load and different input and output voltages. The data for these curves was taken with the LM2599T (TO-220 package) operating as a buck switching regulator in an ambient temperature of 25°C (still air). These temperature rise numbers are all approximate and there are many factors that can affect these temperatures. Higher ambient temperatures require more heat sinking.

The TO-263 surface-mount package tab is designed to be soldered to the copper on a printed-circuit board. The copper and the board are the heat sink for this package and the other heat producing components, such as the catch diode and inductor. The PCB copper area that the package is soldered to must be at least 0.4 in2, and ideally must have 2 or more square inches of 2-oz. (0.0028 in) copper. Additional copper area improves the thermal characteristics, but with copper areas greater than approximately 6 in2, only small improvements in heat dissipation are realized. If further thermal improvements are needed, double-sided, multilayer PCBs with large copper areas or airflow are recommended.

The curves shown in Figure 49 show the LM2599S (TO-263 package) junction temperature rise above ambient temperature with a 2-A load for various input and output voltages. This data was taken with the circuit operating as a buck switching regulator with all components mounted on a PCB to simulate the junction temperature under actual operating conditions. This curve can be used for a quick check for the approximate junction temperature for various conditions, but be aware that there are many factors that can affect the junction temperature. When load currents higher than 2 A are used, double-sided or multilayer PCBs with large copper areas or airflow might be needed, especially for high ambient temperatures and high output voltages.

LM2599 01258238.png
CIRCUIT DATA FOR TEMPERATURE RISE CURVE TO-220 PACKAGE (NDZ)
Capacitors Through-hole electrolytic
Inductor Through-hole Renco
Diode Through-hole, 5-A, 40-V Schottky
PCB 3-square inch, single-sided, 2-oz copper (0.0028″)
Figure 48. Junction Temperature Rise, TO-220
LM2599 01258239.png
CIRCUIT DATA FOR TEMPERATURE RISE CURVE TO-263 PACKAGE (KTW)
Capacitors Surface-mount, tantalum molded D size
Inductor Surface-mount, Pulse engineering, 68 μH
Diode Surface-mount, 5-A, 40-V, Schottky
PCB 9-square inch, single-sided, 2-oz copper (0.0028″)
Figure 49. Junction Temperature Rise, TO-263

For the best thermal performance, wide copper traces and generous amounts of printed-circuit board copper must be used in the board layout. (One exception to this is the output (switch) pin, which must not have large areas of copper). Large areas of copper provide the best transfer of heat (lower thermal resistance) to the surrounding air, and moving air lowers the thermal resistance even further.

Package thermal resistance and junction temperature rise numbers are all approximate, and there are many factors that affects these numbers. Some of these factors include board size, shape, thickness, position, location, and even board temperature. Other factors are, trace width, total printed-circuit copper area, copper thickness, single- or double-sided, multilayer board and the amount of solder on the board. The effectiveness of the PCB to dissipate heat also depends on the size, quantity and spacing of other components on the board, as well as whether the surrounding air is still or moving. Furthermore, some of these components such as the catch diode adds heat to the PCB and the heat can vary as the input voltage changes. For the inductor, depending on the physical size, type of core material and the DC resistance, it could either act as a heat sink taking heat away from the board, or it could add heat to the board.