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
8.2.3.2.8 Type III Feedback Ripple Circuit
Type III ripple circuit as described in Section 7.3.11 is required for the Fly-Buck topology. Type I and Type II ripple circuits use series resistance and the triangular inductor ripple current to generate ripple at VOUT and the FB pin. The primary ripple current of a Fly-Buck is the combination or primary and reflected secondary currents as shown in Figure 8-6. In the Fly-Buck topology, Type I and Type II ripple circuits suffer from large jitter as the reflected load current affects the feedback ripple.
Figure 8-8 Type III Ripple CircuitSelecting the Type III ripple components using the equations from the Section 7.3.11 section ensures that the FB pin ripple is be greater than the capacitive ripple from the primary output capacitor COUT1. The feedback ripple component values are chosen as shown in Equation 30.
The calculated value for Rr is 66 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. For this design, Rr value of 46.4 kΩ is selected.