SLVSA08A February   2010  – September 2015 TPS657052

PRODUCTION DATA.  

  1. Features
  2. Applications
  3. Description
  4. Revision History
  5. Device Options
  6. Pin Configuration and Functions
  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 Dissipation Ratings
    7. 7.7 Typical Characteristics
  8. Detailed Description
    1. 8.1 Overview
    2. 8.2 Functional Block Diagram
    3. 8.3 Feature Description
      1. 8.3.1 DC-DC Converter
      2. 8.3.2 Power Save Mode
        1. 8.3.2.1 Dynamic Voltage Positioning
        2. 8.3.2.2 Soft Start
        3. 8.3.2.3 100% Duty Cycle Low Dropout Operation
      3. 8.3.3 180° Out-of-Phase Operation
        1. 8.3.3.1 Under-Voltage Lockout
      4. 8.3.4 Short-Circuit Protection
      5. 8.3.5 Thermal Shutdown
      6. 8.3.6 LDO
      7. 8.3.7 Enable for DCDC1, DCDC2 and LDO
    4. 8.4 Device Functional Modes
  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 Output Filter Design (Inductor and Output Capacitor)
          1. 9.2.2.1.1 Inductor Selection
          2. 9.2.2.1.2 Output Capacitor Selection
          3. 9.2.2.1.3 Input Capacitor Selection
      3. 9.2.3 Application Curves
  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 Related Links
    3. 12.3 Community Resources
    4. 12.4 Trademarks
    5. 12.5 Electrostatic Discharge Caution
    6. 12.6 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 TPS65705x device is designed for use as a power supply for embedded camera modules or other portable low-power equipment.

9.2 Typical Application

TPS657051 TPS657052 FP_app_cir_lvsa08.gif

9.2.1 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 3.3 V to 6 V
Switching Frequency 2.25 Mhz

9.2.2 Detailed Design Procedure

9.2.2.1 Output Filter Design (Inductor and Output Capacitor)

9.2.2.1.1 Inductor Selection

The converter operates typically with 2.2-µH output inductor. Larger or smaller inductor values can be used to optimize the performance of the device for specific operation conditions. The selected inductor has to be rated for its DC resistance and saturation current. The DC resistance of the inductor 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. TPS657051 TPS657052 EQ1_Dil_lvsa08.gif
Equation 3. TPS657051 TPS657052 EQ2_Ilmax_lvsa08.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.

Notice that the step-down converter has internal loop compensation. As the internal loop compensation is designed to work with a certain output filter corner frequency calculated as follows:

Equation 4. TPS657051 TPS657052 EQ3_fc_lvsa08.gif

This leads to the fact the selection of external L-C filter has to be coped with the above formula. As a general rule of thumb the product of LxCout 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
LPS3015 2.2 µH Coilcraft
LQM21P 2.2 µH Murata

9.2.2.1.2 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 2.2 µH, an output capacitor with 10 µF can be used. Refer to Table 4.

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 5. TPS657051 TPS657052 EQ4_Irms_lvsa08.gif

At nominal load current 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 6. TPS657051 TPS657052 EQ5_Dvout_lvsa08.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

9.2.2.1.3 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 COMPONENT SUPPLIER VALUE VOLTAGE RATING SIZE MATERIAL
DC-DC Output Capacitor Murata
GRM155R60G475ME47D
4.7 µF 4 V 0402 Ceramic X5R
LDO I/O Capacitor Murata
GRM155R60J225ME15D
2.2 µF 6.3 V 0402 Ceramic X5R
DC-DC Output Capacitor Murata
GRM188R60J475K
4.7 µF 6.3 V 0603 Ceramic X5R
DC-DC I/O Capacitor Murata
GRM188R60J106M69D
10 µF 6.3 V 0603 Ceramic X5R

9.2.3 Application Curves

TPS657051 TPS657052 startup_tim_lvsa08.gif Figure 22. Start-Up Timing DC-DC
TPS657051 TPS657052 startup2_tim_lvsa08.gif Figure 23. Start-Up Timing LDO