ZHCS984B June 2012 – May 2019 TPS54678
PRODUCTION DATA.
- 1 特性
- 2 应用
- 3 说明
- 4 修订历史记录
- 5 Pin Configuration and Functions
- 6 Specifications
-
7 Detailed Description
- 7.1 Overview
- 7.2 Functional Block Diagram
- 7.3
Feature Description
- 7.3.1 Fixed Frequency PWM Control
- 7.3.2 Slope Compensation and Output Current
- 7.3.3 Bootstrap Voltage (Boot) and Low Dropout Operation
- 7.3.4 Error Amplifier
- 7.3.5 Voltage Reference
- 7.3.6 Adjusting the Output Voltage
- 7.3.7 Enable and Adjusting Undervoltage Lockout
- 7.3.8 Soft-Start Pin
- 7.3.9 Sequencing
- 7.3.10 Constant Switching Frequency and Timing Resistor (RT/CLK Pin)
- 7.3.11 Overcurrent Protection
- 7.3.12 Safe Start-Up into Prebiased Outputs
- 7.3.13 Synchronize Using the RT/CLK Pin
- 7.3.14 Power Good (PWRGD Pin)
- 7.3.15 Overvoltage Transient Protection
- 7.3.16 Thermal Shutdown
- 7.4 Device Functional Modes
-
8 Application and Implementation
- 8.1 Application Information
- 8.2
Typical Application
- 8.2.1 Design Requirements
- 8.2.2
Detailed Design Procedure
- 8.2.2.1 Custom Design With WEBENCH® Tools
- 8.2.2.2 Step One: Select the Switching Frequency
- 8.2.2.3 Step Two: Select the Output Inductor
- 8.2.2.4 Step Three: Choose the Output Capacitor
- 8.2.2.5 Step Four: Select the Input Capacitor
- 8.2.2.6 Step Five: Choose the Soft-Start Capacitor
- 8.2.2.7 Step Six: Select the Bootstrap Capacitor
- 8.2.2.8 Step Eight: Select Output Voltage and Feedback Resistors
- 8.2.2.9 Step Nine: Select Loop Compensation Components
- 8.2.3 Application Curves
- 9 Power Supply Recommendations
- 10Layout
- 11器件和文档支持
- 12机械、封装和可订购信息
8.2.2.3 Step Two: Select the Output Inductor
The inductor selected works for the entire TPS54678 input voltage range. To calculate the value of the output inductor, use Equation 14. KIND is a coefficient that represents the amount of inductor ripple current relative to the maximum output current. The inductor ripple current is filtered by the output capacitor. Therefore, choosing high-inductor ripple currents impacts the selection of the output capacitor because the output capacitor must have a ripple current rating equal to or greater than the inductor ripple current. In general, the inductor ripple value is at the discretion of the designer, however KIND is usually chosen between 0.1 to 0.3 for the majority of applications.
For this design example, a value of KIND = 0.3 was used at 6 VIN and 6 AOUT, and the inductor value is calculated to be 1.06 μH. For this design, the nearest standard value of 1.2 μH was chosen. For the output filter inductor, it is important that the RMS current and saturation current ratings not be exceeded. The RMS and peak inductor current can be found from Equation 16 and Equation 17.
For this design, the RMS inductor current is 6.02 A and the peak inductor current is 6.8 A. The chosen inductor is a Coilcraft XAL5030-122ME. It has a saturation current rating 0f 11.8 A (20% inductance loss) and a RMS current rating of 8.7 A (20°C temperature rise). The series resistance is 6.78 mΩ typical.
The current flowing through the inductor is the inductor ripple current plus the output current. During power up, faults or transient load conditions, the inductor current can increase above the calculated peak inductor current level calculated above. In transient conditions, the inductor current can increase up to the switch current limit of the device. For this reason, the most conservative approach is to specify an inductor with a saturation current rating equal to or greater than the switch current limit rather than the peak inductor current.
