SBVS135J June   2010  – January 2018 TPS7A80

PRODUCTION DATA.  

  1. 1 Features
  2. 2 Applications
  3. 3 Description
    1.     Device Images
      1.      Typical Application Diagram
  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 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 Internal Current Limit
      2. 7.3.2 Shutdown
      3. 7.3.3 Start-Up
      4. 7.3.4 Undervoltage Lockout (UVLO)
    4. 7.4 Device Functional Modes
  8. 8 Application and Implementation
    1. 8.1 Application Information
    2. 8.2 Typical Application
      1. 8.2.1 Design Requirements
        1. 8.2.1.1 Dropout Voltage
        2. 8.2.1.2 Minimum Load
        3. 8.2.1.3 Input and Output Capacitor Requirements
        4. 8.2.1.4 Transient Response
      2. 8.2.2 Detailed Design Procedure
        1. 8.2.2.1 Output Noise
      3. 8.2.3 Application Curve
  9. 9 Power Supply Recommendations
  10. 10Layout
    1. 10.1 Layout Guidelines
      1. 10.1.1 Board Layout Recommendations to Improve PSRR and Noise Performance
      2. 10.1.2 Thermal Considerations
      3. 10.1.3 Power Dissipation
      4. 10.1.4 Estimating Junction Temperature
    2. 10.2 Layout Example
  11. 11Device and Documentation Support
    1. 11.1 Documentation Support
      1. 11.1.1 Related Documentation
    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

7.3.3 Start-Up

Through a lower resistance, the band-gap reference can quickly charge the noise reduction capacitor (CNR). The TPS7A80 have a quick-start circuit to quickly charge CNR, if present; see the Functional Block Diagram. At start-up, this quick-start switch is closed, with only 33 kΩ of resistance between the band-gap reference and the NR pin. The quick-start switch opens approximately 2ms after any device enabling event, and the resistance between the band-gap reference and the NR pin becomes higher in value (approximately 250 kΩ) to form a very good low-pass (RC) filter. This low-pass filter achieves very good noise reduction for the reference voltage.

Inrush current can be a problem in many applications. The 33-kΩ resistance during the start-up period is intentionally put there to slow down the reference voltage ramp up, thus reducing the inrush current. For example, the capacitance of connecting the recommended CNR value of 0.01 μF along with the 33-kΩ resistance causes approximately 1-ms RC delay. Start-up time with the other CNR values can be calculated as Equation 1:

Equation 1. TPS7A80 q_tstr_bvs135.gif

Equation 1 is valid up to tSTR = 2 ms or CNR = 26 nF, whichever is smaller.

Although the noise reduction effect is nearly saturated at 0.01 μF, connecting a CNR value greater than 0.01 μF can help reduce noise slightly more; however, start-up time will be extremely long because the quick-start switch opens after approximately 2 ms. That is, if CNR is not fully charged during this 2 ms period, CNR finishes charging through a higher resistance of 250 kΩ, and takes much longer to fully charge.

A low leakage CNR should be used; most ceramic capacitors are suitable.