SLVSD26A April   2016  – January 2017 TPS54202

PRODUCTION DATA.  

  1. Features
  2. Applications
  3. Description
  4. Revision History
  5. Pin Configuration and 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 Timing Requirements
    7. 6.7 Typical Characteristics
  7. Detailed Description
    1. 7.1 Overview
    2. 7.2 Functional Block Diagram
    3. 7.3 Feature Description
      1. 7.3.1  Fixed-Frequency PWM Control
      2. 7.3.2  Pulse Skip Mode
      3. 7.3.3  Error Amplifier
      4. 7.3.4  Slope Compensation and Output Current
      5. 7.3.5  Enable and Adjusting Under Voltage Lockout
      6. 7.3.6  Safe Startup into Pre-Biased Outputs
      7. 7.3.7  Voltage Reference
      8. 7.3.8  Adjusting Output Voltage
      9. 7.3.9  Internal Soft-Start
      10. 7.3.10 Bootstrap Voltage (BOOT)
      11. 7.3.11 Overcurrent Protection
        1. 7.3.11.1 High-Side MOSFET Overcurrent Protection
        2. 7.3.11.2 Low-Side MOSFET Overcurrent Protection
      12. 7.3.12 Spread Spectrum
      13. 7.3.13 Output Overvoltage Protection (OVP)
      14. 7.3.14 Thermal Shutdown
    4. 7.4 Device Functional Modes
      1. 7.4.1 Normal Operation
      2. 7.4.2 Eco-mode Operation
  8. Application and Implementation
    1. 8.1 Application Information
    2. 8.2 Typical Application
      1. 8.2.1 TPS54202 8-V to 28-V Input, 5-V Output Converter
      2. 8.2.2 Design Requirements
      3. 8.2.3 Detailed Design Procedure
        1. 8.2.3.1 Input Capacitor Selection
        2. 8.2.3.2 Bootstrap Capacitor Selection
        3. 8.2.3.3 Output Voltage Set Point
        4. 8.2.3.4 Undervoltage Lockout Set Point
        5. 8.2.3.5 Output Filter Components
          1. 8.2.3.5.1 Inductor Selection
          2. 8.2.3.5.2 Output Capacitor Selection
          3. 8.2.3.5.3 Feed-Forward Capacitor
      4. 8.2.4 Application Curves
  9. Power Supply Recommendations
  10. 10Layout
    1. 10.1 Layout Guidelines
    2. 10.2 Layout Example
  11. 11Device and Documentation Support
    1. 11.1 Receiving Notification of Documentation Updates
    2. 11.2 Community Resources
    3. 11.3 Trademarks
    4. 11.4 Electrostatic Discharge Caution
    5. 11.5 Glossary
  12. 12Mechanical, Packaging, and Orderable Information

Package Options

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

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.

Application Information

The TPS54202 are typically used as a step down converter, which convert an input voltage from 8 V - 28 V to fixed output voltage 5 V.

Typical Application

TPS54202 8-V to 28-V Input, 5-V Output Converter

TPS54202 typical_app_circuit_slvsd26.gif Figure 16. 5-V, 2-A Reference Design

Design Requirements

For this design example, use the parameters in Table 1.

Table 1. Design Parameters

PARAMETER VALUE
Input voltage range 8 V to 28 V
Output voltage 5 V
Output current 2 A
Transient response, 1.5 A load step ΔVOUT = ±5 %
Input ripple voltage 400 mV
Output voltage ripple 30 mVpp
Switching frequency 500 kHz

Detailed Design Procedure

Input Capacitor Selection

The device requires an input decoupling capacitor and a bulk capacitor is needed depending on the application. A ceramic capacitor over 10 µF is recommended for the decoupling capacitor. An additional 0.1 µF capacitor (C2) from VIN to GND is optional to provide additional high frequency filtering. The capacitor voltage rating needs to be greater than the maximum input voltage.

Use Equation 4 to calculate the input ripple voltage (ΔVIN).

Equation 4. TPS54202 Eq03_delta_slvsd26.gif

where

  • CBULK is the bulk capacitor value
  • fSW is the switching frequency
  • IOUT(MAX) is the maximum loading current
  • ESRMAX is maximum series resistance of the bulk capacitor

The maximum RMS (root mean square) ripple current must also be checked. For worst case conditions, use Equation 5 to calculate ICIN(RMS).

Equation 5. TPS54202 Eq04_Icin_slvsd26.gif

The actual input-voltage ripple is greatly affected by parasitic associated with the layout and the output impedance of the voltage source. Design Requirements show the actual input voltage ripple for this circuit which is larger than the calculated value. This measured value is still below the specified input limit of 400 mV. The maximum voltage across the input capacitors is VIN (MAX) + ΔVIN/2. The selected bypass capacitor is rated for 35 V and the ripple current capacity is greater than 2 A. Both values provide ample margin. The maximum ratings for voltage and current must not be exceeded under any circumstance.

Bootstrap Capacitor Selection

A 0.1 µF ceramic capacitor must be connected between the BOOT to SW pin for proper operation. It is recommended to use a ceramic capacitor.

Output Voltage Set Point

The output voltage of the TPS54202 device is externally adjustable using a resistor divider network. In the application circuit of Figure 16, this divider network is comprised of R2 and R3. Use Equation 6 and Equation 7 to calculate the relationship of the output voltage to the resistor divider.

Equation 6. TPS54202 Eq05_R5_slvsd26.gif
Equation 7. TPS54202 Eq06_Vout_slvsd26.gif

Select a value of R2 to be approximately 100 kΩ. Slightly increasing or decreasing R3 can result in closer output voltage matching when using standard value resistors. In this design, R2 = 100 kΩ and R3 = 13.3 kΩ which results in a 5-V output voltage. The 49.9-Ω resistor, R1, is provided as a convenient location to break the control loop for stability testing.

Undervoltage Lockout Set Point

The undervoltage lockout (UVLO) set point can be adjusted using the external-voltage divider network of R4 and R5. R4 is connected between the VIN and EN pins of the TPS54202 device. R5 is connected between the EN and GND pins. The UVLO has two thresholds, one for power up when he input voltage is rising and one for power down or brown outs when the input voltage is falling. For the example design, the minimum input voltage is 8 V, so the start-voltage threshold is set to 6.8 V with 1000-mV hysteresis. Use Equation 1 and Equation 2 to calculate the values for the upper and lower resistor values of R4 and R5.

Output Filter Components

Two components must be selected for the output filter, the output inductor (LO) and CO.

Inductor Selection

Use Equation 8 to calculate the minimum value of the output inductor (LMIN).

Equation 8. TPS54202 Eq07_Lmin_slvsd26.gif

Where:

KIND is a coefficient that represents the amount of inductor ripple current relative to the maximum output current.

In general, the value of KIND is at the discretion of the designer; however, the following guidelines may be used. For designs using low-ESR output capacitors, such as ceramics, a value as high as KIND = 0.3 can be used. When using higher ESR output capacitors, KIND = 0.2 yields better results.

For this design example, use KIND = 0.3. The minimum inductor value is calculated as 13.7 μH. For this design, a close standard value of 15 μH was selected for LMIN.

For the output filter inductor, the RMS current and saturation current ratings must not be exceeded. Use Equation 9 to calculate the RMS inductor current (IL(RMS)).

Equation 9. TPS54202 Eq08_ILmax_slvsd26.gif

Use Equation 10 to calculate the peak inductor current (IL(PK)).

Equation 10. TPS54202 Eq09_ILpk_slvsd26.gif

Smaller or larger inductor values can be used depending on the amount of ripple current the designer wants to allow so long as the other design requirements are met. Larger value inductors have lower AC current and result in lower output voltage ripple. Smaller inductor values increase AC current and output voltage ripple.

Output Capacitor Selection

Consider three primary factors when selecting the value of the output capacitor. The output capacitor determines the modulator pole, the output voltage ripple, and how the regulator responds to a large change in load current. The output capacitance must be selected based on the more stringent of these three criteria.

The desired response to a large change in the load current is the first criterion. The output capacitor must supply the load with current when the regulator cannot. This situation occurs if the desired hold-up times are present for the regulator. In this case, the output capacitor must hold the output voltage above a certain level for a specified amount of time after the input power is removed. The regulator is also temporarily unable to supply sufficient output current if a large, fast increase occurs affecting the current requirements of the load, such as a transition from no load to full load. The regulator usually requires two or more clock cycles for the control loop to notice the change in load current and output voltage and to adjust the duty cycle to react to the change. The output capacitor must be sized to supply the extra current to the load until the control loop responds to the load change. The output capacitance must be large enough to supply the difference in current for 2 clock cycles while only allowing a tolerable amount of drop in the output voltage. Use Equation 11 to calculate the minimum required output capacitance.

Equation 11. TPS54202 Eq10_CO_slvsd26.gif

where

  • ∆IOUT is the change in output current
  • ƒSW is the switching frequency of the regulator
  • ∆V(OUT )b is the allowable change in the output voltage

For this example, the transient load response is specified as a 5% change in the output voltage, VOUT, for a load step of 1.5 A. For this example, ΔIOUT = 1.5 A and ΔVOUT = 0.05 × 5 = 0.25 V. Using these values results in a minimum capacitance of 24 μF. This value does not consider the ESR of the output capacitor in the output voltage change. For ceramic capacitors, the ESR is usually small enough to ignore in this calculation.

Equation 12 calculates the minimum output capacitance required to meet the output voltage ripple specification. In this case, the maximum output voltage ripple is 30 mV. Under this requirement, Equation 12 yields 4.56 μF.

Equation 12. TPS54202 Eq11_co_slvsd26.gif

where

  • ƒSW is the switching frequency
  • V(OUTripple) is the maximum allowable output voltage ripple
  • I(ripple) is the inductor ripple current

Use Equation 13 to calculate the maximum ESR an output capacitor can have to meet the output-voltage ripple specification. Equation 13 indicates the ESR should be less than 54.8 mΩ. In this case, the ESR of the ceramic capacitor is much smaller than 54.8 mΩ.

Equation 13. TPS54202 Eq12_RESR_slvsd26.gif

The output capacitor can affect the crossover frequency ƒo. Considering to the loop stability and effect of the internal parasitic parameters, choose the crossover frequency less than 40 kHz without considering the feed forward capacitor. A simple estimation for the crossover frequency without feed forward capacitor C6 is shown in Equation 14, assuming COUT has small ESR.

Equation 14. TPS54202 eq14_slvsd26.gif

Additional capacitance deratings for aging, temperature, and DC bias should be considered which increases this minimum value. For this example, two 22-uF 25-V, X7R ceramic capacitors are used. Capacitors generally have limits to the amount of ripple current they can handle without failing or producing excess heat. An output capacitor that can support the inductor ripple current must be specified. Some capacitor data sheets specify the RMS value of the maximum ripple current. Use Equation 15 to calculate the RMS ripple current that the output capacitor must support. For this application, Equation 15 yields 79 mA for each capacitor.

Equation 15. TPS54202 Eq13_RS_slvsd26.gif

Feed-Forward Capacitor

The TPS54202 is internally compensated and the internal compensation network is composed of two capacitors and one resister shown on the block diagram. Depending on the VOUT, if the output capacitor COUT is dominated by low ESR (ceramic types) capacitors, it could result in low phase margin. To improve the phase boost an external feedforward capacitor C6 can be added in parallel with R2. C6 is chosen such that phase margin is boosted at the crossover frequency.

Equation 16 for C6 was tested:

Equation 16. TPS54202 eq15_slvsd26.gif

For this design, C6 = 75 pF. C6 is not needed when COUT has high ESR, and C6 calculated from Equation 16 should be reduced with medium ESR. Table 2 can be used as a starting point.

Table 2. Recommended Component Values

VOUT (V) L (µH) (1) COUT(µF) R2 (kΩ) R3 (kΩ) C6 (pF)
1.8 5.6 66 100 49.9 47
2.5 8.2 44 100 31.6 33
3.3 10 44 100 22.1 56
5 15 44 100 13.3 75
12 22 44 100 5.23 100
Based on VIN = 28 V

Application Curves

TPS54202 D013_SLVSD26.gif
Figure 17. Efficiency
TPS54202 D015_SLVSD26.gif
Figure 19. Load Regulation
TPS54202 G002_slvsd26.gif
IOUT = 0 A
Figure 21. Output Voltage Ripple
TPS54202 G004_slvsd26.gif
IOUT = 100 mA
Figure 23. Output Voltage Ripple
TPS54202 G006_slvsd26.gif
0.1 to 1 A
Figure 25. Transient Response
TPS54202 G008_slvsd26.gif
Figure 27. Start-Up Relative to VIN
TPS54202 G010_slvsd26.gif
Figure 29. Start-Up Relative to EN
TPS54202 D014_SLVSD26.gif
Figure 18. Line Regulation
TPS54202 G001_slvsd26.gif
IOUT = 2 A
Figure 20. Input Voltage Ripple
TPS54202 G003_slvsd26.gif
IOUT = 10 mA
Figure 22. Output Voltage Ripple
TPS54202 G005_slvsd26.gif
IOUT = 2 A
Figure 24. Output Voltage Ripple
TPS54202 G007_slvsd26.gif
0.5 to 1.5 A
Figure 26. Transient Response
TPS54202 G009_slvsd26.gif
Figure 28. Shutdown Relative to VIN
TPS54202 G011_slvsd26.gif
Figure 30. Shutdown Relative to EN