SLVSAE1B October   2010  – September 2015 TPS65708

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 Parameter Measurement Information
    1. 7.1 Setup
  8. 8 Detailed Description
    1. 8.1 Overview
    2. 8.2 Functional Block Diagram
    3. 8.3 Feature Description
      1. 8.3.1  DC-DC Converters
      2. 8.3.2  Power Save Mode
      3. 8.3.3  Dynamic Voltage Positioning
      4. 8.3.4  Soft Start
      5. 8.3.5  100% Duty Cycle Low Dropout Operation
      6. 8.3.6  180° Out-of-Phase Operation
      7. 8.3.7  Undervoltage Lockout and Enable for DCDC1, DCDC2, LDO1, and LDO2
      8. 8.3.8  Output Voltage Discharge
      9. 8.3.9  Power-Up Sequencing
      10. 8.3.10 Short-Circuit Protection
      11. 8.3.11 Thermal Shutdown
      12. 8.3.12 LDOs
      13. 8.3.13 LED Driver
    4. 8.4 Device Functional Modes
  9. 9 Application and Implementation
    1. 9.1 Application Information
    2. 9.2 Typical Applications
      1. 9.2.1 Powering All Rails from the Input Supply of 5 V
        1. 9.2.1.1 Design Requirements
        2. 9.2.1.2 Detailed Design Procedure
          1. 9.2.1.2.1 Output Filter Design (Inductor and Output Capacitor)
            1. 9.2.1.2.1.1 Inductor Selection
            2. 9.2.1.2.1.2 Output Capacitor Selection
            3. 9.2.1.2.1.3 Input Capacitor Selection
        3. 9.2.1.3 Application Curve
      2. 9.2.2 Powering the LDOs From the Output of the DC-DC Converters to Improve Efficiency
  10. 10Power Supply Recommendations
  11. 11Layout
    1. 11.1 Layout Guidelines
    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 Community Resources
    3. 12.3 Trademarks
    4. 12.4 Electrostatic Discharge Caution
    5. 12.5 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 TPS65708 device is designed for use as a power supply for embedded camera module or other portable low-power equipment.

9.2 Typical Applications

9.2.1 Powering All Rails from the Input Supply of 5 V

TPS65708 fbd_lvsae1.gif

9.2.1.1 Design Requirements

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

Table 2. Design Parameters

DESIGN PARAMETER VALUE
Input Supply Voltage 3.6 V to 6 V
Switching Frequency 2.25 MHz
Output Filter Corner Frequency 40 kHz

9.2.1.2 Detailed Design Procedure

9.2.1.2.1 Output Filter Design (Inductor and Output Capacitor)

9.2.1.2.1.1 Inductor Selection

The converter operates typically with a 1.5-µH or 2.2-µH output inductor. The selected inductor has to be rated for its DC resistance and saturation current. The DC resistance of the inductance will influence directly the efficiency of the converter. Therefore an inductor with lowest DC resistance should be selected for highest efficiency.

Equation 2 calculates the maximum inductor current under static load conditions. The saturation current of the inductor should be rated higher than the maximum inductor current as calculated with Equation 2. This is recommended because during heavy load transient the inductor current will rise above the calculated value.

Equation 2. TPS65708 EQ1_DIL_lvsae1.gif

where

  • f = Switching Frequency (2.25 MHz typical)
  • L = Inductor Value
  • ΔIL = Peak-to-Peak inductor ripple current
  • ILmax = Maximum Inductor current

The highest inductor current will occur at maximum Vin.

Open-core inductors have a soft saturation characteristic and they can usually handle higher inductor currents versus a comparable shielded inductor.

A more conservative approach is to select the inductor current rating just for the maximum switch current of the corresponding converter. It must be considered, that the core material from inductor to inductor differs and will have an impact on the efficiency especially at high switching frequencies.

The step-down converter has internal loop compensation. The internal loop compensation is designed to work with an output filter corner frequency calculated as follows:

Equation 3. TPS65708 EQ2_fc_lvsae1.gif

This leads to the fact the selection of external L-C filter has to be coped with the above equation. As a general rule the product of L × COUT should be constant while selecting smaller inductor or increasing output capacitor value.

Refer to Table 3 and the typical applications for possible inductors.

Table 3. Tested Inductors

INDUCTOR TYPE INDUCTOR VALUE SUPPLIER
BRC1608 1.5 µH Taiyo Yuden
MLP2012 2.2 µH TDK
MIPSA2520 2.2 µH FDK
GLCR1608T1R5M-HC 1.5 µH TDK
LQM21P 2.2 µH Murata

9.2.1.2.1.2 Output Capacitor Selection

The advanced Fast Response voltage mode control scheme of the step-down converter allows the use of small ceramic capacitors with a typical value of 10 µF, without having large output voltage under and overshoots during heavy load transients. Ceramic capacitors having low ESR values result in lowest output voltage ripple and are therefore recommended. For an inductor value of 1.5 µH or 2.2 µH, an output capacitor with 10 µF can be used. See the recommended components.

If ceramic output capacitors are used, the capacitor RMS ripple current rating will always meet the application requirements. Just for completeness the RMS ripple current is calculated as:

Equation 4. TPS65708 EQ3_Irms_lvsae1.gif

At nominal load currents, the inductive converters operate in PWM mode and the overall output voltage ripple is the sum of the voltage spike caused by the output capacitor ESR plus the voltage ripple caused by charging and discharging the output capacitor:

Equation 5. TPS65708 EQ4_Dvout_lvsae1.gif

Where the highest output voltage ripple occurs at the highest input voltage Vin.

At light load currents, the converter operates in Power Save Mode and the output voltage ripple is dependent on the output capacitor value. The output voltage ripple is set by the internal comparator delay and the external capacitor. The typical output voltage ripple is less than 1% of the nominal output voltage.

9.2.1.2.1.3 Input Capacitor Selection

Because of the nature of the buck converter having a pulsating input current, a low ESR input capacitor is required for best input voltage filtering and minimizing the interference with other circuits caused by high input voltage spikes. The converters need a ceramic input capacitor of 10 μF. The input capacitor can be increased without any limit for better input voltage filtering.

Table 4. Tested Capacitors

TYPE VALUE VOLTAGE RATING SIZE SUPPLIER MATERIAL
GRM155R60G475ME47D 4.7 µF 4 V 0402 Murata Ceramic X5R
GRM155R60J225ME15D 2.2 µF 6.3 V 0402 Murata Ceramic X5R
GRM185R60J225 2.2 µF 6.3 V 0603 Murata Ceramic X5R
GRM188R60J475K 4.7 µF 6.3 V 0603 Murata Ceramic X5R
GRM188R60J106ME47D 10 µF 6.3 V 0603 Murata Ceramic X5R

9.2.1.3 Application Curve

TPS65708 tc11_lvsae1.png Figure 14. Start-Up Sequencing

9.2.2 Powering the LDOs From the Output of the DC-DC Converters to Improve Efficiency

See Design Requirements, Detailed Design Procedure, and Application Curve.

TPS65708 fbd2_lvsae1.gif