8.3.3 Adjustable Current Limit

A high-accuracy current limit allows high reliability of the design. It protects the load and the power supply from overstressing during short-circuit-to-GND or power-up conditions. The current limit can also save system cost by reducing the size of PCB traces and connectors, and the capacity of the preceding power stage.

When a current-limit threshold is hit, a closed loop activates immediately. The output current is clamped at the set value, and a fault is reported out. The device heats up due to the high power dissipation on the power FET. If thermal shutdown occurs, the current limit is set to I_CL(TSD) to reduce the power dissipation on the power FET. See Figure 28 for more details.

The device has two current-limit thresholds.

• Internal current limit – The internal current limit is fixed at I_CL(int). Tie the CL pin directly to the device GND for large-transient-current applications.
• External adjustable current limit – An external resistor is used to set the current-limit threshold. Use the Equation 4 to calculate the R_(CL). V_CL(th) is the internal band-gap voltage. K_(CL) is the ratio of the output current and the current-limit set value. It is constant across the temperature and supply voltage. The external adjustable current limit allows the flexibility to set the current limit value by applications.

Equation 4.

Figure 28. Current-Limit Block Diargam

Note that if using a GND network which causes a level shift between the device GND and board GND, the CL pin must be connected with device GND.

For better protection from a hard short-to-GND condition (when the INx pins are enabled, a short to GND occurs suddenly), the device implements a fast-trip protection to turn off the related channel before the current-limit closed loop is set up. The fast-trip response time is less than 1 μs, typically. With this fast response, the device can achieve better inrush current-suppression performance.