The LM5148 operational input voltage range is from 3.5 V to 80 V. The device is intended for step-down conversions from 12-V, 24-V, and 48-V supply rails. The application circuit in Figure 9-28 shows all the necessary components to implement an LM5148 based wide-V_IN single-output step-down regulator using a single supply. The LM5148 uses an internal LDO to provide a 5-V VCC bias rail for the gate drive and control circuits (assuming the input voltage is higher than 5 V with additional voltage margin necessary for the subregulator dropout specification).

In high input voltage applications, take extra care to ensure that the VIN and SW pins do not exceed their absolute maximum voltage rating of 85 V during line or load transient events. Voltage excursions that exceed the applicable voltage specifications can damage the device.

Care must be taken in applications where there are fast input transients that cause the voltage at VIN to suddenly drop more than 2 V below the VOUT setpoint. The LM5148 has an internal ESD diode from the VOUT to the VIN pins that can conduct under such conditions causing the output to discharge. To prevent damage to the internal ESD diode under the said conditions, TI recommends adding a Schottky diode in series with the VIN pin of the LM5148 to prevent reverse current flow from VOUT to VIN.

As V_IN approaches V_OUT, the LM5148 skips t_OFF cycles to allow the controller to extend its duty cycle up to approximately 99%. Refer to Dropout Mode.

Use Equation 1 to calculate when the LM5148 enters dropout mode.

• t_P is the oscillator period

• t_OFF is the minimum off time, typical 90 ns

Figure 8-1 Dropout Mode Operation