SNVS033D May   2004  – November 2015 LM2621

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 Recommended Operating Conditions
    3. 6.3 Thermal Information
    4. 6.4 Electrical Characteristics
    5. 6.5 Typical Characteristics
  7. Detailed Description
    1. 7.1 Overview
    2. 7.2 Functional Block Diagram
    3. 7.3 Feature Description
      1. 7.3.1 Gated Oscillator Control Scheme
      2. 7.3.2 Low Voltage Start-Up
      3. 7.3.3 Output Voltage Ripple Frequency
      4. 7.3.4 Internal Current Limit and Thermal Protection
    4. 7.4 Device Functional Modes
      1. 7.4.1 Shutdown
  8. Application and Implementation
    1. 8.1 Application Information
    2. 8.2 Typical Applications
      1. 8.2.1 Step-Up DC-DC Converter Typical Application Using LM2621
        1. 8.2.1.1 Design Requirements
        2. 8.2.1.2 Detailed Design Procedure
          1. 8.2.1.2.1 Setting the Output Voltage
          2. 8.2.1.2.2 Bootstrapping
          3. 8.2.1.2.3 Setting the Switching Frequency
          4. 8.2.1.2.4 Inductor Selection
          5. 8.2.1.2.5 Output Diode Selection
          6. 8.2.1.2.6 Input and Output Filter Capacitors Selection
        3. 8.2.1.3 Application Curves
      2. 8.2.2 5-V / 0.5-A Step-Up Regulator
        1. 8.2.2.1 Design Requirements
        2. 8.2.2.2 Detailed Design Procedure
      3. 8.2.3 2-mm Tall 5-V / 0.2-A Step-Up Regulator for Low Profile Applications
        1. 8.2.3.1 Design Requirements
        2. 8.2.3.2 Detailed Design Procedure
      4. 8.2.4 3.3-V / 0.5-A SEPIC Regulator
        1. 8.2.4.1 Design Requirements
        2. 8.2.4.2 Detailed Design Procedure
  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 Documentation Support
      1. 11.2.1 Related Documentation
    3. 11.3 Community Resources
    4. 11.4 Trademarks
    5. 11.5 Electrostatic Discharge Caution
    6. 11.6 Glossary
  12. 12Mechanical, Packaging, and Orderable Information

Package Options

Mechanical Data (Package|Pins)
Thermal pad, mechanical data (Package|Pins)
Orderable Information

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

The LM2621 is primarily used as a Boost type step-up converter. The following section provides information regarding connection and component choices to build a successful boost converter. Examples of typical applications are also provided including a SEPIC step-up/step-down topology. More details on designing a SEPIC converter can be found here: SLYT309.

8.2 Typical Applications

8.2.1 Step-Up DC-DC Converter Typical Application Using LM2621

LM2621 10093412.png Figure 13. Typical Circuit

8.2.1.1 Design Requirements

In order to successfully build an application, the designer should have the following parameters:

  • Output voltage to set the feedback voltage divider and to assess the source for biasing the VDD pin.
  • Input voltage range (min and max) to ensure safe operation within absolute max. rating of the IC.
  • Output current to ensure that the system will not hit the internal peak current limit of the IC (2.85 A typical) during normal operation.

8.2.1.2 Detailed Design Procedure

8.2.1.2.1 Setting the Output Voltage

The output voltage of the step-up regulator can be set between 1.24 V and 14 V by connecting a feedback resistive divider made of RF1 and RF2. The resistor values are selected as follows:

Equation 1. RF2 = RF1 /[(VOUT/ 1.24) −1]

A value of 150 kΩ is suggested for RF1. Then, RF2 can be selected using the above equation. A 39-pF capacitor (CF1) connected across RF1 helps in feeding back most of the AC ripple at VOUT to the FB pin. This helps reduce the peak-to-peak output voltage ripple as well as improve the efficiency of the step-up regulator, because a set hysteresis of 30 mV at the FB pin is used for the gated oscillator control scheme.

8.2.1.2.2 Bootstrapping

When the output voltage (VOUT) is between 2.5 V and 5.0 V a bootstrapped operation is suggested. This is achieved by connecting the VDD pin (Pin 6) to VOUT. However if the VOUT is outside this range, the VDD pin should be connected to a voltage source whose range is between 2.5 V and 5 V. This can be the input voltage (VIN), VOUT stepped down using a linear regulator, or a different voltage source available in the system. This is referred to as non-bootstrapped operation. The maximum acceptable voltage at the BOOT pin (Pin 7) is 10 V.

8.2.1.2.3 Setting the Switching Frequency

The switching frequency of the oscillator is selected by choosing an external resistor (RFQ) connected between FREQ and VDD pins. See Figure 9 for choosing the RFQ value to achieve the desired switching frequency. A high switching frequency allows the use of very small surface mount inductors and capacitors and results in a very small solution size. A switching frequency between 300 kHz and 2 MHz is recommended.

8.2.1.2.4 Inductor Selection

The LM2621's high switching frequency enables the use of a small surface mount inductor. A 6.8-µH shielded inductor is suggested. The inductor should have a saturation current rating higher than the peak current it will experience during circuit operation (see Figure 10). Less than 100-mΩ ESR is suggested for high efficiency.

Open-core inductors cause flux linkage with circuit components and interfere with the normal operation of the circuit. They should be avoided. For high efficiency, choose an inductor with a high frequency core material, such as ferrite, to reduce the core losses. To minimize radiated noise, use a toroid, pot core or shielded core inductor. The inductor should be connected to the SW pin as close to the IC as possible. See Table 1 for a list of the inductor manufacturers.

8.2.1.2.5 Output Diode Selection

A Schottky diode should be used for the output diode. The forward current rating of the diode should be higher than the load current, and the reverse voltage rating must be higher than the output voltage. Do not use ordinary rectifier diodes, since slow switching speeds and long recovery times cause the efficiency and the load regulation to suffer. Table 1 shows a list of the diode manufacturers.

8.2.1.2.6 Input and Output Filter Capacitors Selection

Tantalum chip capacitors are recommended for the input and output filter capacitors. A 22-µF capacitor is suggested for the input filter capacitor. It should have a DC working voltage rating higher than the maximum input voltage. A 68-µF tantalum capacitor is suggested for the output capacitor. The DC working voltage rating should be greater than the output voltage. Very high ESR values (> 3Ω) should be avoided.

8.2.1.3 Application Curves

LM2621 1V2_startup_10ms_perdiv_snvs033.png Figure 14. Startup Vin=1.2V,Vout-5V, 10ms/div 1V/div (Ch3:Vin,Ch1:Vout)
LM2621 3V3_startup_10ms_perdiv_snvs033.png Figure 15. Startup Vin=3.3V,Vout-5V, 10ms/div 1V/div (Ch3:Vin,Ch1:Vout)

8.2.2 5-V / 0.5-A Step-Up Regulator

LM2621 10093412.png Figure 16. 5-V/0.5A Step-Up Regulator

8.2.2.1 Design Requirements

Design requirement is the same to the typical application shown earlier. Components have been chosen that comply with the required maximum height. See Design Requirements for the design requirement and following sections for the detailed design procedure.

8.2.2.2 Detailed Design Procedure

Follow the detailed design procedure in Detailed Design Procedure.

Table 1. Bill of Materials

Manufacturer Part Number
U1 TI LM2621MM
C1 Vishay/Sprague 595D226X06R3B2T, Tantalum
C2 Vishay/Sprague 595D686X0010C2T, Tantalum
D1 Motorola MBRS140T3
L Coilcraft DT1608C-682

8.2.3 2-mm Tall 5-V / 0.2-A Step-Up Regulator for Low Profile Applications

LM2621 10093417.png Figure 17. 2-mm Tall 5-V/0.2A Step-Up Regulator for Low Profile Applications

8.2.3.1 Design Requirements

Design requirement is the same to the typical application shown earlier. Components have been chosen that comply with the required maximum height. See Design Requirements for the design requirement and following sections for the detailed design procedure.

8.2.3.2 Detailed Design Procedure

Follow the detailed design procedure in Detailed Design Procedure.

Table 2. Bill of Materials

Manufacturer Part Number
U1 TI LM2621MM
C1 Vishay/Sprague 592D156X06R3B2T, Tantalum
C2 Vishay/Sprague 592D336X06R3C2T, Tantalum
D1 Motorola MBRS140T3
L Vishay/Dale ILS-3825-03

8.2.4 3.3-V / 0.5-A SEPIC Regulator

LM2621 10093422.png Figure 18. 3.3-V/0.5-A SEPIC Regulator

8.2.4.1 Design Requirements

Design requirement for the SEPIC is similar to that of a boost but the current flowing through the switch is the addition of the current flowing through L1 and L2. As a result, the peak current through the main switch is IIN+IOUT+0.5xDeltaIL1+0.5xDeltaIL2. See SLYT309 for detail on the specific design requirement of a SEPIC converter.

8.2.4.2 Detailed Design Procedure

Follow the detailed design procedure in Detailed Design Procedure.

Table 3. Bill of Materials

Manufacturer Part Number Description
U1 TI LM2621MM Low Input Voltage Regulator
C1 Sanyo 10CV220AX, SMT AL-Electrolytic 220 µF
C2 TDK C2012X7R1C225M, MLCC 2.2 µF
C3 Vishay VJ0603A331KXXAT 33 pF
C4 TDK C3225X7R0J107MT 100 µF
C5, C6 Vishay VJ0603Y104KXXAT 0.1 µF
D1 Philips BAT54C VR = 1V
D2 Vishay MBRS120 1A / VR = 20V
L1, L2 Coilcraft DO1813P-682HC 6.8 µH
R1 Vishay CRCW08054990FRT6 499 Ω
R2 Vishay CRCW08051503FRT6 150 kΩ
R3 Vishay CRCW08053923FRT6 392 kΩ
R4 Vishay CRCW08059092FRT6 90.9 kΩ