為啥BUCK芯片EN都拉低了SW還在打波？

01/ 簡介?/

前段時(shí)間群里討論一個(gè)問題，BUCK芯片EN都已經(jīng)拉低了，為啥SW還會(huì)有打波？這是因?yàn)樵撔酒哂小拜敵龇烹姟惫δ?，?duì)應(yīng)英文叫做Output Discharge。這屬于BUCK芯片的輔助功能，有些芯片有，有些芯片沒有。

02/ 為什么需要輸出放電功能?/

輸出放電功能用于確保器件處于禁用狀態(tài)時(shí)輸出電壓快速放電。當(dāng)電力系統(tǒng)需要為不同電路和元件提供精確的電源時(shí)序時(shí)， 此功能非常有必要。如果沒有輸出放電功能， 那么當(dāng)系統(tǒng)被禁用時(shí)， 某些電源軌會(huì)保持懸空， 這可能會(huì)在系統(tǒng)再次啟用時(shí)導(dǎo)致某些問題。

03/ 具有輸出放電功能的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.?該芯片從VOS引腳或者說是VOUT輸出節(jié)點(diǎn)放電。

TPS56873:?The TPS5683x has a built in discharge function by using an integrated MOSFET with 200? 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. 該芯片從SW節(jié)點(diǎn)放電。

04/ 具有輸出放電功能的LoadSwitch?/

上圖所示，TPS22810 Load Switch芯片也具有輸出放電功能；當(dāng)EN / UVLO = Low時(shí)，QOD MOSFET柵極為High，該MOSFET打開，為負(fù)載端放電。

05/ 使用PG引腳也能實(shí)現(xiàn)輸出放電嗎?/

上圖所示，Boost芯片TPS61033可以通過在VOUT端和電源良好引腳PG之間放置電阻實(shí)現(xiàn)快速放電功能。