7.3.4 Current Sensing and Current Limiting

The LM5022 provides a cycle-by-cycle over current protection function. Current limit is accomplished by an internal current sense comparator. If the voltage at the current sense comparator input exceeds 0.5 V, the MOSFET gate drive is immediately terminated. A small RC filter, placed near the controller, is recommended to filter noise from the current sense signal. The CS input has an internal MOSFET which discharges the CS pin capacitance at the conclusion of every cycle. The discharge device remains on an additional 65 ns after the beginning of the new cycle to attenuate leading edge ringing on the current sense signal.

The LM5022 current sense and PWM comparators are very fast, and may respond to short duration noise pulses. Layout considerations are critical for the current sense filter and sense resistor. The capacitor associated with the CS filter must be placed very close to the device and connected directly to the pins of the controller (CS and GND). If a current sense transformer is used, both leads of the transformer secondary must be routed to the sense resistor and the current sense filter network. The current sense resistor can be placed between the source of the primary power MOSFET and power ground, but it must be a low inductance type. When designing with a current sense resistor all of the noise sensitive low-power ground connections must be connected together locally to the controller and a single connection must be made to the high current power ground (sense resistor ground point).