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7.3.2 Programmable Current Limit A high-accuracy current limit allows higher reliability, which protects the power supply during short circuit or power up. Also, it can save system costs by reducing PCB traces, connector size, and the capacity of the preceding power stage. Current limit offers protection from overstressing to the load and integrated power FET. Current limit holds the current at the set value, and pulls up the CS pin to VCS,h as a diagnostic report. The two current-limit thresholds are: • External programmable current limit -- An external resistor is used to convert a proportional load current into a voltage, which is compared with an internal reference voltage, Vth,cl. When the voltage on the CL pin exceeds Vth.cl, a closed loop steps in immediately. VGS voltage regulates accordingly, leading to the Vds voltage regulation. When the closed loop is set up, the current is clamped at the set value. The external programmable current limit provides the capability to set the current-limit value by application.
• Internal current limit -- The internal current limit is fixed and typically 10 A. To use the internal current limit for large-current applications, tie the CL pin directly to the device GND. Both the internal current limit (Ilim,nom) and external programmable current limit are always active when VVS is powered and IN is high. The lower one (of Ilim,nom and the external programmable current limit) is applied as the actual current limit. Note that if a GND network is used (which leads to the level shift between the device GND and board GND), the CL pin must be connected with device GND. Calculate RCL with Equation 2. (2) For better protection from a hard short-to-GND condition (when VS and input are high and a short to GND happens suddenly), an open-loop fast-response behavior is set to turn off the channel, before the current-limit closed loop is set up. The open-loop response time is around 1 µs. With this fast response, the device can achieve better inrush-suppression performance.
