SBVS442A December   2022  – October 2025 TPS748A

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. 6 Detailed Description
    1. 6.1 Overview
    2. 6.2 Functional Block Diagram
    3. 6.3 Feature Description
      1. 6.3.1 Enable and Shutdown
      2. 6.3.2 Active Discharge
      3. 6.3.3 Power-Good Output (PG)
      4. 6.3.4 Internal Current Limit
      5. 6.3.5 Thermal Shutdown Protection (TSD)
    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 Dropout Voltage
      3. 7.1.3 Output Noise
      4. 7.1.4 Estimating Junction Temperature
      5. 7.1.5 Soft Start, Sequencing, and Inrush Current
      6. 7.1.6 Power-Good Operation
    2. 7.2 Typical Application
      1. 7.2.1 Design Requirements
      2. 7.2.2 Detailed Design Procedure
      3. 7.2.3 Application Curve
    3. 7.3 Power Supply Recommendations
    4. 7.4 Layout
      1. 7.4.1 Layout Guidelines
        1. 7.4.1.1 Board Layout
        2. 7.4.1.2 DSQ Package—High CTE Mold Compound
      2. 7.4.2 Layout Example
  9. 8 Device and Documentation Support
    1. 8.1 Device Support
      1. 8.1.1 Device Nomenclature
    2. 8.2 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

Package Options

Mechanical Data (Package|Pins)
Thermal pad, mechanical data (Package|Pins)
Orderable Information

7.1.5 Soft Start, Sequencing, and Inrush Current

Soft-start refers to the ramp-up characteristic of the output voltage during LDO turn-on after EN and UVLO achieve threshold voltage.

To achieve a linear and monotonic soft-start, the TPS748A error amplifier tracks the voltage ramp of the external soft-start capacitor. This tracking continues until the voltage exceeds the internal reference. The soft-start ramp time depends on the soft-start charging current (ISS), soft-start capacitance (CSS), and the internal reference voltage (VREF). Equation 7 estimates the start up.

Equation 7. TPS748A

If large output capacitors are used, the device current limit (ICL) and the output capacitor potentially sets the start-up time. In this case, Equation 8 gives the start-up time.

Equation 8. TPS748A

The maximum recommended soft-start capacitor is 100nF. Larger soft-start capacitors do not damage the device; however, the soft-start capacitor discharge circuit is potentially unable to fully discharge the soft-start capacitor when enabled. Soft-start capacitors larger than 100nF are potentially a problem in applications where rapidly pulsing the enable pin is required but the device soft-starts from ground. Confirm CSS is 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 (nF) DELAY TIME (ms) RAMP-UP TIME (ms) START-UP TIME (ms)
1 0.2 0.1 0.3
4.7 0.2 0.30 0.50
10 0.2 0.7 0.9
22 0.2 1.5 1.7
47 0.2 3.1 3.3
100 0.2 6.7 6.9

VOUT(NOM) is the nominal set output voltage, 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 7 cannot be greater than Equation 8.

Although the device does not have sequencing requirements, following the sequencing order of BIAS, IN, and EN verifies the soft-start starts from zero.

Inrush current is defined as the current into the LDO at the IN pin during start-up. Inrush current then consists primarily of the sum of load current and the current used to charge the output capacitor. This current is difficult to measure because the input capacitor is removed, which is not recommended. However, Equation 9 estimates this soft-start current:

Equation 9. TPS748A

where:

  • VOUT(t) is the instantaneous output voltage of the turn-on ramp
  • dVOUT(t) / dt is the slope of the VOUT ramp
  • RLOAD is the resistive load impedance