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
7.3.8 Soft-Start Pin
TPS54678 effectively uses the lower voltage of the internal voltage reference or the SS/TR pin voltage as the reference voltage of the power supply and regulates the output accordingly. A capacitor on the SS/TR pin to ground will implement a slow-start time. The TPS54678 has an internal pullup current source of 47 µA when V(SS/TR) is less than 0.15 V and 2.2 µA when V(SS/TR) is higher than 0.15 V. The ISS charges the external slow-start capacitor. The equation for the slow-start time is shown in Equation 4 considering the fact the first 47 µA charges the SS to 0.15 V. The 2.2 µA then charges the SS from 0.15 V to about 0.8 V for the handoff of the SS voltage to reference voltage.
If during normal operation, the VIN UVLO is exceeded, EN pin pulled below 1.2 V, or a thermal shutdown event occurs, the TPS54678 will stop switching and the SS/TR must be discharged to about 60 mV before reinitiating a powering-up sequence.
The VSENSE voltage will follow the SS/TR pin voltage up to 90% of the internal voltage reference. When the SS/TR voltage is greater than 90% of the internal voltage, the effective system reference voltage will transit from the SS/TR voltage to the internal voltage reference.