7.3.1 Internal Current Limit

The TPS7A8101 internal current limit helps protect the regulator during fault conditions. During current limit, the output sources a fixed amount of current that is largely independent of output voltage. For reliable operation, the device should not be operated in a current limit state for extended periods of time.

The PMOS pass element in the TPS7A8101 has a built-in body diode that conducts current when the voltage at OUT exceeds the voltage at IN. This current is not limited, so if extended reverse voltage operation is anticipated, external limiting may be appropriate.

7.3.2 Shutdown

The enable pin (EN) is active high and is compatible with standard and low voltage, TTL-CMOS levels. When shutdown capability is not required, EN can be connected to IN.

7.3.3 Start-Up

Through a lower resistance, the bandgap reference can quickly charge the noise reduction capacitor (C_NR). The TPS7A8101 has a quick-start circuit to quickly charge C_NR, if present; see the . At start-up, this quick-start switch is closed, with only 33 kΩ of resistance between the bandgap reference and the NR pin. The quick-start switch opens approximately 100 ms after any device enabling event, and the resistance between the bandgap reference and the NR pin becomes higher in value (approximately 250 kΩ) to form a very good low-pass (RC) filter. This low-pass filter achieves very good noise reduction for the reference voltage.

Inrush current can be a problem in many applications. The 33-kΩ resistance during the start-up period is intentionally put there to slow down the reference voltage ramp up, thus reducing the inrush current. For example, the capacitance of connecting the recommended C_NR value of 0.47 μF along with the 33-kΩ resistance causes approximately 80-ms RC delay. Start-up time with the other C_NR values can be calculated as:

Equation 1.

Although the noise reduction effect is nearly saturated at 0.47 μF, connecting a C_NR value greater than 0.47 μF can help reduce noise slightly more; however, start-up time will be extremely long because the quick-start switch opens after approximately 100 ms. That is, if C_NR is not fully charged during this 100-ms period, C_NR finishes charging through a higher resistance of 250 kΩ, and takes much longer to fully charge.

A low leakage C_NR should be used; most ceramic capacitors are suitable.

7.3.4 Undervoltage Lock-Out (UVLO)

The TPS7A8101 uses an undervoltage lock-out circuit to keep the output shut off until the internal circuitry is operating properly. The UVLO circuit has a de-glitch feature so that it typically ignores undershoot transients on the input if they are less than 50-μs duration.