SLUSA41B JUNE   2010  – September 2016 TPS53311

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 Soft Start
      2. 7.3.2 Power Good
      3. 7.3.3 Undervoltage Lockout (UVLO) Function
      4. 7.3.4 Overcurrent Protection
      5. 7.3.5 Overvoltage Protection
      6. 7.3.6 Undervoltage Protection
      7. 7.3.7 Overtemperature Protection
      8. 7.3.8 Output Discharge
      9. 7.3.9 Master and Slave Operation and Synchronization
    4. 7.4 Device Functional Modes
      1. 7.4.1 Operation Mode
      2. 7.4.2 Light Load Operation
      3. 7.4.3 Forced Continuous Conduction Mode
  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 the Value of R1 and R2
        2. 8.2.2.2 Choose the Inductor
        3. 8.2.2.3 Choose the Output Capacitor(s)
        4. 8.2.2.4 Choose the Input Capacitor
        5. 8.2.2.5 Compensation Design
      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 Documentation Support
      1. 11.1.1 Related Documentation
    2. 11.2 Receiving Notification of Documentation Updates
    3. 11.3 Community Resources
    4. 11.4 Trademarks
    5. 11.5 Electrostatic Discharge Caution
    6. 11.6 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 TPS53311 device is a high-efficiency synchronous-buck converter. The device suits low-output voltage point-of-load applications with 3-A or lower output current in computing and similar digital consumer applications.

8.2 Typical Application

This design example describes a voltage-mode, 3-A synchronous buck converter with integrated MOSFETs. The TPS53311 device provides a fixed 1.5-V output at up to 3 A from a 3.3-V input bus.

TPS53311 v10029_lusa41.gif Figure 13. Typical 3.3-V Input Application Circuit Diagram

8.2.1 Design Requirements

Table 2 lists the parameters for this design example.

Table 2. TPS53311 Design Example Specifications

PARAMETER TEST CONDITIONS MIN TYP MAX UNIT
INPUT CHARACTERISTICS
Input voltage, VIN VIN 2.9 3.3 6 V
Maximum input current VIN = 3.3 V, 1.5 V or 3 A 2.82 A
No load input current VIN = 3.3 V, 1.5 V or 0 A 40 mA
OUTPUT CHARACTERISTICS
Output voltage, VO 1.485 1.5 1.515 V
Output voltage regulation Line regulation 0.1%
Load regulation 1%
Output voltage ripple VIN = 3.3 V, 1.5 V or 0 A to 3 A 20 mVpp
Output load current 0 3 A
Output over current 4.5 A
SYSTEMS CHARACTERISTICS
Switching frequency Fixed 1.1 MHz
1.5-V full load efficiency VIN = 3.3 V, 1.5 V or 3 A 88.82%
VIN = 5 V, 1.5 V or 3 A 89.5%
Operating temperature 25 °C

8.2.2 Detailed Design Procedure

Select the external components using the following steps.

8.2.2.1 Determine the Value of R1 and R2

The output voltage is programmed by the voltage-divider resistor, R1 and R2 shown in Figure 13. R1 is connected between the FB pin and the output, and R2 is connected between the FB pin and GND. The recommended value for R1 is from 1 kΩ to 5 kΩ. Determine R2 using equation in Equation 1.

Equation 1. TPS53311 q_r2_lusa41.gif

8.2.2.2 Choose the Inductor

The inductance value must be determined to give the ripple current of approximately 20% to 40% of maximum output current. The inductor ripple current is determined by Equation 2.

Equation 2. TPS53311 q_ilripple_lusa41.gif

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

8.2.2.3 Choose the Output Capacitor(s)

The output capacitor selection is determined by output ripple and transient requirement. When operating in CCM, the output ripple has three components calculated with Equation 3 through Equation 6.

Equation 3. TPS53311 vripple_lusa41.gif
Equation 4. TPS53311 vripplec_lusa41.gif
Equation 5. TPS53311 vrippleesr_lusa41.gif
Equation 6. TPS53311 vrippleesl_lusa41.gif

When ceramic output capacitors are used, the ESL component is usually negligible. In the case when multiple output capacitors are used, ESR and ESL must be the equivalent of ESR and ESL of all the output capacitor in parallel.

When operating in DCM, the output ripple is dominated by the component determined by capacitance. It also varies with load current and can be expressed as shown in Equation 7.

Equation 7. TPS53311 vrippledcm_lusa41.gif

where

  • α is the DCM on-time coefficient and can be expressed in Equation 8 (typical value 1.25)
Equation 8. TPS53311 q_alpha_lusa41.gif
TPS53311 v10055_lusa41.gif Figure 14. DCM VOUT Ripple Calculation

8.2.2.4 Choose the Input Capacitor

The selection of input capacitor must be determined by the ripple current requirement. The ripple current generated by the converter needs to be absorbed by the input capacitors as well as the input source. The RMS ripple current from the converter can be expressed in Equation 9.

Equation 9. TPS53311 q_iinripple_lusa41.gif

where

  • D is the duty cycle and can be expressed as shown in Equation 10
Equation 10. TPS53311 q_d_lusa41.gif

To minimize the ripple current drawn from the input source, sufficient input decoupling capacitors must be placed close to the device. TI recommends the ceramic capacitor because it provides low ESR and low ESL. The input voltage ripple can be calculated as shown in Equation 11 when the total input capacitance is determined.

Equation 11. TPS53311 q_vinvripple_lusa41.gif

8.2.2.5 Compensation Design

The TPS53311 uses voltage mode control. To effectively compensate the power stage and ensure fast transient response, Type III compensation is typically used.

The control to output transfer function can be described in Equation 12.

Equation 12. TPS53311 q_gco_lusa41.gif

The output L-C filter introduces a double pole which can be calculated as shown in Equation 13.

Equation 13. TPS53311 q_fdp_lusa41.gif

The ESR zero can be calculated as shown in Equation 14.

Equation 14. TPS53311 q_fesr_lusa41.gif

Figure 15 and Figure 16 show the configuration of Type III compensation and typical pole and zero locations. Equation 16 through Equation 20 describe the compensator transfer function and poles and zeros of the Type III network.

TPS53311 v10058_lusa41.gif Figure 15. Type III Compensation Network
Configuration Schematic
TPS53311 v10057_lusa41.gif Figure 16. Type III Compensation Gain Plot
and Zero/Pole Placement
Equation 15. TPS53311 q_gea_lusa41.gif
Equation 16. TPS53311 q_fz1_lusa41.gif
Equation 17. TPS53311 q_fz2_lusa41.gif
Equation 18. TPS53311 q_fp1_lusa41.gif
Equation 19. TPS53311 q_fp2_lusa41.gif
Equation 20. TPS53311 q_fp3_lusa41.gif

The two zeros can be placed near the double pole frequency to cancel the response from the double pole. One pole can be used to cancel ESR zero, and the other non-zero pole can be placed at half switching frequency to attenuate the high frequency noise and switching ripple. Suitable values can be selected to achieve a compromise between high phase margin and fast response. A phase margin higher than 45 degrees is required for stable operation.

For DCM operation, a C3 between 56 pF and 150 pF is recommended for output capacitance between 20 µF to 200 µF.

Figure 17 shows the master and slave configuration schematic for a design with a 3.3-V input.

TPS53311 v10059_lusa41.gif Figure 17. Master and Slave Configuration Schematic

8.2.3 Application Curves

TPS53311 TPS53311EVM_561_Output_Ripple.gif
3.3 VIN, 1.5 V/3 A
Figure 18. 1.5-V Output Ripple
TPS53311 TPS53311EVM_561_1.5_Output_Trans.gif
5 VIN, 1.5 V/0 A-3 A
Figure 20. 1.5-V Output Transient
TPS53311 TPS53311EVM_561_Enable_Turns_Off.gif
3.3 VIN, 1.5 V/3 A
Figure 22. 1.5-V Turnoff Waveform
TPS53311 TPS53311EVM_561_Synch.gif
3.3 VIN, 1.5 V/3 A and 1.2 V/3 A
Figure 19. Master-Slave 180° Synchronization
TPS53311 TPS53311EVM_561_Enable_Turns_On.gif
3.3 VIN, 1.5 V/3 A
Figure 21. 1.5-V Turnon Waveform