ZHCSD64G December 2012 – May 2021 LM25018
PRODUCTION DATA
- 1 特性
- 2 应用
- 3 说明
- 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 Control Overview
- 7.3.2 VCC Regulator
- 7.3.3 Regulation Comparator
- 7.3.4 Overvoltage Comparator
- 7.3.5 On-Time Generator
- 7.3.6 Current Limit
- 7.3.7 N-Channel Buck Switch and Driver
- 7.3.8 Synchronous Rectifier
- 7.3.9 Undervoltage Detector
- 7.3.10 Thermal Protection
- 7.3.11 Ripple Configuration
- 7.3.12 Soft Start
- 7.4 Device Functional Modes
-
8 Application and Implementation
- 8.1 Application Information
- 8.2
Typical Applications
- 8.2.1 Application Circuit: 12.5-V to 48-V Input and 10-V, 325-mA Output Buck Converter
- 8.2.2 Application Curves
- 8.2.3
Typical Isolated DC-DC Converter Using LM25018
- 8.2.3.1 Design Requirements
- 8.2.3.2
Detailed Design Procedure
- 8.2.3.2.1 Transformer Turns Ratio
- 8.2.3.2.2 Total IOUT
- 8.2.3.2.3 RFB1, RFB2
- 8.2.3.2.4 Frequency Selection
- 8.2.3.2.5 Transformer Selection
- 8.2.3.2.6 Primary Output Capacitor
- 8.2.3.2.7 Secondary Output Capacitor
- 8.2.3.2.8 Type III Feedback Ripple Circuit
- 8.2.3.2.9 Secondary Diode
- 8.2.3.2.10 VCC and Bootstrap Capacitor
- 8.2.3.2.11 Input Capacitor
- 8.2.3.2.12 UVLO Resistors
- 8.2.3.2.13 VCC Diode
- 8.2.3.3 Application Curves
- 9 Power Supply Recommendations
- 10Layout
- 11Device and Documentation Support
- 12Mechanical, Packaging, and Orderable Information
8.2.1.2.6 Type III Ripple Circuit
Type III ripple circuit as described in Section 7.3.11 is chosen for this example. For a constant on-time converter to be stable, the injected in-phase ripple must be larger than the capacitive ripple on COUT.
Using type III ripple circuit equations, the injected ripple at the FB pin is set to a level greater than the capacitive ripple as follows:
Cr = C6 = 3300 pF
Cac = C8 = 100 nF
For TON, refer to Equation 3.
Ripple resistor Rr is calculated to be 57.6 kΩ. This value provides the minimum ripple for stable operation. A smaller resistance must be selected to allow for variations in TON, COUT1, and other components. Rr = R4 = 46.4 kΩ is selected for this example application.