为啥BUCK芯片EN都拉低了SW还在打波？

01/ 简介 /

前段时间群里讨论一个问题，BUCK芯片EN都已经拉低了，为啥SW还会有打波？这是因为该芯片具有“输出放电”功能，对应英文叫做Output Discharge。这属于BUCK芯片的辅助功能，有些芯片有，有些芯片没有。

02/ 为什么需要输出放电功能 /

输出放电功能用于确保器件处于禁用状态时输出电压快速放电。当电力系统需要为不同电路和元件提供精确的电源时序时，此功能非常有必要。如果没有输出放电功能，那么当系统被禁用时，某些电源轨会保持悬空，这可能会在系统再次启用时导致某些问题。

03/ 具有输出放电功能的BUCK芯片 /

RT6585C Discharge Mode (Soft Discharge): When ENx is low the output under-voltage fault latch is set, the output discharge mode will be triggered. During discharge mode, an internal switch creates a path for discharging the output capacitors' residual charge to GND.

MP1605: When the MP1605 is disabled, the part goes into output discharge mode automatically. The internal discharge MOSFET provides a resistive discharge path for the output capacitor.

TPS62866: An internal MOSFET switch smoothly discharges the output through the VOS pin in shutdown mode (EN = Low or Software Enable Device bit = 0). The output discharge is also active when the device is in thermal shutdown and UVLO. 该芯片从VOS引脚或者说是VOUT输出节点放电。

TPS56873: The TPS5683x has a built in discharge function by using an integrated MOSFET with 200Ω RDS(on), which is connected to the output terminal SW. The discharge is slow due to the lower current capability of the MOSFET. The discharge path turns on when the device is turned off due to UV, OV, OT, and EN shutdown conditions. 该芯片从SW节点放电。