SLUS772G MARCH   2008  – June 2020 TPS40210 , TPS40211

PRODUCTION DATA.  

  1. Features
  2. Applications
  3. Description
    1.     Device Images
      1.      Simplified Schematic
  4. Revision History
  5. Pin Configuration and Functions
    1.     Pin Functions
  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. 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  BP Regulator
      3. 7.3.3  Shutdown (DIS/EN Pin)
      4. 7.3.4  Minimum On-Time and Off-Time Considerations
      5. 7.3.5  Setting the Oscillator Frequency
      6. 7.3.6  Synchronizing the Oscillator
      7. 7.3.7  Current Sense and Overcurrent
      8. 7.3.8  Current Sense and Subharmonic Instability
      9. 7.3.9  Current Sense Filtering
      10. 7.3.10 Control Loop Considerations
      11. 7.3.11 Gate Drive Circuit
      12. 7.3.12 TPS40211
    4. 7.4 Device Functional Modes
      1. 7.4.1 Operation Near Minimum Input Voltage
      2. 7.4.2 Operation With DIS/EN Pin
  8. Application and Implementation
    1. 8.1 Application Information
    2. 8.2 Typical Applications
      1. 8.2.1 12-V to 24-V Nonsynchronous Boost Regulator
        1. 8.2.1.1 Design Requirements
        2. 8.2.1.2 Detailed Design Procedure
          1. 8.2.1.2.1  Custom Design with WEBENCH Tools
          2. 8.2.1.2.2  Duty Cycle Estimation
          3. 8.2.1.2.3  Inductor Selection
          4. 8.2.1.2.4  Rectifier Diode Selection
          5. 8.2.1.2.5  Output Capacitor Selection
          6. 8.2.1.2.6  Input Capacitor Selection
          7. 8.2.1.2.7  Current Sense and Current Limit
          8. 8.2.1.2.8  Current Sense Filter
          9. 8.2.1.2.9  Switching MOSFET Selection
          10. 8.2.1.2.10 Feedback Divider Resistors
          11. 8.2.1.2.11 Error Amplifier Compensation
          12. 8.2.1.2.12 RC Oscillator
          13. 8.2.1.2.13 Soft-Start Capacitor
          14. 8.2.1.2.14 Regulator Bypass
          15. 8.2.1.2.15 Bill of Materials
        3. 8.2.1.3 Application Curves
      2. 8.2.2 12-V Input, 700-mA LED Driver, Up to 35-V LED String
        1. 8.2.2.1 Design Requirements
        2. 8.2.2.2 Detailed Design Procedure
  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. 11.1.2 Related Devices
      3. 11.1.3 Development Support
        1. 11.1.3.1 Custom Design with WEBENCH Tools
    2. 11.2 Documentation Support
      1. 11.2.1 Related Documentation
    3. 11.3 Related Links
    4. 11.4 Receiving Notification of Documentation Updates
    5. 11.5 Trademarks
    6. 11.6 Electrostatic Discharge Caution
    7. 11.7 Glossary
  12. 12Mechanical, Packaging, and Orderable Information

Package Options

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

Control Loop Considerations

There are two methods to design a suitable control loop for the TPS4021x. The first and preferred if equipment is available is to use a frequency response analyzer to measure the open loop modulator and power stage gain and to then design compensation to fit that. The usage of these tools for this purpose is well documented with the literature that accompanies the tool and is not be discussed here.

The second option is to make an initial guess at compensation, and then evaluate the transient response of the system to see if the compensation is acceptable to the application or not. For most systems, an adequate response can be obtained by simply placing a series resistor and capacitor (RFB and CFB) from the COMP pin to the FB pin as shown in Figure 27. The initial compensation selection can be done more accurately with aid of WEBENCH® to select the components or the average Spice model to simulate the open loop modulator and power stage gain.

TPS40210 TPS40211 v07177_lus772.gifFigure 27. Basic Compensation Network

The natural phase characteristics of most capacitors used for boost outputs combined with the current mode control provide adequate phase margin when using this type of compensation. To determine an initial starting point for the compensation, the desired crossover frequency must be considered when estimating the control to output gain. The model used is a current source into the output capacitor and load.

When using these equations, the loop bandwidth should be no more than 20% of the switching frequency, fSW. A more reasonable loop bandwidth would be 10% of the switching frequency. Be sure to evaluate the transient response of the converter over the expected load range to ensure acceptable operation.

Equation 22. TPS40210 TPS40211 q_kco_v2_lus772.gif
Equation 23. TPS40210 TPS40211 q_gm_v2_lus772.gif
Equation 24. TPS40210 TPS40211 q_zout_lus772.gif

where

  • KCO is the control to output gain of the converter, in V/V
  • gM is the transconductance of the power stage and modulator, in S
  • ROUT is the output load equivalent resistance, in Ω
  • ZOUT is the output impedance, including the output capacitor, in Ω
  • RISNS is the value of the current sense resistor, in Ω
  • L is the value of the inductor, in H
  • COUT is the value of the output capacitance, in F
  • RESR is the equivalent series resistance of COUT, in Ω
  • f SW is the switching frequency, in Hz
  • f L is the desired crossover frequency for the control loop, in Hz

These equations assume that the operation is discontinuous and that the load is purely resistive. The gain in continuous conduction can be found by evaluating Equation 23 at the resistance that gives the critical conduction current for the converter. Loads that are more like current sources give slightly higher gains than predicted here. To find the gain of the compensation network required for a control loop of bandwidth fL, take the reciprocal of Equation 22.

Equation 25. TPS40210 TPS40211 q_kcomp_v2_lus772.gif

The GBWP of the error amplifier is only guaranteed to be at least 1.5MHz. If KCOMP multiplied by fL is greater than 750 kHz, reduce the desired loop crossover frequency until this condition is satisfied. This ensures that the high-frequency pole from the error amplifier response with the compensation network in place does not cause excessive phase lag at fL and decreased phase margin in the loop.

The RC network connected from COMP to FB places a zero in the compensation response. That zero should be approximately 1/10th of the desired crossover frequency, fL. With that being the case, RFB and CFB can be found from Equation 26 and Equation 27.

Equation 26. TPS40210 TPS40211 q_rfb_lus772.gif
Equation 27. TPS40210 TPS40211 q_cfb_lus772.gif

where

  • R1 is the high side feedback resistor in Figure 27, in Ω
  • fL is the desired loop crossover frequency, in Hz

Thought not strictly necessary, it is recommended that a capacitor be added between COMP and FB to provide high-frequency noise attenuation in the control loop circuit. This capacitor introduces another pole in the compensation response. The allowable location of that pole frequency determines the capacitor value. As a starting point, the pole frequency should be 10 × fL. The value of CHF can be found from Equation 28.

Equation 28. TPS40210 TPS40211 q_chf01_lus772.gif

While the error amplifier GBWP will usually be higher, it can be as low as 1.5MHz. If 10 × KComp × fL > 1.5MHz, the error amplifier gain-bandwidth product may limit the high-frequency response below that of the high-frequency capacitor. To maintain a consistent high-frequency gain roll-off, CHF can be calculated by Equation 29.

Equation 29. TPS40210 TPS40211 q_chf02_lus772.gif

where

  • CHF is the high-frequency roll-off capacitor value in F
  • RFB is the mid band gain setting resistor value in Ω