SLVSHT1 August   2024 TPS55287-Q1

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 I2C Timing Characteristics
    7. 5.7 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  VCC Power Supply
      2. 6.3.2  EXTVCC Power Supply
      3. 6.3.3  I2C Address Selection
      4. 6.3.4  Input Undervoltage Lockout
      5. 6.3.5  Enable and Programmable UVLO
      6. 6.3.6  Soft Start
      7. 6.3.7  Shutdown and Load Discharge
      8. 6.3.8  Switching Frequency
      9. 6.3.9  Switching Frequency Dithering
      10. 6.3.10 Inductor Current Limit
      11. 6.3.11 Internal Charge Path
      12. 6.3.12 Output Voltage Setting
      13. 6.3.13 Output Current Monitoring and Cable Voltage Droop Compensation
      14. 6.3.14 Output Current Limit
      15. 6.3.15 Overvoltage Protection
      16. 6.3.16 Output Short Circuit Protection
      17. 6.3.17 Thermal Shutdown
    4. 6.4 Device Functional Modes
      1. 6.4.1 PWM Mode
      2. 6.4.2 Power Save Mode
    5. 6.5 Programming
      1. 6.5.1 Data Validity
      2. 6.5.2 START and STOP Conditions
      3. 6.5.3 Byte Format
      4. 6.5.4 Acknowledge (ACK) and Not Acknowledge (NACK)
      5. 6.5.5 target Address and Data Direction Bit
      6. 6.5.6 Single Read and Write
      7. 6.5.7 Multi-Read and Multi-Write
  8. 7 Register Maps
    1. 7.1 REF Register (Address = 0h, 1h) [reset = 10100100b, 00000001b]
    2. 7.2 IOUT_LIMIT Register (Address = 2h) [reset = 11100100b]
    3. 7.3 VOUT_SR Register (Address = 3h) [reset = 00000001b]
    4. 7.4 VOUT_FS Register (Address = 4h) [reset = 00000011b]
    5. 7.5 CDC Register (Address = 5h) [reset = 11100000b]
    6. 7.6 MODE Register (Address = 6h) [reset = 00100000b]
    7. 7.7 STATUS Register (Address = 7h) [reset = 00000011b]
    8. 7.8 Register Summary
  9. 8 Application and Implementation
    1. 8.1 Application Information
    2. 8.2 Typical Application
      1. 8.2.1 Design Requirements
      2. 8.2.2 Detailed Design Procedure
        1. 8.2.2.1 Switching Frequency
        2. 8.2.2.2 Output Voltage Setting
        3. 8.2.2.3 Inductor Selection
        4. 8.2.2.4 Input Capacitor
        5. 8.2.2.5 Output Capacitor
        6. 8.2.2.6 Output Current Limit
        7. 8.2.2.7 Loop Stability
      3. 8.2.3 Application Curves
    3. 8.3 Power Supply Recommendations
    4. 8.4 Layout
      1. 8.4.1 Layout Guidelines
      2. 8.4.2 Layout Example
  10. 9 Device and Documentation Support
    1. 9.1 Device Support
      1. 9.1.1 Third-Party Products Disclaimer
      2. 9.1.2 Development Support
    2. 9.2 Receiving Notification of Documentation Updates
    3. 9.3 Support Resources
    4. 9.4 Trademarks
    5. 9.5 Electrostatic Discharge Caution
    6. 9.6 Glossary
  11. 10Revision History
  12. 11Mechanical, Packaging, and Orderable Information

8.2.2.3 Inductor Selection

Since the selection of the inductor affects steady state operation, transient behavior, and loop stability, the inductor is the most important component in power regulator design. There are three important inductor specifications: inductance, saturation current, and DC resistance.

The TPS55287-Q1 is designed to work with inductor values between 1µH and 10µH. The inductor selection is based on consideration of both buck and boost modes of operation.

For buck mode, the inductor selection is based on limiting the peak-to-peak current ripple to the maximum inductor current at the maximum input voltage. In CCM, Equation 9 shows the relationship between the inductance and the inductor ripple current.

Equation 9. TPS55287-Q1

where

  • VIN(MAX) is the maximum input voltage
  • VOUT is the output voltage
  • ΔIL(P-P) is the peak to peak ripple current of the inductor
  • fSW is the switching frequency

For a certain inductor, the inductor ripple current achieves maximum value when VOUT equals half of the maximum input voltage. Choosing higher inductance gets smaller inductor current ripple while smaller inductance gets larger inductor current ripple.

For boost mode, the inductor selection is based on limiting the peak-to-peak current ripple to the maximum inductor current at the maximum output voltage. In CCM, Equation 10 shows the relationship between the inductance and the inductor ripple current.

Equation 10. TPS55287-Q1

where

  • VIN is the input voltage
  • VOUT(MAX) is the maximum output voltage
  • ΔIL(P-P) is the peak to peak ripple current of the inductor
  • fSW is the switching frequency

For a certain inductor, the inductor ripple current achieves maximum value when VIN equals to the half of the maximum output voltage. Choosing higher inductance gets smaller inductor current ripple while smaller inductance gets larger inductor current ripple.

For this application example, a 4.7µH inductor is selected, which produces approximate maximum inductor current ripple of 50% of the highest average inductor current in buck mode and 50% of the highest average inductor current in boost mode.

In buck mode, the inductor DC current equals to the output current. In boost mode, the inductor DC current can be calculated with Equation 11.

Equation 11. TPS55287-Q1

where

  • VOUT is the output voltage
  • IOUT is the output current
  • VIN is the input voltage
  • η is the power conversion efficiency

For a given maximum output current of the buck-boost converter TPS55287-Q1, the maximum inductor DC current happens at the minimum input voltage and maximum output voltage. Set the inductor current limit of the TPS55287-Q1 higher than the calculated maximum inductor DC current to make sure the TPS55287-Q1 has the desired output current capability.

In boost mode, the inductor ripple current is calculated with Equation 12.

Equation 12. TPS55287-Q1

where

  • ΔIL(P-P) is the inductor ripple current
  • L is the inductor value
  • fSW is the switching frequency
  • VOUT is the output voltage
  • VIN is the input voltage

Therefore, the inductor peak current is calculated with Equation 13.

Equation 13. TPS55287-Q1

Normally, it is advisable to work with an inductor peak-to-peak current of less than 40% of the average inductor current for maximum output current. A smaller ripple from a larger valued inductor reduces the magnetic hysteresis losses in the inductor and EMI, but in the same way, load transient response time is increased. The selected inductor must have higher saturation current than the calculated peak current.

The conversion efficiency is dependent on the resistance of its current path. The switching loss associated with the switching MOSFETs, and the inductor core loss. Therefore, the overall efficiency is affected by the inductor DC resistance (DCR), equivalent series resistance (ESR) at the switching frequency, and the core loss. Table 8-2 lists recommended inductors for the TPS55287-Q1. In this application example, the Coilcraft inductor XAL7070-472 is selected for its small size, high saturation current, and small DCR.

Table 8-2 Recommended Inductors
PART NUMBERL (µH)DCR (MAXIMUM) (mΩ)SATURATION CURRENT / HEAT RATING CURRENT (A)SIZE (L x W x H mm)VENDOR(1)
XAL7070-472ME4.714.315.2/10.57.5 × 7.2 × 7.0Coilcraft
VCHA085D-4R7MS64.715.616.0/8.88.7 × 8.2 × 5.2Cyntec
IHLP4040DZER4R7M014.716.517/9.510.2 × 10.2 × 4.0Vishay
(1) See the Third-party Products disclaimer.