SLVSGP3A May   2023  – February 2024 TPS54KB20

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  Internal VCC LDO and Using External Bias On the VCC Pin
      2. 6.3.2  Enable
      3. 6.3.3  Adjustable Soft Start
      4. 6.3.4  Power Good
      5. 6.3.5  Output Voltage Setting
      6. 6.3.6  Remote Sense
      7. 6.3.7  D-CAP4 Control
      8. 6.3.8  Multifunction Select (MSEL) Pin
      9. 6.3.9  Low-side MOSFET Zero-Crossing
      10. 6.3.10 Current Sense and Positive Overcurrent Protection
      11. 6.3.11 Low-side MOSFET Negative Current Limit
      12. 6.3.12 Overvoltage and Undervoltage Protection
      13. 6.3.13 Output Voltage Discharge
      14. 6.3.14 UVLO Protection
      15. 6.3.15 Thermal Shutdown
    4. 6.4 Device Functional Modes
      1. 6.4.1 Auto-Skip Eco-mode Light Load Operation
      2. 6.4.2 Forced Continuous-Conduction Mode
      3. 6.4.3 Powering the Device From a Single Bus
      4. 6.4.4 Powering the Device From a Split-rail Configuration
  8. 7 Application and Implementation
    1. 7.1 Application Information
    2. 7.2 Typical Application
      1. 7.2.1 Design Requirements
      2. 7.2.2 Detailed Design Procedure
        1. 7.2.2.1  Output Voltage Setting Point
        2. 7.2.2.2  Choose the Switching Frequency and the Operation Mode
        3. 7.2.2.3  Choose the Inductor
        4. 7.2.2.4  Set the Current Limit (ILIM)
        5. 7.2.2.5  Choose the Output Capacitor
        6. 7.2.2.6  RAMP Selection
        7. 7.2.2.7  Choose the Input Capacitors (CIN)
        8. 7.2.2.8  Soft-Start Capacitor (SS Pin)
        9. 7.2.2.9  EN Pin Resistor Divider
        10. 7.2.2.10 VCC Bypass Capacitor
        11. 7.2.2.11 BOOT Capacitor
        12. 7.2.2.12 RC Snubber
        13. 7.2.2.13 PG Pullup Resistor
      3. 7.2.3 Application Curves
    3. 7.3 Power Supply Recommendations
    4. 7.4 Layout
      1. 7.4.1 Layout Guidelines
      2. 7.4.2 Layout Example
  9. 8 Device and Documentation Support
    1. 8.1 Documentation Support
      1. 8.1.1 Related Documentation
    2. 8.2 Receiving Notification of Documentation Updates
    3. 8.3 Support Resources
    4. 8.4 Trademarks
    5. 8.5 Electrostatic Discharge Caution
    6. 8.6 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

7.2.2.5 Choose the Output Capacitor

There are three considerations for selecting the value of the output capacitor:

  1. Stability
  2. Steady state output voltage ripple
  3. Regulator transient response to a change load current
First, calculate the minimum output capacitance based on these three requirements. Equation 22 calculates the minimum capacitance to keep the LC double pole below the fP(MAX) to meet stability requirements. This requirement helps to keep the LC double pole close to the internal zero. Equation 23 calculates the minimum capacitance to meet the steady state output voltage ripple requirement of 33mV. This calculation is for CCM operation and does not include the portion of the output voltage ripple caused by the ESR or ESL of the output capacitors.

Equation 22. C O U T _ S T A B I L I T Y > 1 2 π × f P ( RAMP4 ) × 1 + V O U T V I N T Y P 2 2 × 1 L O U T = 1 2 π × 20.3 k H z × 1 + 3.3 V 12 V 2 2 × 1 0.47 μ H = 113 μ F
Equation 23. C O U T _ R I P P L E > I R I P P L E 8 × V R I P P L E × f S W = 7   A 8 × 33   m V × 800   k H z = 33   μ F

Equation 25 and Equation 26 calculate the minimum capacitance to meet the transient response requirement of 99mV with a 10A step. These equations calculate the necessary output capacitance to hold the output voltage steady while the inductor current ramps up or ramps down after a load step. Calculations determine only 418.5μF is needed to meet the transient response requirement. This calculation assumes instant load step.

Equation 24. C O U T _ U N D E R S H O O T > L × I S T E P 2 × V O U T V I N m i n × f S W + t O F F _ M I N m a x 2 × V T R A N S × V O U T × V I N m i n - V O U T V I N m i n × f S W - t O F F _ M I N m a x
Equation 25. C O U T _ U N D E R S H O O T > 0.47   μ H × 10   A 2 × 3.3   V 4.5   V × 800   k H z + 150   n s 2 × 99   m V × 3.3   V × 4.5   V - 3.3   V 4.5   V × 800   k H z - 150   n s = 418.5   μ F
Equation 26. C O U T _ O V E R S H O O T > L × I S T E P 2 2 × V T R A N S × V O U T = 0.47   μ H × 10   A 2 2 × 99   m V × 3.3   V = 71.9   μ F

The output capacitance needed to meet the undershoot requirement is the highest value, so this sets the required minimum output capacitance for this example. Stability requirements can also limit the maximum output capacitance. Equation 27 calculates the recommended maximum output capacitance. This calculation keeps the LC double pole above 1/100th the fSW. Using more output capacitance is possible, but the stability must be checked through a bode plot or transient response measurement. The selected output capacitance is 7 × 22μF, 10V ceramic capacitors. When using ceramic capacitors, the capacitance must be derated due to DC and AC bias effects. The selected capacitors derate to 58% the nominal value giving an effective total capacitance of 89μF. Additionally, 2 × 220μF, 6.3V bulk capacitors are also selected, increasing the effective total output capacitance to 529μF. This effective capacitance meets the minimum and maximum requirements.

Equation 27. C O U T _ S T A B I L I T Y < 50 π × f S W 2 × 1 L = 50 π × 800   k H z 2 × 1 0.47   μ H = 842   μ F

This application uses all ceramic capacitors so the effects of ESR on the ripple and transient were ignored. If using non ceramic capacitors, as a starting point, the ESR must be below the values calculated in Equation 28 to meet the ripple requirement and Equation 29 to meet the transient requirement. For more accurate calculations or if using mixed output capacitors, the impedance of the output capacitors must be used to determine if the ripple and transient requirements can be met.

Equation 28. R E S R _ R I P P L E < V R I P P L E I R I P P L E = 33   m V 7   A = 4.7   m Ω
Equation 29. R E S R _ T R A N S < V T R A N S I S T E P = 99   m V 10   A = 9.9   m Ω