SBVS082K June   2007  – June 2024 TPS74901

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: IOUT = 50mA
    7. 5.7 Typical Characteristics: IOUT = 1 A
  7. 6 Detailed Description
    1. 6.1 Overview
    2. 6.2 Functional Block Diagrams
    3. 6.3 Feature Description
      1. 6.3.1 Enable and Shutdown
      2. 6.3.2 Power-Good
      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
  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
      5. 7.1.5 Programmable Soft-Start
      6. 7.1.6 Sequencing Requirements
    2. 7.2 Typical Application
      1. 7.2.1 Design Requirements
      2. 7.2.2 Detailed Design Procedure
      3. 7.2.3 Application Curves
    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 Device Support
      1. 8.1.1 Device Nomenclature
      2. 8.1.2 Development Support
        1. 8.1.2.1 Evaluation Modules
        2. 8.1.2.2 Spice Models
    2. 8.2 Documentation Support
      1. 8.2.1 Related Documentation
    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

7.1.5 Programmable Soft-Start

The TPS74901 features a programmable, monotonic, voltage-controlled soft-start that is set with an external capacitor (CSS). This feature is important for many applications because power-up initialization problems are eliminated 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 transient events to the input power bus.

To achieve a linear and monotonic soft-start, the TPS74901 error amplifier tracks the voltage ramp of the external soft-start capacitor until the voltage exceeds the internal reference. The soft-start ramp time is dependent on the soft-start charging current (ISS), soft-start capacitance (CSS), and the internal reference voltage (VREF), and can be calculated using Equation 2.

Equation 2. TPS74901

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 3:

Equation 3. TPS74901

where:

  • VOUT(NOM) is the nominal set output voltage
  • COUT is the output capacitance
  • 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 2 must be set to be greater than Equation 3.

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 possibly be unable to fully discharge the soft-start capacitor when enabled. Soft-start capacitors larger than 0.015µF can be a problem in applications where the enable pin must be rapidly pulsed while still requiring the device to soft-start from ground. CSS must be low-leakage; X7R, X5R, or C0G dielectric materials are preferred. See Table 7-1 for suggested soft-start capacitor values.

Table 7-1 Standard Capacitor Values for Programming the Soft-Start Time
CSS(1) SOFT-START TIME (Legacy Chip) SOFT-START TIME (New Chip)
Open 0.1ms 0.25ms
270pF 0.5ms 0.4ms
560pF 1ms 0.8ms
2.7nF 5ms 4.1ms
5.6nF 10ms 8.5ms
0.01µF 18ms 15ms
(1) tSS(s) = 0.8 × CSS(F) / ISS, where tSS(s) = soft-start time in seconds.

Another option for setting the start-up rate is to use a feedforward capacitor. See the Pros and Cons of Using a Feedforward Capacitor with a Low-Dropout Regulator application note for more information.