The LM5145 implements a lossless current sense scheme designed to limit the inductor current during an overload or short-circuit condition. Figure 8-8 portrays the popular current sense method using the on-state resistance of the low-side MOSFET. Meanwhile, Figure 8-9 shows an alternative implementation with current shunt resistor, R_S. The LM5145 senses the inductor current during the PWM off-time (when LO is high).

Figure 8-8 MOSFET R_DS(on) Current Sensing

Figure 8-9 Shunt Resistor Current Sensing

The ILIM pin of the LM5145 sources a reference current that flows in an external resistor, designated R_ILIM, to program of the current limit threshold. A current limit comparator on the ILIM pin prevents further SW pulses if the ILIM pin voltage goes below GND. Figure 8-10 shows the implementation.

Resistor R_ILIM is tied to SW to use the R_DS(on) of the low-side MOSFET as a sensing element (termed R_DS(on) mode). Alternatively, R_ILIM is tied to a shunt resistor connected at the source of the low-side MOSFET (termed R_SENSE mode). The LM5145 detects the appropriate mode at start-up and sets the source current amplitude and temperature coefficient (TC) accordingly.

The ILIM current with R_DS-ON sensing is 200 μA at 27°C junction temperature and incorporates a TC of +4500 ppm/°C to generally track the R_DS(on) temperature variation of the low-side MOSFET. Conversely, the ILIM current is a constant 100 μA in R_SENSE mode. This controls the valley of the inductor current during a steady-state overload at the output. Depending on the chosen mode, select the resistance of R_ILIM using Equation 6.

Equation 6.

where

• ΔI_L is the peak-to-peak inductor ripple current
• R_DS(on)Q2 is the on-state resistance of the low-side MOSFET
• I_RDSON is the ILIM pin current in R_DS-ON mode
• R_S is the resistance of the current-sensing shunt element, and
• I_RS is the ILIM pin current in R_SENSE mode.

Given the large voltage swings of ILIM in R_DS(on) sensing mode, a capacitor designated C_ILIM connected from ILIM to PGND is essential to the operation of the valley current limit circuit. Choose this capacitance such that the time constant R_ILIM · C_ILIM is approximately 6 ns.

Figure 8-10 OCP Setpoint Defined by Current Source I_RDSON and Resistor R_ILIM in R_DS-ON Mode

Note that current sensing with a shunt component is typically implemented at lower output current levels to provide accurate overcurrent protection. Burdened by the unavoidable efficiency penalty, PCB layout, and additional cost implications, this configuration is not usually implemented in high-current applications (except where OCP setpoint accuracy and stability over the operating temperature range are critical specifications).