A high-accuracy current limit allows higher reliability, which protects the power supply during short circuit or power up. Also, a current limit can save system costs by reducing PCB traces, connector size, and the capacity of the preceding power stage.

Current limit offers protection from over-stressing to the load and integrated power FET. Current limit holds the current at the set value, and pulls up the SNS pin to V_SNSFH and asserts the FLT pin as diagnostic reports. The three current-limit thresholds are:

• External programmable current limit – An external resistor, R_ILIM, is used to set the channel current limit. When the current through the device exceeds I_CL (current limit threshold), a closed loop steps in immediately. V_GS voltage regulates accordingly, leading to the V_DS 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.

  Additionally, this value can be dynamically changed by changing the resistance on the ILIM pin. This information can be seen in the Applications section.

• Internal current limit: I_LIM pin shorted to ground – If the external current limit is out of range on the lower end or the I_LIM pin is shorted to ground, the internal current limit is fixed and typically 12 A. To use the internal current limit for large-current applications, tie the I_LIM pin directly to the device GND.
• Internal current limit: I_LIM pin open – If the external resistor is out of range on the higher end or the ILIM pin is open, the current limit reverts to 6 A or half the current limit range. This level is still above the nominal operation for the device to operate in DC STEADY state, but is low enough that if a pin fault occurs and the R_ILIM opens up, the current does not default to the highest rating and put additional stress on the power supply.

Both the internal current limit (I_lim,nom) and external programmable current limit are always active when V_BB is powered and EN is high. The lower value one (of I_LIM and the external programmable current limit) is applied as the actual current limit. The typical deglitch time for the current limit to assert is 2.5 µs.

Note that if a GND network is used (which leads to the level shift between the device GND and board GND), the ILIM pin must be connected with device GND. Use Equation 2 to calculate R_ILIM.

For better protection from a "hot short" condition (when V_BB is high, channel is on, and a short to GND happens suddenly), an overcurrent protection, OVCR, circuit is triggered that makes sure to limit the maximum current the device allows to go through. With this OVCR, the device is protected during "hot short" events.

For more information about the current limiting feature, see the Short-Circuit and Overload Protection section.

Current Limit Accuracy Across V_DS

The TPS1HC30-Q1 has very tight accuracy of the current limit regulation level across the full range of currents and temperature. This accuracy is defined at several defined RILIM values, 7.15 kΩ, 25 kΩ, and 71.5 kΩ specified in the Electrical Characteristics at VDS = 3 V. However, as V_DS (V_BB – V_OUT) increases , the current regulation value also slightly increases. Taking a typical device, at the 3 different current limits ranges specified, sweeping the VDS voltage, and plotting the regulation value gives the graphs below.

Using a point during the regulation time of each of the different RILIM settings, the graph can be normalized to the specification in the electrical characteristics of V_DS = 3 V which results in graph below.

Using this figure, the current limit regulation value can be estimated for any current limit value desired based on the VDS value seen in the application. These graphs were taken on a typical device and should be used as reference when accounting for current limit tolerances. As an example see table below for regulation values based on setting the current limit close to the maximum load current. Note that RILIM tolerances are not factored into analysis below.