7.3.1 Internal Current Limit

The TPS780 is internally current-limited to 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, do not operate the device in a current-limit state for extended periods of time.

The PMOS pass element in the TPS780 family 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 to 5% of rated output current may be appropriate.

7.3.2 Shutdown

The enable pin (EN) is active high and is compatible with standard and low-voltage CMOS levels. When shutdown capability is not required, connect EN to the IN pin, as shown in Figure 54.

Figure 54. Circuit Showing EN Tied High When Shutdown Capability is not Required

7.3.3 Active V_OUT Pulldown

In the TPS780 series, the active pulldown discharges V_OUT when the device is off. However, the input voltage must be greater than 2.2 V for the active pulldown to work.

Table 1. Device Functional Mode Comparison

7.4.1 Normal Operation

The device regulates to the nominal output voltage under the following conditions:

• The input voltage is greater than the nominal output voltage plus the dropout voltage (V_OUT(nom) + V_DO).
• The enable voltage has previously exceeded the enable rising threshold voltage (V_EN > V_EN(HI)) and not yet decreased below the enable falling threshold.
• The output current is less than the current limit (I_OUT < I_CL).
• The device junction temperature is less than the thermal shutdown temperature (T_J < T_SD).

7.4.2 Dropout Operation

If the input voltage is lower than the nominal output voltage plus the specified dropout voltage, but all other conditions are met for normal operation, the device operates in dropout mode. In this mode, the output voltage tracks the input voltage. During this mode, the transient performance of the device becomes significantly degraded because the pass device is in a triode state and no longer controls the current through the LDO. Line or load transients in dropout can result in large output-voltage deviations.

7.4.3 Disabled

The device is disabled under the following conditions:

• The enable voltage is less than the enable falling threshold voltage (V_EN < V_EN(LO)) or has not yet exceeded the enable rising threshold.
• The device junction temperature is greater than the thermal shutdown temperature (T_J > T_SD).

7.5.1 Programming the TPS78001 Adjustable LDO Regulator

The output voltage of the TPS78001 adjustable regulator is programmed using an external resistor divider as shown in Figure 55. The output voltage operating range is 1.2 V to 5.1 V, and is calculated using Equation 1:

Equation 1.

where

• V_FB = 1.216 V typical (the internal reference voltage)

Resistors R₁ and R₂ should be chosen for approximately 1.2-μA divider current. Lower value resistors can be used for improved noise performance, but the solution consumes more power. Higher resistor values should be avoided because leakage current into/out of FB across R₁/R₂ creates an offset voltage that artificially increases/decreases the feedback voltage and thus erroneously decreases/increases V_OUT. Table 2 lists several common output voltages and resistor values. The recommended design procedure is to choose R₂ = 1 MΩ to set the divider current at 1.2 μA, and then calculate R₁ using Equation 2:

Equation 2.

Figure 55. TPS78001 Adjustable LDO Regulator Programming