SLUS786H OCTOBER   2007  – January 2015 TPS51125

PRODUCTION DATA.  

  1. 1 Features
  2. 2 Applications
  3. 3 Description
  4. 4 Revision History
  5. 5 Pin Configuration and 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  PWM Operations
      2. 7.3.2  Adaptive On-Time Control and PWM Frequency
      3. 7.3.3  Loop Compensation
      4. 7.3.4  Ramp Signal
      5. 7.3.5  Light-Load Condition in Auto-Skip Operation
      6. 7.3.6  Out-of-Audio Light-Load Operation
      7. 7.3.7  VREG5/VREG3 Linear Regulators
      8. 7.3.8  VREG5 Switch Over
      9. 7.3.9  VREG3 Switch Over
      10. 7.3.10 Powergood
      11. 7.3.11 Output Discharge Control
      12. 7.3.12 Low-Side Driver
      13. 7.3.13 High-Side Driver
      14. 7.3.14 VCLK for Charge Pump
      15. 7.3.15 Current Protection
      16. 7.3.16 Overvoltage and Undervoltage Protection
      17. 7.3.17 UVLO Protection
      18. 7.3.18 Thermal Shutdown
    4. 7.4 Device Functional Modes
      1. 7.4.1 Enable and Soft-Start
  8. 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 Determine Output Voltage
        2. 8.2.2.2 Choose the Inductor
        3. 8.2.2.3 Choose the Output Capacitors
        4. 8.2.2.4 Choose the Low-Side MOSFET
      3. 8.2.3 Application Curves
  9. 9 Power Supply Recommendations
  10. 10Layout
    1. 10.1 Layout Guidelines
    2. 10.2 Layout Example
  11. 11Device and Documentation Support
    1. 11.1 Device Support
      1. 11.1.1 Third-Party Products Disclaimer
    2. 11.2 Trademarks
    3. 11.3 Electrostatic Discharge Caution
    4. 11.4 Glossary
  12. 12Mechanical, Packaging, and Orderable Information

Package Options

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

8 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.

8.1 Application Information

The TPS51125 is typically used as a dual-synchronous buck controller, which convert an input voltage ranging from 5.5 V to 28 V, to output voltage 5 V and 3.3 V respectively, targeted for notebook system-power supply solutions.

8.2 Typical Application

fig42_lus786.gifFigure 34. 5-V/8-A, 3.3-V/8-A Application Circuit (245-kHz/305-kHz Setting)

8.2.1 Design Requirements

Table 3. Design Parameters

PARAMETER VALUE
Input voltage range 5.5 V to 28 V
Channel 1 output voltage 5 V
Channel 1 output current 8 A
Channel 2 output voltage 3.3 V
Channel 2 output current 8 A

8.2.2 Detailed Design Procedure

Table 4. List of Materials for 5-V / 8-A, 3.3-V / 8-A Application Circuit

SYMBOL SPECIFICATION MANUFACTURER PART NUMBER
C1, C2, C8, C9 10 μ F, 25 V Taiyo Yuden TMK325BJ106MM
C3 10 μF, 6.3 V TDK C2012X5R0J106K
C11 33 μF, 6.3 V TDK C3216X5RBJ336M
C5, C10 330 μF, 6.3 V, 25 mΩ Sanyo 6TPE330ML
L1, L2 3.3 μH, 15.6 A, 5.92 mΩ TOKO FDA1055-3R3M
Q1, Q3 30 V, 9.5 mΩ IR IRF7821
Q2, Q4(1) 30 V, 12 mΩ Fairchild FDS6690AS
(1) Please use MOSFET with integrated Schottky barrier diode (SBD) for low side, or add SBD in parallel with normal MOSFET.

8.2.2.1 Determine Output Voltage

The output voltage is programmed by the voltage-divider resistor, R1 and R2 shown in Figure 31. R1 is connected between VFBx pin and the output, and R2 is connected betwen the VFBx pin and GND. Recommended R2 value is from 10 kΩ to 20 kΩ. Determine R1 using equation as below.

Equation 5. q_r2_lus786.gif

8.2.2.2 Choose the Inductor

The inductance value should be determined to give the ripple current of approximately 1/4 to 1/2 of maximum output current. Larger ripple current increases output ripple voltage and improves S/N ratio and helps stable operation.

Equation 6. q_l_lus786.gif

The inductor also needs to have low DCR to achieve good efficiency, as well as enough room above peak inductor current before saturation. The peak inductor current can be estimated as follows.

Equation 7. q_iindpeak_lus786.gif

8.2.2.3 Choose the Output Capacitors

Organic semiconductor capacitors or specialty polymer capacitors are recommended. Determine ESR to meet required ripple voltage. A quick approximation is as shown in Equation 8.

Equation 8. q_esr_lus786.gif

where

  • D is the duty cycle
  • the required output ripple slope is approximately 20 mV per tSW (switching period) in terms of VFB terminal voltage

8.2.2.4 Choose the Low-Side MOSFET

It is highly recommended that the low-side MOSFET should have an integrated Schottky barrier diode, or an external Schottky barrier diode in parallel to achieve stable operation.

8.2.3 Application Curves

fig29_lus786.gif
Figure 35. 5-V Load Transient Response
fig31_lus786.gif
Figure 37. 5-V Start-up Waveforms
fig33_lus786.gif
Figure 39. 5-V Switchover Waveforms
fig35_lus786.gif
Figure 41. 5-V Soft-Stop Waveforms
fig30_lus786.gif
Figure 36. 3.3-V Load Transient Response
fig32_lus786.gif
Figure 38. 3.3-V Start-up Waveforms
fig34_lus786.gif
Figure 40. 3.3-V Switchover Waveforms
fig36_lus786.gif
Figure 42. 3.3-V Soft-Stop Waveforms