The LM25117 is a synchronous buck controller intended for step-down regulator applications from a high voltage or widely varying input supply. The control method is based upon current mode control utilizing an emulated current ramp. Current mode control provides inherent line feed-forward, cycle-by-cycle current limiting and ease of loop compensation. The use of an emulated control ramp reduces noise sensitivity of the pulse-width modulation circuit, allowing reliable control of very small duty cycles necessary in high input voltage applications.
LM5117 / LM25117 — Powerwise Wide Input Range Synchronous Buck Controller with Analog Current Monitor - [Link]
Traditionally, current limiting of a buck regulator is accomplished by monitoring the switch current.This method protects the switch effectively, but the maximum output current limit will vary widely. In some emerging applications such as battery/supercapacitor chargers, USB power supplies, or LED drivers, very tight control of the maximum output current is now mandatory. Achieving higher accuracy of current limit/regulation is a competitive advantage.
In this article, a unique average output current monitor feature of National Semiconductor’s LM5117 synchronous buck controller will be introduced and a method to use this feature in order to implement precise current limiting will be shown. An example of a constant current regulator providing output overvoltage protection will also be presented.
Precision Output Current Limiting Using Average Current Monitor – [Link]