ZHCSI00D April 2018 – September 2020 LMR36015
PRODUCTION DATA
- 1 特性
- 2 應用
- 3 說明
- 4 Revision History
- 5 說明（續）
- 6 Device Comparison Table
- 7 Pin Configuration and Functions
- 8 Specifications
- 9 Detailed Description
-
10Application and Implementation
- 10.1 Application Information
- 10.2
Typical Application
- 10.2.1
Design 1: Low Power 24-V, 1.5-A PFM Converter
- 10.2.1.1 Design Requirements
- 10.2.1.2
Detailed Design Procedure
- 10.2.1.2.1 Custom Design With WEBENCH Tools
- 10.2.1.2.2 Choosing the Switching Frequency
- 10.2.1.2.3 Setting the Output Voltage
- 10.2.1.2.4 Inductor Selection
- 10.2.1.2.5 Output Capacitor Selection
- 10.2.1.2.6 Input Capacitor Selection
- 10.2.1.2.7 CBOOT
- 10.2.1.2.8 VCC
- 10.2.1.2.9 CFF Selection
- 10.2.1.2.10 Maximum Ambient Temperature
- 10.2.2 Application Curves
- 10.2.3 Design 2: High Density 24-V, 1.5-A FPWM Converter
- 10.2.1
Design 1: Low Power 24-V, 1.5-A PFM Converter
- 10.3 What to Do and What Not to Do
- 11Power Supply Recommendations
- 12Layout
- 13Device and Documentation Support
- 14Mechanical, Packaging, and Orderable Information
10.2.1.2.6 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 LMR36015. 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, maintain the input voltage during load transients, or both. In addition, a small case size 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 220 nF must also be rated at 100 V with an X7R dielectric. The VQFN package provides two input voltage pins and two power ground pins on opposite sides of the package. This allows the input capacitors to be split, and placed optimally with respect to the internal power MOSFETs, thus improving the effectiveness of the input bypassing. In this example, place two 220-nF ceramic capacitors at each VIN-PGND location.
It is often desirable to use an electrolytic capacitor on the input in parallel with the ceramics. This is especially true if long leads/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 8 and should be checked against the manufacturers' maximum ratings.
