ZHCSI04E September 2009 – April 2018 TPS54418
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 Frequency Shift
- 7.3.13 Reverse Overcurrent Protection
- 7.3.14 Synchronize Using the RT/CLK Pin
- 7.3.15 Power Good (PWRGD Pin)
- 7.3.16 Overvoltage Transient Protection
- 7.3.17 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 Step One: Select the Switching Frequency
- 8.2.2.2 Step Two: Select the Output Inductor
- 8.2.2.3 Step Three: Choose the Output Capacitor
- 8.2.2.4 Step Four: Select the Input Capacitor
- 8.2.2.5 Step Five: Minimum Load DC COMP Voltage
- 8.2.2.6 Step Six: Choose the Soft-Start Capacitor
- 8.2.2.7 Step Seven: Select the Bootstrap Capacitor
- 8.2.2.8 Step Eight: Undervoltage Lockout Threshold
- 8.2.2.9 Step Nine: Select Output Voltage and Feedback Resistors
- 8.2.2.10 Step 10: Select Loop Compensation Components
- 8.2.2.11 Power Dissipation Estimate
- 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
8.2.2.6 Step Six: Choose the Soft-Start Capacitor
The soft-start capacitor determines the minimum amount of time it takes for the output voltage to reach its nominal programmed value during power up. This is useful if a load requires a controlled voltage slew rate. This is also used if the output capacitance is very large and would require large amounts of current to quickly charge the capacitor to the output voltage level. The large currents necessary to charge the capacitor may make the device reach the current limit or excessive current draw from the input power supply may cause the input voltage rail to sag. Limiting the output voltage slew rate solves both of these problems.
The soft-start capacitor value can be calculated using Equation 31. For the example circuit, the soft-start time is not too critical since the output capacitor value is 44 µF which does not require much current to charge to 1.8 V. The example circuit has the soft-start time set to an arbitrary value of 4 ms which requires a 10 nF capacitor. In the device, ISS is 2 μA and VREF is 0.8 V. For this application, maintain the soft-start time in the range between 1 ms and 10 ms.
where
- CSS is in nF
- ISS is in µA
- tSS is in ms
- VREF is in V