The TPS20xxE responds to overloads by limiting output current to the static I_OS levels shown in Section 6.5. When an overload condition is present, the device maintains a constant output current, with the output voltage determined by (I_OS × R_LOAD). Two possible overload conditions can occur. The first overload condition occurs when either:

1. input voltage is first applied, enable is true, and a short circuit is present (load which draws I_OUT > I_OS)
2. input voltage is present and the TPS20xxE are enabled into a short circuit.

The output voltage is held near zero potential with respect to ground and the TPS20xxE ramps the output current to I_OS. The TPS20xxE limits the current to I_OS until the overload condition is removed or the device begins to thermal cycle. This is demonstrated in Figure 8-4 where the device was enabled into a short, and subsequently cycles current OFF and ON as the thermal protection engages.

The second condition is when an overload occurs while the device is enabled and fully turned on. The device responds to the overload condition within t_IOS (Figure 6-5 and Figure 6-6) when the specified overload (see Section 6.5) is applied. The response speed and shape varies with the overload level, input circuit, and rate of application. The current limit response vary between simply settling to I_OS, or turnoff and controlled return to I_OS. Similar to the previous case, the TPS20xxE limits the current to I_OS until the overload condition is removed or the device begins to thermal cycle.

The TPS20xxE thermal cycles if an overload condition is present long enough to activate thermal limiting in any of the above cases. This is due to the relatively large power dissipation [(V_IN – V_OUT) × I_OS] driving the junction temperature up. The device turns off when the junction temperature exceeds 135°C (minimum) while in current limit. The device remains off until the junction temperature cools 10°C and then restarts.

There are two kinds of current limit profiles typically available in TI switch products that are similar to the TPS20xxE. Many older designs have an output I vs V characteristic similar to the plot labeled Current Limit with Peaking in Figure 7-2. This type of limiting can be characterized by two parameters, the current limit corner (I_OC), and the short circuit current (I_OS). I_OC is often specified as a maximum value. The TPS20xxE family of parts does not present noticeable peaking in the current limit, corresponding to the characteristic labeled Flat Current Limit in Figure 7-2. This is why the I_OC parameter is not present in Section 6.5.

Figure 7-2 Current Limit Profiles