8.3.5 Current Sense and Current Limit

The LM3421 and LM3423 achieve peak current mode control using a comparator that monitors the main MOSFET (Q1) transistor current, comparing it with the COMP pin voltage as shown in Figure 15. The controller incorporates a cycle-by-cycle overcurrent protection function. Aredundant internal current sense comparator provides the current limit functionality . If the voltage at the current sense comparator input (IS pin) exceeds 245 mV (typical), the on cycle is immediately terminated. The IS input pin has an internal N-channel MOSFET which pulls it down at the conclusion of every cycle. The discharge device remains on for an additional 210 ns (typical) after the beginning of a new cycle to blank the leading edge spike on the current sense signal. The leading edge blanking (LEB) determines the minimum achievable on-time (t_ON-MIN).

Figure 15. Current Sense / Current Limit Circuitry

There are two possible methods to sense the transistor current. The R_DS-ON of the main power MOSFET can be used as the current sense resistance because the IS pin was designed to withstand the high voltages present on the drain when the MOSFET is in the off state. Alternatively, a sense resistor located in the source of the MOSFET may be used for current sensing; however, TI suggests a low inductance (ESL) type. The cycle-by-cycle current limit (I_LIM) can be calculated using either method as the limiting resistance (R_LIM):

Equation 11.