ZHCSHO5C December 2017 – March 2023 LMZM23601
PRODUCTION DATA
- 1 特性
- 2 應用
- 3 說明
- 4 Revision History
- 5 Pin Configuration and Functions
- 6 Specifications
- 7 Detailed Description
- 8 Application and Implementation
- 9 Device and Documentation Support
- 10Mechanical, Packaging, and Orderable Information
8.4.3.2 Stability
The added inductance of long input cables together with the ceramic (and low ESR) input capacitor can result in an underdamped RLC network at the input of the buck converter. This circuit can cause instability, or overvoltage transients at the VIN pin each time the input supply is cycled on and off. If long wires are used, TI recommends adding some electrolytic bulk capacitance in parallel with the ceramic input capacitor. The ESR of the bulk capacitor improves the damping. Use an electrolytic capacitor with a capacitance at least four times larger than the ceramic input capacitance.
The required ESR from the bulk capacitor depends on the cable inductance.
For example, two cables (one for VIN and one for GND), each 1 meter (approximately 3 feet) long with approximately 1-mm diameter (18 AWG), placed 1 cm (approximately 0.4 inch) apart forms a rectangular loop resulting in about 1.2 μH of inductance. The inductance in this example can be decreased to almost half if the input wires are twisted. Based on a 10-μF ceramic input capacitor, the recommended parallel CBULK is ≥ 40 μF. Using a 47-μF capacitor is sufficient. Based on about 1.2 μH of inductance and 10 μF of ceramic input capacitance, the recommended ESR of the bulk capacitor is 0.35 Ω or larger. See TI User Guide, Simple Success with Conducted EMI for DC/DC Converters for more details on input filter design.