Capacitive Charging Circuit shows the typical set up for a capacitive load application and the internal blocks that function when the device is used. Note that all capacitive loads will have an associated "load" in parallel with the capacitor that is described as a resistive load but in reality it can be inductive or resistive.

Figure 8-7 Capacitive Charging Circuit

The first thing to check is that the nominal DC current, I_NOM, is acceptable for the TPS281C30 device. This can easily be done by taking the R_θJA from the Thermal Section and multiplying the RON of the TPS281C30 and the INOM with it, add the ambient temperature and if that value is below the thermal shutdown value the device can operate with that load current. For an example of this calculation see the Applications Section.

The second key care about for this application is to make sure that the capacitive load can be charged up completely without the device hitting thermal shutdown. This is because if the device hits thermal shutdown during the charging, the resistive nature of the load in parallel with the capacitor will start to discharge the capacitor over the duration the TPS281C30x is off. Note that there are some application with high enough load impedance that the TPS281C30 hitting thermal shutdown and trying again is acceptable; however, for the majority of applications the system should be designed so that the TPS281C30x does not hit thermal shutdown while charging the capacitor.

With the current clamping feature of the TPS281C30x, capacitors can be charged up at a lower inrush current than other high current limit switches. This lower inrush current means that the capacitor will take a little longer to charge all the way up. However, to minimize this longer charge time during startup, TPS281C30 implements an inrush current handling feature described in On-State Short Circuit Behavior. When the EN pin goes high to turn on the high side switch, the device will default its current limit threshold to I_{LIM_STARTUP} for a duration of I_{LIM_STARTUP_DELAY.} During this delay period, a capacitive load can be charged at a higher rate than what typical I_CL would allow and FAULT will be masked to prevent unwanted Fault triggers. After I_{LIM_STARTUP_DELAY}, the current limit will default back to I_CL and Fault will work normally.

Figure 8-8 Inrush Current Handling

The initial inrush current period when the current limit is higher enables two different system advantages when driving loads:

• Enables higher load current to be supported for a period of time of the order of milliseconds to drive high inrush current loads like incandescent bulb loads.
• Enables fast capacitive load charging. In some situations, it is ideal to charge capacitive loads at a higher current than the DC current to ensure quick supply bring up. This architecture allows a module to quickly charge a capacitive load using the initial higher inrush current limit and then use a lower current limit to reliably protect the module under overload or short circuit conditions.

Figure 8-9 Auto-retry Behavior After ILIM_STARTUP_DELAY Period Expires

Figure 8-10 Auto-retry Behavior Before ILIM_STARTUP_DELAY Period Expires

While in current limiting mode, at any level, the device will have a high power dissipation. If the FET temperature exceeds the over-temperature shutdown threshold, the device will turn off just the channel that is overloaded. After cooling down, the device will re-try. If the device is turning off prematurely on start-up, it is recommended to improve the PCB thermal layout, lower the current limit to lower power dissipation, or decrease the inrush current (capacitive loading).

For more information about capacitive charging with high side switches see the How to Drive Capacitive Loads application note. This application note has information about the thermal modeling available along with quick ways to estimate if a high side switch will be able to charge a capacitor to a given voltage.