SLVSAU1E May 2011 – August 2016 TPS54228
PRODUCTION DATA.
- 1 Features
- 2 Applications
- 3 Description
- 4 Revision History
- 5 Pin Configuration and Functions
- 6 Specifications
- 7 Detailed Description
- 8 Application and Implementation
- 9 Power Supply Recommendations
- 10Layout
- 11Device and Documentation Support
- 12Mechanical, Packaging, and Orderable Information
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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 TPS54228 is typically used as step-down converters, which convert a voltage to a lower voltage, 4.5 V to
18 V. WEBENCH™ software is available to aid in the design and analysis of circuits.
8.2 Typical Application
Figure 11. Example Design Schematic
8.2.1 Design Requirements
Table 1 shows the parameters for this design example.
Table 1. Design Parameters
| PARAMETER | VALUE |
|---|---|
| Input voltage | 4.5 V to 18 V |
| Output voltage | 1.05 V |
| Output current | 2 A |
| Output voltage ripple | 20 mVpp |
8.2.2 Detailed Design Procedure
8.2.2.1 Output Voltage Resistors Selection
The output voltage is set with a resistor divider from the output node to the VFB pin. TI recommends using 1% tolerance or better divider resistors. Start by using Equation 3 to calculate VOUT.
To improve efficiency at light loads, consider using larger value resistors. High resistance is more susceptible to noise, and the voltage errors from the VFB input current are more noticeable.
8.2.2.2 Output Filter Selection
The output filter used with the TPS54228 is an LC circuit. This LC filter has double pole at:
At low frequencies, the overall loop gain is set by the output set-point resistor divider network and the internal gain of the TPS54228. The low frequency phase is 180°. At the output filter pole frequency, the gain rolls off at a –40 dB per decade rate and the phase drops rapidly. D-CAP2™ introduces a high frequency zero that reduces the gain roll off to –20 dB per decade and increases the phase to 90° one decade above the zero frequency. The inductor and capacitor selected for the output filter must be selected so that the double pole of Equation 4 is located below the high frequency zero but close enough that the phase boost provided be the high frequency zero provides adequate phase margin for a stable circuit. To meet this requirement use the values recommended in Table 2.
Table 2. Recommended Component Values
| OUTPUT VOLTAGE (V) | R1 (kΩ) | R2 (kΩ) | C4 (pF)(1) | L1 (µH) | C8 + C9 (µF) |
|---|---|---|---|---|---|
| 1 | 6.81 | 22.1 | — | 1.5 to 2.2 | 22 to 68 |
| 1.05 | 8.25 | 22.1 | — | 1.5 to 2.2 | 22 to 68 |
| 1.2 | 12.7 | 22.1 | — | 2.2 | 22 to 68 |
| 1.5 | 21.5 | 22.1 | — | 2.2 | 22 to 68 |
| 1.8 | 30.1 | 22.1 | 5 to 22 | 3.3 | 22 to 68 |
| 2.5 | 49.9 | 22.1 | 5 to 22 | 3.3 | 22 to 68 |
| 3.3 | 73.2 | 22.1 | 5 to 22 | 3.3 | 22 to 68 |
| 5 | 124 | 22.1 | 5 to 22 | 4.7 | 22 to 68 |
| 6.5 | 165 | 22.1 | 5 to 22 | 4.7 | 22 to 68 |
Because the DC gain is dependent on the output voltage, the required inductor value increases as the output voltage increases. For higher output voltages at or above 1.8 V, additional phase boost can be achieved by adding a feedforward capacitor (C4) in parallel with R1
The inductor peak-to-peak ripple current, peak current and RMS current are calculated using Equation 5, Equation 6, and Equation 7. The inductor saturation current rating must be greater than the calculated peak current, and the RMS or heating current rating must be greater than the calculated RMS current.
Use 700 kHz for fSW. Make sure the chosen inductor is rated for the peak current of Equation 6 and the RMS current of Equation 7.
For this design example, the calculated peak current is 2.311 A and the calculated RMS current is 2.008 A. The inductor used is a TDK CLF7045T-2R2N with a peak current rating of 5.5 A and an RMS current rating of 4.3 A.
The capacitor value and ESR determines the amount of output voltage ripple. The TPS54228 is intended for use with ceramic or other low-ESR capacitors. Recommended values range from 22 µF to 68 µF. Use Equation 8 to determine the required RMS current rating for the output capacitor.
For this design, two TDK C3216X5R0J226M 22-µF output capacitors are used. The typical ESR is 2 mΩ each. The calculated RMS current is 0.18 A and each output capacitor is rated for 4 A.
8.2.2.3 Input Capacitor Selection
The TPS54228 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 (C3) from pin 8 to ground is optional to provide additional high frequency filtering. The capacitor voltage rating must be greater than the maximum input voltage.
8.2.2.4 Bootstrap Capacitor Selection
A 0.1-µF ceramic capacitor must be connected between the VBST to SW pin for proper operation. TI recommends using a ceramic capacitor.
8.2.2.5 VREG5 Capacitor Selection
A 1-µF ceramic capacitor must be connected between the VREG5 to GND pin for proper operation. TI recommends using a ceramic capacitor.
8.2.3 Application Curves
Figure 12. 1.05-V, Load Transient Response
Figure 14. Efficiency vs Output Current
Figure 16. Voltage Ripple at Output (IO = 2 A)
Figure 18. Voltage Ripple at Input (IO = 2 A)
Figure 13. Start-Up Wave Form
Figure 15. Light Load Efficiency vs Output Current
Figure 17. DCM Voltage Ripple at Output (IO = 30 mA)