SNVS008L September   1998  – June 2016 LM2671

PRODUCTION DATA.  

  1. 1 Features
  2. 2 Applications
  3. 3 Description
  4. 4 Revision History
  5. 5 Description (continued)
  6. 6 Pin Configuration and Functions
  7. 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. 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. 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)

Package Options

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

8 Detailed Description

8.1 Overview

The LM2671 provides all of the active functions required for a step-down (buck) switching regulator. The internal power switch is a DMOS power MOSFET to provide power supply designs with high current capability, up to
0.5 A, and highly efficient operation.

The LM2671 is part of the SIMPLE SWITCHER® family of power converters. A complete design uses a minimum number of external components, which have been predetermined from a variety of manufacturers. Using either this data sheet or TI's WEBENCH® design tool, a complete switching power supply can be designed quickly. Also, see LM2670 SIMPLE SWITCHER® High Efficiency 3A Step-Down Voltage Regulator with Sync for additional applications information.

8.2 Functional Block Diagram

LM2671 10004217.png

8.3 Feature Description

8.3.1 Switch Output

This is the output of a power MOSFET switch connected directly to the input voltage. The switch provides energy to an inductor, an output capacitor and the load circuitry under control of an internal pulse-width-modulator (PWM). The PWM controller is internally clocked by a fixed 260-kHz oscillator. In a standard step-down application the duty cycle (Time ON/Time OFF) of the power switch is proportional to the ratio of the power supply output voltage to the input voltage. The voltage on the VSW pin cycles between VIN (switch ON) and below ground by the voltage drop of the external Schottky diode (switch OFF).

8.3.2 Input

The input voltage for the power supply is connected to the VIN pin. In addition to providing energy to the load the input voltage also provides bias for the internal circuitry of the LM2671. For ensured performance the input voltage must be in the range of 6.5 V to 40 V. For best performance of the power supply the VIN pin must always be bypassed with an input capacitor placed close to this pin and GND.

8.3.3 C Boost

A capacitor must be connected from the CB pin to the VSW pin. This capacitor boosts the gate drive to the internal MOSFET above VIN to fully turn it ON. This minimizes conduction losses in the power switch to maintain high efficiency. The recommended value for C Boost is 0.01 μF.

8.3.4 Ground

This is the ground reference connection for all components in the power supply. In fast-switching, high-current applications such as those implemented with the LM2671, TI recommends that a broad ground plane be used to minimize signal coupling throughout the circuit.

8.3.5 Sync

This input allows control of the switching clock frequency. If left open-circuited the regulator is switched at the internal oscillator frequency, typically 260 kHz. An external clock can be used to force the switching frequency and thereby control the output ripple frequency of the regulator. This capability provides for consistent filtering of the output ripple from system to system as well as precise frequency spectrum positioning of the ripple frequency which is often desired in communications and radio applications. This external frequency must be greater than the LM2671 internal oscillator frequency, which could be as high as 275 kHz, to prevent an erroneous reset of the internal ramp oscillator and PWM control of the power switch. The ramp oscillator is reset on the positive going edge of the sync input signal. TI recommends that the external TTL or CMOS compatible clock (between
0 V and a level greater than 3 V) be ac coupled to the SYNC pin through a 100-pF capacitor and a 1-kΩ resistor to ground.

When the SYNC function is used, current limit frequency foldback is not active. Therefore, the device may not be fully protected against extreme output short-circuit conditions.

8.3.6 Feedback

This is the input to a two-stage high gain amplifier, which drives the PWM controller. Connect the FB pin directly to the output for proper regulation. For the fixed output devices (3.3-V, 5-V and 12-V outputs), a direct wire connection to the output is all that is required as internal gain setting resistors are provided inside the LM2671. For the adjustable output version two external resistors are required to set the DC output voltage. For stable operation of the power supply it is important to prevent coupling of any inductor flux to the feedback input.

8.3.7 ON/OFF

This input provides an electrical ON/OFF control of the power supply. Connecting this pin to ground or to any voltage less than 0.8 V is completely turn OFF the regulator. The current drain from the input supply when OFF is only 50 μA. The ON/OFF input has an internal pullup current source of approximately 20 μA and a protection clamp Zener diode of 7 V to ground. When electrically driving the ON/OFF pin the high voltage level for the ON condition must not exceed the 6 V absolute maximum limit. When ON/OFF control is not required this pin must be left open.

8.4 Device Functional Modes

8.4.1 Shutdown Mode

The ON/OFF pin provides electrical ON and OFF control for the LM2671. When the voltage of this pin is lower than 1.4 V, the device enters shutdown mode. The typical standby current in this mode is 50 μA.

8.4.2 Active Mode

When the voltage of the ON/OFF pin is higher than 1.4 V, the device starts switching and the output voltage rises until it reaches a normal regulation voltage.