SBVS066S December   2005  – November 2024 TPS74401

PRODUCTION DATA  

  1.   1
  2. 1 Features
  3. 2 Applications
  4. 3 Description
  5. 4 Pin Configuration and Functions
  6. 5 Specifications
    1. 5.1 Absolute Maximum Ratings
    2. 5.2 ESD Ratings
    3. 5.3 Recommended Operating Conditions
    4. 5.4 Thermal Information
    5. 5.5 Electrical Characteristics
    6. 5.6 Typical Characteristics
  7. 6 Detailed Description
    1. 6.1 Overview
    2. 6.2 Functional Block Diagram
    3. 6.3 Feature Description
      1. 6.3.1 Enable, Shutdown
      2. 6.3.2 Power-Good (VQFN Package Only)
      3. 6.3.3 Internal Current Limit
      4. 6.3.4 Thermal Protection
    4. 6.4 Device Functional Modes
      1. 6.4.1 Normal Operation
      2. 6.4.2 Dropout Operation
      3. 6.4.3 Disabled
    5. 6.5 Programming
      1. 6.5.1 Programmable Soft-Start
      2. 6.5.2 Sequencing Requirements
  8. 7 Application and Implementation
    1. 7.1 Application Information
      1. 7.1.1 Input, Output, and Bias Capacitor Requirements
      2. 7.1.2 Transient Response
      3. 7.1.3 Dropout Voltage
      4. 7.1.4 Output Noise
    2. 7.2 Typical Applications
      1. 7.2.1 Setting the TPS74401
        1. 7.2.1.1 Design Requirements
        2. 7.2.1.2 Detailed Design Procedure
        3. 7.2.1.3 Application Curves
      2. 7.2.2 Using an Auxiliary Bias Rail
      3. 7.2.3 Without an Auxiliary Bias
    3. 7.3 Power Supply Recommendations
    4. 7.4 Layout
      1. 7.4.1 Layout Guidelines
        1. 7.4.1.1 Power Dissipation
        2. 7.4.1.2 Thermal Considerations
      2. 7.4.2 Layout Example
  9. 8 Device and Documentation Support
    1. 8.1 Documentation Support
      1. 8.1.1 Related Documentation
      2. 8.1.2 Device Nomenclature
    2. 8.2 Device Support
      1. 8.2.1 Development Support
        1. 8.2.1.1 Evaluation Modules
        2. 8.2.1.2 Spice Models
    3. 8.3 Receiving Notification of Documentation Updates
    4. 8.4 Support Resources
    5. 8.5 Trademarks
    6. 8.6 Electrostatic Discharge Caution
    7. 8.7 Glossary
  10. 9 Revision History
  11. 10Mechanical, Packaging, and Orderable Information

6.5.1 Programmable Soft-Start

The TPS74401 features a programmable, monotonic, voltage-controlled soft-start that is set with an external capacitor (CSS). This feature is important for many applications to eliminate power-up initialization problems when powering FPGAs, DSPs, or other processors. The controlled voltage ramp of the output also reduces peak inrush current during start-up, minimizing start-up transients to the input power bus.

To achieve a linear and monotonic soft-start, the TPS74401 error amplifier tracks the voltage ramp of the external soft-start capacitor until the voltage exceeds the internal reference. The soft-start ramp time depends on the soft-start charging current (ISS), the soft-start capacitance (CSS), and the internal reference voltage (VREF), and can be calculated using Equation 1:

Equation 1. t S S = V R E F × C S S I S S

If large output capacitors are used, the device current limit (ICL) and the output capacitor can set the start-up time. In this case, the start-up time is given by Equation 2:

Equation 2. t S S C L = V O U T n o m × C O U T I C L m i n

where

  • VOUT(nom) is the nominal set output voltage as set by the user,
  • COUT is the output capacitance,
  • and ICL(min) is the minimum current limit for the device.

In applications where monotonic start-up is required, the soft-start time given by Equation 1 must be set to be greater than Equation 2.

The maximum recommended soft-start capacitor is 0.015μF. Larger soft-start capacitors can be used and do not damage the device; however, the soft-start capacitor discharge circuit can not be able to fully discharge the soft-start capacitor when re-enabled. Soft-start capacitors larger than 0.015μF can be a problem in applications where the user must rapidly pulse the enable pin and also require the device to soft-start from ground. CSS must be low-leakage; X7R, X5R, or C0G dielectric materials are preferred. Table 6-2 lists suggested soft-start capacitor values.

Table 6-2 Standard Capacitor Values for Programming the Soft-Start Time
CSS SOFT-START TIME
(LEGACY CHIP)(1)		 SOFT-START TIME
(NEW CHIP)(1)
Open 0.1ms 0.25ms
470pF 0.5ms 0.7ms
1000pF 1ms 1.5ms
4700pF 5ms 7ms
0.01μF 10ms 15ms
0.015μF 16ms 22.6ms
(1)
t S S s = 0.8 × C S S F ÷ I S S