ZHCSNR4B May 2021 – April 2022 TPS25946
PRODUCTION DATA
- 1 特性
- 2 應用
- 3 說明
- 4 Revision History
- 5 Device Comparison Table
- 6 Pin Configuration and Functions
- 7 Specifications
-
8 Detailed Description
- 8.1 Overview
- 8.2 Functional Block Diagram
- 8.3
Feature Description
- 8.3.1 Undervoltage Lockout (UVLO and UVP)
- 8.3.2 Overvoltage Lockout (OVLO)
- 8.3.3 Inrush Current, Overcurrent, and Short-Circuit Protection
- 8.3.4 Analog Load Current Monitor
- 8.3.5 Reverse Current Protection
- 8.3.6 Overtemperature Protection (OTP)
- 8.3.7 Fault Response and Indication (FLT)
- 8.3.8 Power Good Indication (PG)
- 8.3.9 Input Supply Good Indication (SPLYGD)
- 8.4 Device Functional Modes
-
9 Application and Implementation
- 9.1 Application Information
- 9.2
Typical Application
- 9.2.1 Design Requirements
- 9.2.2
Detailed Design Procedure
- 9.2.2.1 Device Selection
- 9.2.2.2 Setting Overvoltage Threshold
- 9.2.2.3 Setting Output Voltage Rise Time (tR)
- 9.2.2.4 Setting Power Good Assertion Threshold
- 9.2.2.5 Setting Overcurrent Threshold (ILIM)
- 9.2.2.6 Setting Overcurrent Blanking Interval (tITIMER)
- 9.2.2.7 Selecting External Bias Resistor (R5)
- 9.2.2.8 Selecting External Diode (D1)
- 9.2.3 Application Curve
- 10Power Supply Recommendations
- 11Layout
- 12Device and Documentation Support
- 13Mechanical, Packaging, and Orderable Information
9.2.2.3 Setting Output Voltage Rise Time (tR)
For a successful design, the junction
temperature of device must be kept below the absolute maximum rating during both dynamic
(start-up) and steady-state conditions. Dynamic power stresses often are an order of
magnitude greater than the static stresses, so it is important to determine the right
start-up time and inrush current limit required with system capacitance to avoid thermal
shutdown during start-up.
The slew rate (SR) needed to achieve the desired output rise time can be calculated as:
Equation 9. 
The CdVdt needed to achieve this slew rate can be calculated as:
Equation 10. 
Choose the nearest standard capacitor value as 1100 pF.
For this slew rate, the inrush current can be calculated as:
Equation 11. 
The average power dissipation inside the part during inrush can be calculated as:
Equation 12. 
For the given
power dissipation, the thermal shutdown time of the device must be greater than the
ramp-up time tR to avoid start-up failure. Figure 9-4 shows the thermal
shutdown limit, for 0.38 W of power, the shutdown time is over 100 ms which is very
large as compared to tR = 5 ms. Therefore, it is safe to use 5 ms as the
start-up time for this application.
Figure 9-4 Thermal Shutdown Plot During
Inrush