SNVSAU1 March   2017 LM25141

PRODUCTION DATA.  

  1. 1 Features
  2. 2 Applications
  3. 3 Description
    1.     Device Images
      1.      Simplified Schematic
  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 Switching Characteristics
    7. 6.7 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  High Voltage Start-up Regulator
      2. 7.3.2  VCC Regulator
      3. 7.3.3  Oscillator
      4. 7.3.4  Synchronization
      5. 7.3.5  Frequency Dithering (Spread Spectrum)
      6. 7.3.6  Enable
      7. 7.3.7  Power Good
      8. 7.3.8  Output Voltage
        1. 7.3.8.1 Minimum Output Voltage Adjustment
      9. 7.3.9  Current Sense
      10. 7.3.10 DCR Current Sensing
      11. 7.3.11 Error Amplifier and PWM Comparator
      12. 7.3.12 Slope Compensation
      13. 7.3.13 Hiccup Mode Current Limiting
      14. 7.3.14 Standby Mode
      15. 7.3.15 Soft-Start
      16. 7.3.16 Diode Emulation
      17. 7.3.17 High and Low Side Drivers
  8. 8 Application and Implementation
    1. 8.1 Application Information
    2. 8.2 Typical Application
      1. 8.2.1 Design Requirements
      2. 8.2.2 Detailed Design Procedure
        1. 8.2.2.1 Custom Design With WEBENCH® Tools
      3. 8.2.3 Inductor Calculation
      4. 8.2.4 Current Sense Resistor
      5. 8.2.5 Output Capacitor
      6. 8.2.6 Input Filter
        1. 8.2.6.1 EMI Filter Design
        2. 8.2.6.2 MOSFET Selection
        3. 8.2.6.3 Driver Slew Rate Control
        4. 8.2.6.4 Frequency Dithering
      7. 8.2.7 8.9 Control Loop
        1. 8.2.7.1 Feedback Compensator
      8. 8.2.8 Application Curves
  9. 9 Power Supply Recommendations
  10. 10Layout
    1. 10.1 Layout Guidelines
      1. 10.1.1 Layout Procedure
    2. 10.2 Layout Examples
  11. 11Device and Documentation Support
    1. 11.1 Custom Design With WEBENCH® Tools
    2. 11.2 Receiving Notification of Documentation Updates
    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

7.3.3 Oscillator

The LM5141 has an internal trimmed oscillator with two frequency options: 2.2 MHz, or 440 kHz. With the OSC pin connected to VDDA the oscillator frequency is 2.2 MHz. With the OSC pin grounded, the oscillator frequency is 440 kHz. The state of the OSC pin is read and latched during VCC power-up and cannot be changed until VCC drops below the VCC(UVLO) threshold.

The oscillator frequency can be modulated up or down from the nominal oscillator frequency (2.2 MHz or 440 kHz) on demand by connecting a resistor from the RT pin to ground (refer to Figure 19). To disable the frequency modulation option, the RT pin can be grounded or left open. If the RT pin is connected to ground during power-up the frequency modulation option is latch-off and cannot be changed unless VCC is allowed to drop below the VCC(UVLO) threshold. If the RT pin is left open during power-up the frequency modulation option will be disabled, but it can be enabled at a later time by switching in a valid RT resistor. When the frequency modulation option is disabled, the LM5141 will operate at the internal oscillator frequency (2.2 MHz or 440 kHz).

On power up, after soft-start is complete and the output voltage is in regulation, a 16 µs timer is initiated. If a valid RT resistor is connected, the LM5141 will switch to the frequency set by the RT resistor n the completion of the 16 µs time delay.

The modulation range for 2.2 MHz is 1.8 MHz to 2.53 MHz (refer to Table 1). If an RT resistor value > 95 kΩ (typical) is placed on the RT pin, the LM5141 controller will assume that the RT pin is open, and will use the internal oscillator. If an RT resistor < 27 kΩ (typical) is connected, the controller will use the internal oscillator. To calculate an RT resistor for a specific oscillator frequency, use Equation 1 for the 2.2 MHz frequency range or Equation 2 for the 440 kHz frequency range.

Equation 1. LM25141 equation_01_snvsaj6.gif

where

  • RT is kΩ and Fsw is in MHz
Equation 2. LM25141 equation_02_snvsaj6.gif

where

  • RT is in kΩ and Fsw is in kHz

Table 1. RT Resistance vs Oscillator Frequency

S1S2RT Resistance (Typical) 2.2 MHz 2.2 MHZ Oscillator Range (Typical) RT Resistance (Typical) 440 kHz 440 kHz Oscillator Range (Typical)
X X > 95 kΩ Internal Oscillator > 95 kΩ Internal Oscillator
OFF OFF 61.98 kΩTotal 1.8 MHz 73.8 kΩTotal 300 kHz
OFF ON 50.18 kΩTotal 2.2 MHz 50.1 kΩTotal 440 kHz
ON OFF 43.2 kΩ 2.53 MHz 44.2 kΩ 500 kHz
X X < 27 kΩ Internal Oscillator < 27 kΩ Internal Oscillator
LM25141 rt_connection_circuit_2pt2mhz_snvsaj6.gifFigure 19. RT Connection Circuit, 2.2 MHz
LM25141 rt_connection_circuit_440mhz_snvsaj6.gifFigure 20. RT Connection Circuit, 440 kHz

An alternative method to modulate the oscillator frequency is to use an analog voltage connected to the RT pin through a resistor. See Figure 21. An analog voltage of 0.0 V to 0.6 V will modulate the oscillator frequency between 1.8 MHz to 2.53 MHz (OSC at 2.2 MHz), or 300 kHz to 500 kHz (OSC at 440 kHz). The analog voltage source must be able to sink current.

LM25141 analog_voltage_control_oscillator_frequency_snvsaj6.gifFigure 21. Analog Voltage Control of the Oscillator Frequency

When the LM5141 is in the low IQ standby mode, the controller will set the RT pin to a high impedance state and ignore the RT resistor. After coming out of standby mode, the controller will monitor the RT pin. If a valid resistor is connected, and there have been 16 µs of continuous switching without a zero-crossing event, the LM5141 will switch to the frequency set by the RT resistor.