ZHCSO70A march 2023 – may 2023 LMR36501 , LMR36502
PRODUCTION DATA
- 1
- 1 特性
- 2 应用
- 3 说明
- 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 Enable, Shutdown, and Start-up
- 8.3.2 Adjustable Switching Frequency (with RT)
- 8.3.3 Power-Good Output Operation
- 8.3.4 Internal LDO, VCC UVLO, and VOUT/FB Input
- 8.3.5 Bootstrap Voltage and VBOOT-UVLO (BOOT Terminal)
- 8.3.6 Output Voltage Selection
- 8.3.7 Soft Start and Recovery from Dropout
- 8.3.8 Current Limit and Short Circuit
- 8.3.9 Thermal Shutdown
- 8.3.10 Input Supply Current
- 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
- 9.2.3 Application Curves
- 9.3 Best Design Practices
- 9.4 Power Supply Recommendations
- 9.5 Layout
- 10Device and Documentation Support
- 11Mechanical, Packaging, and Orderable Information
8.3.7.1 Soft Start
The soft-start feature allows the converter to gradually reach the steady state output voltage, reducing the startup stress in the system. Soft start is triggered by any of the following conditions:
- Voltage is applied to the VIN pin of the device, releasing undervoltage lockout.
- EN/UVLO voltage is sufficient to enter active mode.
- Recovery from shutdown due to over temperature protection.
After soft start is triggered, the internal reference is slowly ramped up. Assuming the output voltage is initially 0 V, the reference is ramped to 90% of the target output voltage in tSS. During the soft-start time the switching mode is set to AUTO mode. AUTO mode activates diode emulation for the low-side MOSFET, not allowing negative inductor current. This allows the output voltage to be pre-biased, voltage already present on the output, during startup without discharging the output capacitor. Figure 8-6 shows the difference between a non biased soft start and a pre-biased soft start.
Figure 8-6 Soft Start with and Without
Pre-biased Voltage