SLVS616C November   2005  – December 2014 TPS51124

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 Recommended Operating Conditions
    3. 6.3 Thermal Information
    4. 6.4 Electrical Characteristics
    5. 6.5 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 Operation
      2. 7.3.2  Light-Load Condition
      3. 7.3.3  Low-Side Driver
      4. 7.3.4  High-Side Driver
      5. 7.3.5  PWM Frequency and Adaptive On-Time Control
      6. 7.3.6  Powergood
      7. 7.3.7  Output Discharge Control
      8. 7.3.8  Current Protection
      9. 7.3.9  Over and Undervoltage Protection
      10. 7.3.10 UVLO Protection
      11. 7.3.11 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
      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 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 TPS51124 is typically used as a dual-synchronous buck controller, which convert an input voltage ranging from 3V to 28 V, to output voltage ranging 0.76 V to 5.5 V, targeted for notebook I/O and low voltage system bus supply solutions.

8.2 Typical Application

typ_app_lvs616.gifFigure 18. Typical Application Circuit

Table 2. Typical Application Circuit Components

SYMBOL SPECIFICATION MANUFACTURER PART NUMBER
C1 330 μF, 2.5 V, 15 mΩ SANYO 2R5TPE330MF
C4 330 μF, 2.5 V, 18 mΩ SANYO 2R5TPE330MI
L1, L2 1 μH, 2 mΩ TOKO FDA1254-1R0M
C3, C6 10 μF, 25 V TDK C3225X5R1E106
Q1, Q3 30 V, 13 mΩ International Rectifier IRF7821
Q2, Q4 30 V, 7 mΩ International Rectifier IRF8113

8.2.1 Design Requirements

Table 3. Design Parameters

PARAMETER VALUE
Input voltage range 3 V to 28 V
Channel 1 output voltage 1.05 V
Channel 1 output current 10 A
Channel 2 output voltage 1.5 V
Channel 2 output current 10 A

8.2.2 Detailed Design Procedure

Figure 19 shows a simplified buck converter system using D-CAP Mode.

modu_sch_lvs616.gifFigure 19. Simplifying the Modulator

The output voltage is compared with an internal reference voltage after divider resistors, R1 and R2. The PWM comparator determines the timing to turn on the high-side MOSFET. The gain and speed of the comparator is high enough to keep the voltage at the beginning of each on cycle (or the end of off cycle) substantially constant. The DC output voltage may have line regulation due to ripple amplitude that slightly increases as the input voltage increase.

For the loop stability, the 0-dB frequency, f0, defined in Equation 4 needs to be lower than 1/4 of the switching frequency.

Equation 4. q4_fo_lvs616.gif

As f0 is determined solely by the output capacitor’s characteristics, loop stability of D-CAP Mode is determined by the capacitor’s chemistry. For example, specialty polymer capacitors (SP-CAP) have Co in the order of several 100 μF and ESR in range of 10 mΩ. These make f0 in the order of 100 kHz or less and the loop is stable. However, ceramic capacitors have f0 at more than 700 kHz, which is not suitable for this operational mode.

Although D-CAP Mode provides many advantages such as ease-of-use, minimum external components configuration, and extremely short response time, a sufficient amount of feedback signal needs to be provided by an external circuit to reduce jitter level. This is due to not employing an error amplifier in the loop. The required signal level is approximately 10 mV at the comparing point (VFB terminal). This gives Vripple at the output node as shown in the following equation.

Equation 5. q5_vripple_lvs616.gif

The output capacitor's ESR should meet this requirement.

The external components selection is much simpler in D-CAP Mode.

  1. Determine the value of R1 and R2.

    2. Recommended R2 value is from 10 kΩ to 100 kΩ. Determine R1 using the following equation.

    Equation 6. q6_r1_lvs616.gif
  2. Choose inductor.

    3. 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 the output ripple voltage, improves S/N ratio, and contributes to a stable operation.

    Equation 7. q7_l_lvs616.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 8. q8_iind_lvs616.gif
  3. Choose output capacitor(s).

    4. Organic semiconductor capacitor(s) or specialty polymer capacitor(s) are recommended. Determine ESR to meet the required ripple voltage indicated previously. A quick approximation is shown here:

    Equation 9. eq9_esr_lvs616.gif

8.2.3 Application Curves

trns_res_lvs616.gif
Figure 20. 1.05-V Load Transient Response
startup_wf_lvs616.gif
Figure 22. 1.05-V Start-Up Waveforms
dischg_wf_lvs616.gif
Figure 24. 1.05-V Discharge Waveforms
trns2_res_lvs616.gif
Figure 21. 1.5-V Load Transient Response
startup2_wf_lvs616.gif
Figure 23. 1.5-V Start-Up Waveforms
(1) The data of – are measured from the Typical Application Circuit of and
dischg2_wf_lvs616.gif
Figure 25. 1.5-V Discharge Waveforms