ZHCSO65B January 2023 – January 2024 LMR38020-Q1
PRODUCTION DATA
- 1
- 1 特性
- 2 應用
- 3 說明
- 4 Device Comparison Table
- 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 Fixed Frequency Peak Current Mode Control
- 7.3.2 Adjustable Output Voltage
- 7.3.3 Enable
- 7.3.4 Switching Frequency and Synchronization (RT/SYNC)
- 7.3.5 Power-Good Flag Output
- 7.3.6 Minimum On Time, Minimum Off Time, and Frequency Foldback
- 7.3.7 Bootstrap Voltage
- 7.3.8 Overcurrent and Short Circuit Protection
- 7.3.9 Soft Start
- 7.3.10 Thermal Shutdown
- 7.4 Device Functional Modes
- 8 Application and Implementation
- 9 Device and Documentation Support
- 10Revision History
- 11Mechanical, Packaging, and Orderable Information
8.2.2.6 Input Capacitor Selection
The ceramic input capacitors provide a low impedance source to the regulator in addition to supplying the ripple current and isolating switching noise from other circuits. A minimum ceramic capacitance of 4.7 μF is required on the input of the LMR38020-Q1. This must be rated for at least the maximum input voltage that the application requires, preferably twice the maximum input voltage. This capacitance can be increased to help reduce input voltage ripple and maintain the input voltage during load transients. In addition, a small case size 100-nF to 220-nF ceramic capacitor must be used at the input, as close a possible to the regulator. This provides a high frequency bypass for the control circuits internal to the device. For this example, a 4.7-μF, 100-V, X7R (or better) ceramic capacitor is chosen. The 100 nF must also be rated at 100 V with an X7R dielectric.
Using an electrolytic capacitor on the input in parallel with the ceramics is desirable. This is especially true if long leads or traces are used to connect the input supply to the regulator. The moderate ESR of this capacitor can help damp any ringing on the input supply caused by the long power leads. The use of this additional capacitor also helps with voltage dips caused by input supplies with unusually high impedance.
Most of the input switching current passes through the ceramic input capacitor or capacitors. The approximate RMS value of this current can be calculated from Equation 12 and must be checked against the manufacturers' maximum ratings.
