ZHCSKZ5A March 2020 – August 2020 LM25184-Q1
PRODUCTION DATA
- 1 特性
- 2 應用
- 3 說明
- 4 Revision History
- 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 Integrated Power MOSFET
- 7.3.2 PSR Flyback Modes of Operation
- 7.3.3 Setting the Output Voltage
- 7.3.4 Control Loop Error Amplifier
- 7.3.5 Precision Enable
- 7.3.6 Configurable Soft Start
- 7.3.7 External Bias Supply
- 7.3.8 Minimum On-Time and Off-Time
- 7.3.9 Overcurrent Protection
- 7.3.10 Thermal Shutdown
- 7.4 Device Functional Modes
-
8 Application and Implementation
- 8.1 Application Information
- 8.2
Typical Applications
- 8.2.1
Design 1: Wide VIN, Low IQ PSR Flyback Converter Rated at 12 V, 1 A
- 8.2.1.1 Design Requirements
- 8.2.1.2
Detailed Design Procedure
- 8.2.1.2.1 Custom Design With WEBENCH? Tools
- 8.2.1.2.2 Custom Design With Excel Quickstart Tool
- 8.2.1.2.3 Flyback Transformer – T1
- 8.2.1.2.4 Flyback Diode – DFLY
- 8.2.1.2.5 Leakgae Inductance Clamp Circuit – DF, DCLAMP
- 8.2.1.2.6 Output Capacitor – COUT
- 8.2.1.2.7 Input Capacitor – CIN
- 8.2.1.2.8 Feedback Resistor – RFB
- 8.2.1.2.9 Thermal Compensation Resistor – RTC
- 8.2.1.2.10 UVLO Resistors – RUV1, RUV2
- 8.2.1.2.11 Soft-Start Capacitor – CSS
- 50
- 8.2.2 Application Curves
- 8.2.3 Design 2: PSR Flyback Converter With Dual Outputs of 15 V and –8 V at 0.5 A
- 8.2.1
Design 1: Wide VIN, Low IQ PSR Flyback Converter Rated at 12 V, 1 A
- 9 Power Supply Recommendations
- 10Layout
- 11Device and Documentation Support
- 12Mechanical, Packaging, and Orderable Information
7.3.6 Configurable Soft Start
The LM25184-Q1 has a flexible and easy-to-use soft-start control pin, SS/BIAS. The soft-start feature prevents inrush current impacting the LM25184-Q1 and the input supply when power is first applied. This is achieved by controlling the voltage at the output of the internal error amplifier. Soft start is achieved by slowly ramping up the target regulation voltage when the device is first enabled or powered up. Selectable and adjustable start-up timing options include a 6-ms internally-fixed soft start and an externally-programmable soft start.
The simplest way to use the LM25184-Q1 is to leave SS/BIAS open. The LM25184-Q1 employs an internal soft-start control ramp and starts up to the regulated output voltage in 6 ms.
However, in applications with a large amount of output capacitance, higher VOUT, or other special requirements, the soft-start time can be extended by connecting an external capacitor CSS from SS/BIAS to GND. A longer soft-start time further reduces the supply current needed to charge the output capacitors while sourcing the required load current. When the EN/UVLO voltage exceeds the UVLO rising threshold and a delay of 20 μs expires, an internal current source ISS of 5 μA charges CSS and generates a ramp to control the primary current amplitude. Calculate the soft-start capacitance for a desired soft-start time, tSS, using Equation 12.
CSS is discharged by an internal FET when switching is disabled by EN/UVLO or thermal shutdown.