SNVSA45B June   2014  – March 2018 LM46000

PRODUCTION DATA.  

  1. 1 Features
  2. 2 Applications
  3. 3 Description
    1.     Device Images
      1.      Simplified Schematic
      2.      Radiated Emission Graph VOUT = 3.3 V, VIN = 24 V, FS= 500 kHz, IOUT = 0.5 A
  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 Requirements
    7. 6.7 Switching Characteristics
    8. 6.8 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 Controlled Step-Down Regulator
      2. 7.3.2  Light Load Operation
      3. 7.3.3  Adjustable Output Voltage
      4. 7.3.4  Enable (EN)
      5. 7.3.5  VCC, UVLO, and BIAS
      6. 7.3.6  Soft Start and Voltage Tracking (SS/TRK)
      7. 7.3.7  Switching Frequency (RT) and Synchronization (SYNC)
      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 (CBOOT)
      11. 7.3.11 Power Good (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 Light Load Operation
      6. 7.4.6 Self-Bias Mode
  8. 8 Applications 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 Capacitor
        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
      3. 8.2.3 Application Performance Curves
  9. 9 Power Supply Recommendations
  10. 10Layout
    1. 10.1 Layout Guidelines
      1. 10.1.1 Compact Layout for EMI Reduction
      2. 10.1.2 Ground Plane and Thermal Considerations
      3. 10.1.3 Feedback Resistors
    2. 10.2 Layout Example
  11. 11Device and Documentation Support
    1. 11.1 Device Support
      1. 11.1.1 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

7.3.6 Soft Start and Voltage Tracking (SS/TRK)

The LM46000 has a flexible and easy-to-use start-up rate control pin: SS/TRK. The soft-start feature is there to prevent inrush current impacting the LM46000 and its supply when power is first applied. Soft-start is achieved by slowly ramping up the target regulation voltage when the device is first enabled or powered up.

The simplest way to use the device is to leave the SS/TRK pin open circuit or floating. The LM46000 employs the internal soft-start control ramp and starts up to the regulated output voltage in 4.1 ms typically.

In applications with a large amount of output capacitors, or higher VOUT, or other special requirements, the soft-start time can be extended by connecting an external capacitor CSS from SS/TRK pin to AGND. Extended soft-start time further reduces the supply current required to charge up output capacitors and supply any output loading. An internal current source (ISSC = 2.2 µA) charges CSS and generates a ramp from 0 V to VFB to control the ramp-up rate of the output voltage. VFB value is typically 1 V, and therefore it is not mentioned in the Equation 2. For a desired soft-start time tSS, the capacitance for CSS can be found by:

Equation 2. LM46000 eq02_snvsa13.gif

The soft-start capacitor CSS is discharged by an internal FET when VOUT is shut down by hiccup protection due to excessive load, temperature shutdown due to overheating or ENABLE = logic low. A large CSS capacitor takes a long time to discharge when EN is toggled low. If EN is toggled high again before the CSS is completely discharged, then the next resulting soft-start ramp follows the internal soft-start ramp. Only when the soft-start voltage reaches the leftover voltage on CSS, does the output follow the ramp programmed by CSS. This behavior looks as if there are two slopes at start-up. If this is not acceptable by a certain application, a R-C low-pass filter can be added to EN to slow down the shutting down of VCC, which allows more time to discharge CSS.

The LM46001 is capable of start-up into prebiased output conditions. When the inductor current reaches zero, the LS switch is turned off to avoid negative current conduction. This operation mode is also called diode emulation mode. It is built-in by the DCM operation at light loads. With a prebiased output voltage, the LM46001 waits until the soft-start ramp allows regulation above the prebiased voltage. It then follows the soft-start ramp to the regulation level.

When an external voltage ramp is applied to the SS/TRK pin, the LM46001 FB voltage follows the external ramp if the ramp magnitude is lower than the internal soft-start ramp. A resistor divider pair can be used on the external control ramp to the SS/TRK pin to program the tracking rate of the output voltage. The final external ramp voltage applied at the SS/TRK pin must be above 1.2 V to avoid abnormal operation.

LM46000 soft_start_track_snvsa13.gifFigure 37. Soft Start Tracking External Ramp

VOUT tracked to an external voltage ramp has the option of ramping up slower or faster than the internal voltage ramp. VFB always follows the lower potential of the internal voltage ramp and the voltage on the SS/TRK pin. Figure 38 shows the case when VOUT ramps slower than the internal ramp, while Figure 39 shows when VOUT ramps faster than the internal ramp. Faster start up time may result in inductor current tripping current protection during start-up. Use with special care.

LM46000 tracking_slow_snvsa13.gifFigure 38. Tracking with Longer Start-up Time Than The Internal Ramp
LM46000 tracking_fast_snvsa13.gifFigure 39. Tracking With Shorter Start-up Time Than the Internal Ramp