7.3.1 Dual-Rail Supply

The LP5952 requires two different supply voltages:

• V_IN, the power input voltage, is regulated to the fixed output voltage.
• V_BATT, the bias input voltage, supplies internal circuitry.

It is important that V_IN does not exceed V_BATT at any time. If the device is used in the typical post-regulation application as shown in Figure 18, the sequencing of the two power supplies is not an issue because V_BATT supplies both the DC-DC regulator and the LP5952 device. The output voltage of the DC-DC regulator takes some time to rise up and supply the input voltage of the device. In this application V_IN always ramps up more slowly than V_BATT.

If V_IN is shorted to V_BATT, the voltages at the two supply pins ramp up simultaneously causing no problems.

However, in applications with two independent supplies connected to the LP5952, special care must be taken to ensure that V_IN is always ≤ V_BATT.

7.3.2 No-Load Stability

The LP5952 remains stable and in regulation with no external load. This is an important consideration in some circuits, for example, CMOS RAM keep-alive applications.

7.3.3 Fast Turnon

Fast turnon is ensured by an optimized architecture allowing a fast ramp of the output voltage to reach the target voltage while the inrush current is controlled low at 120 mA typical (for a C_OUT of 2.2 µF).

7.3.4 Short-Circuit Protection

The LP5952 is short-circuit protected and, in the event of a peak overcurrent condition, the output current through the NFET pass device is limited.

If the overcurrent condition exists for a longer time, the average power dissipation increases depending on the input-to-output voltage difference until the thermal shutdown circuitry turns the NFET off. Refer to Power Dissipation and Device Operation for power dissipation calculations.

7.3.5 Thermal-Overload Protection

Thermal-overload protection limits the total power dissipation in the LP5952. When the junction temperature exceeds T_J = 165°C typical, the shutdown logic is triggered, and the NFET is turned off, allowing the device to cool down. After the junction temperature dropped by 20°C (temperature hysteresis) typical, the NFET is activated again. This results in a pulsed output voltage during continuous thermal-overload conditions.

The thermal-overload protection is designed to protect the LP5952 in the event of a fault condition. For normal continuous operation, do not exceed the absolute maximum junction temperature rating of T_J = 150°C (see Absolute Maximum Ratings).

7.3.6 Reverse Current Path

The internal NFET pass device in LP5952 has an inherent parasitic body diode. During normal operation, the input voltage is higher than the output voltage, and the parasitic diode is reverse biased. However, if the output is pulled above the input in an application, the current flows from the output to the input as the parasitic diode gets forward biased. The output can be pulled above the input as long as the current in the parasitic diode is limited to 50 mA. For currents above this limit an external Schottky diode must be connected from V_OUT to V_IN (cathode on V_IN, anode on V_OUT).

7.4.1 Enable Operation

The LP5952 may be switched to an ON or OFF state by a logic input at the EN pin, V_EN. A logic high at this pin turns the device on. When the enable pin is low, the regulator output is off, and the device typically consumes 0.1 µA.

If the application does not require the enable switching feature, the EN pin must be tied to V_BATT to keep the regulator output permanently on.

To ensure proper operation, the signal source used to drive the EN input must be able to swing above and below the specified turnon and turnoff voltage thresholds shown in V_IL and V_IH in Electrical Characteristics: Enable Control.