SNVSAK0A October   2017  – October 2019 LM76002 , LM76003

PRODUCTION DATA.  

  1. 1 Features
  2. 2 Applications
  3. 3 Description
    1.     Simplified Schematic
    2.     Efficiency vs Output Current (VOUT = 5 V, fSW = 400 kHz, Auto Mode)
  4. 4 Revision History
  5. 5 Pin Configuration and Functions
    1.     Pin Functions
  6. 6 Specifications
    1. 6.1 Absolute Maximum Ratings
    2. 6.2 ESD Ratings
    3. 6.3 Recommended Operating Conditions
    4. 6.4 Thermal Information
    5. 6.5 Electrical Characteristics
    6. 6.6 Timing Characteristics
    7. 6.7 Switching Characteristics
    8. 6.8 System Characteristics
    9. 6.9 Typical Characteristics
  7. 7 Detailed Description
    1. 7.1 Overview
    2. 7.2 Functional Block Diagram
    3. 7.3 Feature Description
      1. 7.3.1  Fixed-Frequency, Peak-Current-Mode Control
      2. 7.3.2  Light Load Operation Modes — PFM and FPWM
      3. 7.3.3  Adjustable Output Voltage
      4. 7.3.4  Enable (EN Pin) and UVLO
      5. 7.3.5  Internal LDO, VCC UVLO, and Bias Input
      6. 7.3.6  Soft Start and Voltage Tracking (SS/TRK)
      7. 7.3.7  Adjustable Switching Frequency (RT) and Frequency Synchronization
      8. 7.3.8  Minimum On-Time, Minimum Off-Time, and Frequency Foldback at Dropout Conditions
      9. 7.3.9  Internal Compensation and CFF
      10. 7.3.10 Bootstrap Voltage and VBOOT UVLO (BOOT Pin)
      11. 7.3.11 Power Good and Overvoltage Protection (PGOOD)
      12. 7.3.12 Overcurrent and Short-Circuit Protection
      13. 7.3.13 Thermal Shutdown
    4. 7.4 Device Functional Modes
      1. 7.4.1 Shutdown Mode
      2. 7.4.2 Standby Mode
      3. 7.4.3 Active Mode
      4. 7.4.4 CCM Mode
      5. 7.4.5 DCM Mode
      6. 7.4.6 Light Load Mode
      7. 7.4.7 Foldback Mode
      8. 7.4.8 Forced Pulse-Width-Modulation Mode
      9. 7.4.9 Self-Bias Mode
  8. 8 Application and Implementation
    1. 8.1 Application Information
    2. 8.2 Typical Applications
      1. 8.2.1 Design Requirements
      2. 8.2.2 Detailed Design Procedure
        1. 8.2.2.1  Custom Design With WEBENCH® Tools
        2. 8.2.2.2  Output Voltage Setpoint
        3. 8.2.2.3  Switching Frequency
        4. 8.2.2.4  Input Capacitors
        5. 8.2.2.5  Inductor Selection
        6. 8.2.2.6  Output Capacitor Selection
        7. 8.2.2.7  Feed-Forward Capacitor
        8. 8.2.2.8  Bootstrap Capacitors
        9. 8.2.2.9  VCC Capacitors
        10. 8.2.2.10 BIAS Capacitors
        11. 8.2.2.11 Soft-Start Capacitors
        12. 8.2.2.12 Undervoltage Lockout Setpoint
        13. 8.2.2.13 PGOOD
        14. 8.2.2.14 Synchronization
      3. 8.2.3 Application Curves
  9. 9 Power Supply Recommendations
  10. 10Layout
    1. 10.1 Layout Guidelines
      1. 10.1.1 Layout Highlights
      2. 10.1.2 Compact Layout for EMI Reduction
      3. 10.1.3 Ground Plane and Thermal Considerations
      4. 10.1.4 Feedback Resistors
    2. 10.2 Layout Example
    3. 10.3 Thermal Design
  11. 11Device and Documentation Support
    1. 11.1 Device Support
      1. 11.1.1 Development Support
        1. 11.1.1.1 Custom Design With WEBENCH® Tools
    2. 11.2 Receiving Notification of Documentation Updates
    3. 11.3 Support Resources
    4. 11.4 Trademarks
    5. 11.5 Electrostatic Discharge Caution
    6. 11.6 Glossary
  12. 12Mechanical, Packaging, and Orderable Information

7.3.7 Adjustable Switching Frequency (RT) and Frequency Synchronization

The switching frequency of the LM76002/LM76003 can be programmed by the impedance RT from the RT pin to ground. The frequency is inversely proportional to the RT resistance. The RT pin can be left floating, and the LM76002/LM76003 operates at 500-kHz default switching frequency. The RT pin is not designed to be shorted to ground.

For an desired frequency, RT can be found by:

Equation 6. LM76002 LM76003 frequencysync_eq.gif
LM76002 LM76003 RT_FREQ.gifFigure 17. Switching Frequency vs RT

Table 1. Switching Frequency vs RT

SWITCHING FREQUENCY (kHz) RT RESISTANCE (kΩ)
300 134.42
400 99.57
500 79.07
750 52.20
1000 38.96
1500 25.85
2000 19.34
2200 17.57

The LM76002/LM76003 switching action can also be synchronized to an external clock from 300 kHz to 2.2 MHz. TI recommends connecting an external clock to the SYNC pin with appropriate termination resistor. Ground the SYNC pin if not used.

LM76002 LM76003 freq_sync.gifFigure 18. Frequency Synchronization

The recommendations for the external clock include high level no lower than 2 V, low level no higher than 0.4 V, duty cycle between 10% and 90%, and both positive and negative pulse width no shorter than 80 ns. When the external clock fails at logic high or low, the LM76002/LM76003 switches at the frequency programmed by the RT resistor after a time-out period. TI recommends connecting a resistor RT to the RT pin so that the internal oscillator frequency is the same as the target clock frequency when the LM76002/LM76003 is synchronized to an external clock. This allows the regulator to continue operating at approximately the same switching frequency if the external clock fails.

The choice of switching frequency is usually a compromise between conversion efficiency and the size of the circuit. Lower switching frequency implies reduced switching losses (including gate charge losses, switch transition losses, etc.) and usually results in higher overall efficiency. However, higher switching frequency allows use of smaller LC output filters and hence a more compact design. Lower inductance also helps transient response (higher large signal slew rate of inductor current), and reduces the DCR loss. The optimal switching frequency is usually a trade-off in a given application and thus needs to be determined on a case-by-case basis. It is related to the input voltage, output voltage, most frequent load current level(s), external component choices, and circuit size requirement. The choice of switching frequency may also be limited if an operating condition triggers tON-MIN or tOFF-MIN.