SNVSA44B August   2014  – January 2018 LM43601

PRODUCTION DATA.  

  1. 1 Features
  2. 2 Applications
  3. 3 Description
    1.     Device Images
      1.      Simplified Schematic
      2.      Radiated Emission Graph VIN = 12 V, VOUT = 3.3 V, FS= 500 kHz, IOUT = 1 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 (ENABLE)
      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 (BOOT)
      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 Application
      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  Feedforward 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 Set-Point
        13. 8.2.2.13 PGOOD
      3. 8.2.3 Application 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 Development Support
        1. 11.1.1.1 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 LM43601 has a flexible and easy-to-use start up rate control pin: SS/TRK. The soft-start feature is to prevent inrush current impacting the LM43601 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. The LM43601 employs the internal soft-start control ramp and start-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 needed 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. For a desired soft-start time tSS, the capacitance for CSS can be found by:

Equation 2. LM43601 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 ENABLE is toggled low. If ENABLE is toggled high again before the CSS is completely discharged, then the next resulting soft-start ramp follows the internal soft-start ramp. The output follows the ramp programmed by CSS only when the soft-start voltage reaches the leftover voltage on CSS. This behavior appears 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 ENABLE to slow down the shutting down of VCC, which allows more time to discharge CSS.

The LM43601 is capable of start-up into prebiased output conditions. When the inductor current reaches zero, the LS switches 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 LM43601 waits until the soft-start ramp allows regulation above the prebiased voltage. The device then follows the soft-start ramp to the regulation level.

When an external voltage ramp is applied to the SS/TRK pin, the LM43601 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 not fall below 1.2 V to avoid abnormal operation.

LM43601 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.

LM43601 tracking_slow_snvsa13.gifFigure 38. Tracking With Start-up Time Longer Than the Internal Ramp
LM43601 tracking_fast_snvsa13.gifFigure 39. Tracking With Start-up Time Shorter Than the Internal Ramp