一、How does Over Current Protection (OCP)work?

When over-current protection (OCP) is enabled, the power system turns off the output if the output current reaches the current limit setting and transitions from constant voltage (CV) operation to constant current (CC) operation.

With OCP turned on, the OCP takes over after a specified time delay and shuts down the output of the power supply. The delay time is programmable. This prevents OCP from shutting down the DC power supply from short current spikes and other acceptably short overloads that are not considered harmful. In comparison, with Over Voltage Protection (OVP), the output is disabled immediately. OVP is usually always enabled and cannot be turned off. However, unlike OVP, OCP can be turned on and off and its default is usually off. A typical OCP event is illustrated here:

Wecan specify an OCP delay to prevent momentary output settings, load, and status changes from tripping the over-current protection. In most cases these momentary conditions would not be considered an over-current protection fault, and having an OCP condition disable the output when they occur would be a nuisance. Specifying an OCP delay lets the OCP circuit ignore these momentary changes during the specified delay period. Once the OCP delay time has expired and the over-current condition persists, the output will shut down. Other factors include the difference between old output value and new output value, the current limit setting, and the load capacitance in CV operation or load inductance in CC operation. The delay required must be determined experimentally on a case-by-case basis.

二、N-MOS & P-MOS 工藝芯片對比：

P-MOSOCP原理簡介

N-MOSOCP原理簡介

性能指標對比

三、鏡像電流采樣技術：

1、外部設置Rlimit，確定觸發Rlimit時的電流為

2、隨著負載電流的增加，成比例的鏡像電流也逐漸增加

3、當鏡像電流達到滿足， ? ?此時，觸發限流機制

4、低延時的驅動單元將功率管的柵極電壓拉低，達到限制PowerNMOS電流的大小的目的

四、PCB Layout注意事項

1、輸入輸出電容靠近芯片PIN腳擺放

2、輸入輸出電容兩端端就近打足夠的GND-VIA

3、芯片及周邊元件盡量靠近負載端擺放，減小路徑上的寄生參數

4、TVS靠近芯片PIN腳擺放，電流流過順序為先TVS，再過電容，最后到芯片PIN腳

5、Rlimit電阻選用的精度至少±1%

五、OVP 與 OCP 對比：

This is quite different than the almost immediate response time of an OVP system. Even if the OCP delay time was set to zero, the response is still on the order of milliseconds instead of microseconds compared to an OVP event. And when the amount of overdrive is small, as is the case for the 6 A loading, providing just 1 A of overdrive, the total response time is much greater.
Unlike the OVP system, which operates totally independent of the voltage limit control system, the OCP system is triggered off the current limit control system. Thus the total response time includes the response time of the current limit as well. The behavior of a current limit is quite different than a simple “go/no go” threshold detector like OVP. A limit system, or circuit, needs to regulate the power supply’s output at a certain level, making it a feedback control system. The stability of this system is important, both with crossing over from constant voltage operation as well as maintaining a stable output current after crossing over. This leads to the slower and overdrive dependent response characteristics that are typical of current limit systems.

審核編輯：劉清