SLVSCT3 March   2015 TPS51275B-1

PRODUCTION DATA.  

  1. 1 Features
  2. 2 Applications
  3. 3 Description
  4. 4 Typical Application Diagram
  5. 5 Revision History
  6. 6 Pin Configuration and Functions
  7. 7 Specifications
    1. 7.1 Absolute Maximum Ratings
    2. 7.2 ESD Ratings
    3. 7.3 Recommended Operating Conditions
    4. 7.4 Thermal Information
    5. 7.5 Electrical Characteristics
    6. 7.6 Timing Requirements
    7. 7.7 Switching Characteristics
    8. 7.8 Typical Characteristics
  8. 8 Detailed Description
    1. 8.1 Overview
    2. 8.2 Functional Block Diagram
    3. 8.3 Feature Description
      1. 8.3.1  PWM Operations
      2. 8.3.2  Adaptive On-Time and PWM Frequency Control
      3. 8.3.3  Light-Load Condition in Out-of-Audio Operation
      4. 8.3.4  Enable and Power Good
      5. 8.3.5  Soft-Start and Discharge
      6. 8.3.6  VREG5 and VREG3 Linear Regulators
      7. 8.3.7  VCLK for Charge Pump
      8. 8.3.8  Overcurrent Protection
      9. 8.3.9  Output Overvoltage and Undervoltage Protection
      10. 8.3.10 Undervoltage Lockout Protection
      11. 8.3.11 Over-Temperature Protection (OTP)
    4. 8.4 Device Functional Modes
      1. 8.4.1 D-CAP Mode
  9. 9 Application and Implementation
    1. 9.1 Application Information
    2. 9.2 Typical Application
      1. 9.2.1 Design Requirements
      2. 9.2.2 Detailed Design Procedure
        1. 9.2.2.1 External Components Selection
          1. 9.2.2.1.1 Step 1. Determine the Value of R1 and R2
          2. 9.2.2.1.2 Step 2. Select the Inductor
          3. 9.2.2.1.3 Step 3. Select Output Capacitors
      3. 9.2.3 Application Curves
  10. 10Power Supply Recommendations
  11. 11Layout
    1. 11.1 Layout Guidelines
      1. 11.1.1 Placement
      2. 11.1.2 Routing (Sensitive Analog Portion)
      3. 11.1.3 Routing (Power portion)
    2. 11.2 Layout Example
  12. 12Device and Documentation Support
    1. 12.1 Device Support
      1. 12.1.1 Third-Party Products Disclaimer
    2. 12.2 Trademarks
    3. 12.3 Electrostatic Discharge Caution
    4. 12.4 Glossary
  13. 13Mechanical, Packaging, and Orderable Information

Package Options

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

9 Application and Implementation

NOTE

Information in the following applications sections is not part of the TI component specification, and TI does not warrant its accuracy or completeness. TI’s customers are responsible for determining suitability of components for their purposes. Customers should validate and test their design implementation to confirm system functionality.

9.1 Application Information

The TPS51275B-1 device is typically used as a dual-synchronous buck controller, which converts an input voltage ranging from 5 to 24 V, to output voltage of 5 V and 3.3 V (respectively). The device is targeted for power-supply solutions for notebook and desktop computer systems.

9.2 Typical Application

TPS51275B-1 typ_app_slvsct3.gifFigure 18. Detailed Application Schematic

9.2.1 Design Requirements

For this design example, use the parameters listed in Table 2.

Table 2. Design Parameters

PARAMETER VALUE
Input voltage range 5.5 to 24 V
Channel 1 output voltage 5 V
Channel 1 output voltage 8 A
Channel 2 output voltage 3.3 V
Channel 2 output voltage 8 A

9.2.2 Detailed Design Procedure

9.2.2.1 External Components Selection

The external components selection is relatively simple for a design using D-CAP mode. Table 3 lists the key external components that are recommended for this application design (see Figure 18).

Table 3. Key External Components

REFERENCE
DESIGNATOR		 FUNCTION MANUFACTURER PART NUMBER
L1 Output Inductor (5 VOUT) Alps GLMC3R303A
L2 Output Inductor (3.3 VOUT) Alps GLMC2R203A
C1 Output Capacitor (5 VOUT) SANYO 6TPE220MAZB × 2
C2 Output Capacitor (3.3 VOUT) SANYO 6TPE220MAZB × 2
Q1 MOSFET (5 VOUT) TI CSD87330Q3D
Q2 MOSFET (3.3 VOUT) TI CSD87330Q3D

9.2.2.1.1 Step 1. Determine the Value of R1 and R2

The recommended value of R2 is between 10 kΩ and 20 kΩ. Use Equation 5 to calculate the value of R1.

Equation 5. TPS51275B-1 q_r1_slvsct3.gif

9.2.2.1.2 Step 2. Select the Inductor

The inductance value should be determined to give the ripple current of approximately ½ to ¼ of maximum output current and less than half of OCL (valley) threshold. A larger ripple current increases the output ripple voltage, improves signal-to-noise ratio, and helps ensure stable operation.

Equation 6. TPS51275B-1 q_l_slvsct3.gif

The calculated inductance for channel1 and channel2 is 3.3 µH and 2 µH, respectively. For this design, select the inductance values of 3.3 µH and 2.2 µH for these two channels.

The inductor must also have low DCR to achieve good efficiency, as well as enough room above the peak inductor current before saturation. Use Equation 7 to calculate the peak inductor current.

Equation 7. TPS51275B-1 q_iindpeak_slvsct3.gif

9.2.2.1.3 Step 3. Select Output Capacitors

Organic semiconductor capacitors or specialty polymer capacitors are recommended. Determine the ESR to meet the required ripple voltage. Use Equation 8 to quickly calculate the ESR.

Equation 8. TPS51275B-1 q_esr_slvsct3.gif

where

  • D as the duty-cycle factor
  • the required output ripple voltage slope is approximately 20 mV per tSW (switching period) in terms of the VFBx pin

The calculated minimum-required ESR for channel1 and channel2 is 9.9 mΩ and 7.8 mΩ, respectively. For this design, use two 220-µF, 35-mΩ polymer capacitors in parallel for each channel. The equivalent ESR is 17.5 mΩ which meets the minimum ESR requirement. Using a value of 440 µF for the output capacitor and 17.5 mΩ of ESR, the resulting value of the 0-dB frequency, f0 (see Equation 4), is approximately 21 kHz which is much less than fSW / 4 for both channels.

TPS51275B-1 rip_slope_jitter_slvsct3.gifFigure 19. Ripple Voltage Slope and Jitter Performance

9.2.3 Application Curves

TPS51275B-1 app_curve_startup_slvsct3.gif
Figure 20. Startup
TPS51275B-1 app_curve_5-V_load_transient_slvsct3.gif
IOUT1 = 0 to 3 A IOUT2 = 0 A VVIN = 7.4 V
Figure 22. 5-V Load Transient
TPS51275B-1 app_curve_5-V_switch_over_slvsct3.gif
Figure 24. 5-V Switch Over
TPS51275B-1 app_curve_shutdown_slvsct3.gif
Figure 21. Shutdown
TPS51275B-1 app_curve_3p3-V_load_transient_slvsct3.gif
IOUT1 = 0 A IOUT2 = 0 to 3 A VVIN = 7.4 V
Figure 23. 3.3-V Load Transient
TPS51275B-1 app_curve_3p3-V_switchover_slvsct3.gif
Figure 25. 3.3-V Switch Over